Whether made of Nb₃Sn or higher-performance superconductors, such devices promise to substantially improve the discovery potential of hadron colliders. Since their energy reach scales as the dipole field times the size of the tunnel, each additional tesla directly expands the energy frontier.

What makes these magnets unique is their compactness. Superconducting coils can carry a current density of order 500 A/mm², a factor 100 higher than what can be tolerated by copper with active cooling. A magnet based on superconductivity can therefore have coils that are narrower and lighter.

No application of superconductivity pushes this limit harder than an accelerator magnet. Larger coils mean larger magnets and an unaffordably large tunnel to accommodate them. Accelerator magnets must therefore be highly optimised in space and cost – the capsule hotels of superconductivity – and this extreme optimisation creates opportunities for spinoff applications, from lightweight motors for electric aircraft to power transmission beneath the pavement of a crowded metropolis. Superconducting accelerator devices have already paved the way for societal applications in medical imaging and advanced accelerators for cancer therapy, and the field continues to benefit from strong research synergies with fusion tokamaks, though their toroidal coils don’t need to push the limits of current densities in the same way.

Superconductors also save energy. At the LHC, more than a thousand niobium–titanium alloy (Nb–Ti) superconducting dipoles are powered by only 40 MW. This is much less than what is consumed by the LHC’s injectors.

As dipoles based on Nb-Ti superconductors are limited to a maximum achievable field of nearly 10 tesla, corresponding to an operational field of about 8 tesla with acceptable margins, accelerator physicists and engineers are exploring the use of better superconductors to roughly double their field. The options include Nb₃Sn, which will soon be used in an accelerator for the first time at the HL-LHC, and “high temperature” superconductors that promise much higher performance and a simplified accelerator infrastructure. But dipoles are much more difficult to design than solenoids. Though 30 tesla solenoid magnets are already available on the market, no one has yet succeeded in building a 20 tesla dipole magnet.

Shear complexity

An accelerator dipole poses several challenges compared to a solenoid. While a solenoid’s current loops generate an axial magnetic field, a dipole must use vertically separated coils to generate a vertical magnetic field; for the same total coil thickness and current density, a solenoid can provide twice the field strength of a dipole; and the field distribution and the forces exerted on the coils are much more difficult to control. In a solenoid, electromagnetic forces are perpendicular to the conductor, but in a dipole they push the coil towards the midplane and outwards, with a two-dimensional distribution that includes shear stresses.

The engineering challenge is increased by the need for dipoles to operate precisely during the ramp, when particles gain energy with every turn after being injected into the collider, requiring increasingly strong magnetic fields to bend them. To ensure that accelerator physicists can make tightly focused beams collide with high luminosity inside the experiments, the field must be uniform to better than one part in 10⁴ across two thirds of a dipole’s aperture as the field increases up to a factor 15. These challenges are not present in either medical-imaging magnets or the toroidal coils used for fusion, which must operate at a constant current, though the toroidal coils used for fusion are subject to rapidly varying external magnetic fields.

In the context of the 2026 update to the European Strategy for Particle Physics (ESPP), advanced high-field dipole magnets would be needed by the hadron-collider phase of the Future Circular Collider (FCC-hh) and the proposed muon collider. Due to its exceptionally large and unstable beams, a muon collider would also require a kilometre-long channel of superconducting solenoids with alternating gradient, and a final superconducting cooling solenoid with a strength of roughly 40 tesla before the collider ring. These challenges are complementary to what is required by the FCC-hh, and the community is devoting significant research and development in this direction.

The targets initially set for the FCC-hh in 2014 were based on round numbers: a 100 km tunnel and a centre-of-mass energy of 100 TeV. This required 16 tesla dipoles, one or two tesla above what can be done with adequate margins and costs with present technology. After a decade of studies, the tunnel size was reduced to 91 km to fit geological constraints, and the field was brought down to 14 tesla, allowing a centre-of-mass energy of 85 TeV after some optimisation of the lattice. This 15% reduction in the energy in the centre-of-mass frame has had a major effect on the energy consumption of the collider, as synchrotron radiation reduced by 50%. A similar tuning occurred for the LHC, which was initially imagined at 16 TeV with 10 tesla magnets rather than today’s 13.6 TeV and 8.1 tesla.

The baseline design for the FCC-hh dipole magnets is Nb₃Sn technology operated at 1.9 K, though the ESPP documents also note three other possibilities: hybrid magnets that use substitute Nb–Ti for Nb₃Sn in the lower field regions; operation at 4.5 K; and a high-temperature-superconductor option operating between 4.5 and 20 K with magnetic fields in the range 14 to 20 tesla.

The Nb₃Sn path

Nb₃Sn was discovered a few years before Nb-Ti and has the advantage of providing current densities in excess of 500 A/mm² up to 16 T (see “Superconductors for high-field accelerator magnets” figure). After 35 years of research, fields have now reached 14.5 tesla, close to the 15–16 tesla target needed to have magnets operating at 14 tesla in the FCC-hh with adequate margins (see “Niobium dipoles” figure). The main goal today is to produce a double-aperture short-model Nb₃Sn magnet with all features specified in the FCC-hh design. This should be achieved by 2030 and then scaled up in length.

A key challenge is to reduce the quantity of Nb₃Sn, thereby lowering both the cost and hysteresis losses during field ramping. As the magnetic field changes, currents are induced within the superconducting filaments, leading to energy dissipation that must be carefully controlled. Minimising these losses is one reason for the complex, multi-filamentary architecture of superconducting wires. The smaller filaments of Nb-Ti can significantly reduce the losses, and Nb-Ti costs five to 10 times less than Nb₃Sn.

A second engineering challenge is to achieve a mechanical structure capable of keeping the coil in compression during powering but not overstressing it. The stress limits of Nb₃Sn are of the order of 200 MPa, and the required precompression for a 14 tesla dipole is about 150 MPa.

Another challenge of the low-temperature path would be logistical: the production of roughly 5000 tonnes of Nb₃Sn. This corresponds to a 1 kA cable from the Earth to the Moon at a cost of several billions of dollars. These numbers are an order of magnitude larger than what was needed for the Nb-Ti coils of the LHC.

Despite these challenges, Nb₃Sn technology is now well established for small series, and will soon play a key role at the High-Luminosity LHC – the technology’s first use in a working accelerator, though for focusing beams rather than bending them (see “Nb₃Sn quadrupoles” figure). But newer superconductors may well prove competitive.

The high-temperature path

In 1986, Johannes Georg Bednorz and Karl Alexander Müller announced the discovery of superconductivity above 35 K, something not foreseen by theory, and well above the boiling point of liquid helium. “High-temperature” superconductors (HTS) not only remain superconducting at high temperatures, in many cases above the boiling point of liquid nitrogen (though at 77 K HTS performance is not yet adequate for our needs ), but also at high fields. HTS solenoids have been constructed with fields up to 40 tesla, and though the problem of degradation is not yet totally solved, progress has been outstanding.

Three families of superconducting conductors are currently available or emerging on the market: rare-earth barium copper oxides (REBCO), bismuth strontium calcium copper oxides (BSCCO) and iron-based superconductors (IBS).

REBCO is of strong interest in the world of fusion. Billions of dollars of investment have reduced the cost by more than an order of magnitude in the past decades. REBCO comes in tapes (see “Frontier superconductors” figure). A 12 mm-wide tape has thickness of 0.1 mm and can carry 1500 A at 4.5 K, or about half that at 20 K. 20 tesla peak field coils have been built and tested for fusion applications, and private investors plan to build reactors that are much more compact than ITER, which is based on Nb₃Sn technology. 

Manufacturing REBCO coils is greatly simplified compared to Nb₃Sn as the tape needs no temperature treatment; but the technology used to wind the tapes is not easy to adapt for accelerator dipole magnets, which are radically different from the toroidal coils designed for tokamaks. The challenge here is not to develop a conductor for accelerator magnets, but to adapt our magnet designs to this amazing tape. There is a long way to the 15–16 tesla target, but the potential is huge, with progress being made in Europe, the US and China (see “HTS dipoles” figure).

And what of the other HTS superconductors? BSCCO has the great advantage of round wires, but must be treated at 800 °C and it does not profit from synergies with fusion. At present, this path is only being pursued in the US, with achieved fields of just 1.8 T. IBS is being actively developed in China and Europe, but its current density has not yet matched the performance of REBCO, and the best results were obtained for tapes rather than wires.

HTS would allow operation at 20 K, with a simplification of the cooling scheme and a possible reduction in the energy consumption of the collider, though at 85 TeV half of the heat loads are due to synchrotron radiation, which does not depend on the operational temperature of the magnets. Moreover, REBCO tape has a single filament, as wide as the tape, and therefore the saving from the higher operational temperature could be compensated by larger heat losses. Estimating the energy balance is far from trivial: do not draw easy conclusions!

Optimal solution

Addressing these challenges is the work of the High Field Magnet (HFM) programme, an international collaboration with 15 institutes steered by CERN that was founded in 2021. HFM is exploring multiple different designs to find the optimal solution, from the most classical to the more exotic, and novel ideas should be explored in parallel to the most conservative paths. Though there are major challenges ahead, solving them promises societal benefits via a number of diverse spinoff applications.

High-field magnets remain one of the hardest problems in applied superconductivity. The next decade will be decisive for understanding the feasibility and cost of the FCC-hh.

The post Superconductors for the energy frontier appeared first on CERN Courier.

Coming back to Fermilab is, for me, a little like coming home. My family and I moved to the United States in 1998, and Fermilab was the first place I worked in the Department of Energy (DOE) system. It was also a place where people really took me in. Fermilab, like many national laboratories, is built on the shoulders of giants – and Robert Wilson was one of them.

He got this huge site, more than 6000 acres, with a real vision for expansion and growth in science. He was also a genuine fan of architecture, truly inspired by it. Our Wilson Hall is a tribute to that. It echoes what people call the folding hands of Beauvais Cathedral in France. Having that building stand out from the prairie was a statement.

That’s Robert Wilson’s legacy at Fermilab: a science of statements and the ability to do things fast, effectively, things that people thought could not be done. So, honestly, sitting in that chair feels good.

Wilson’s 1969 Congressional testimony is one of the most celebrated defences of fundamental science. What do you make of his case today?

He told Congress that high-energy physics had to do with dignity and all the things that we really venerate and honour in our country. That is still true. Despite the strain on science funding and all the questions about whether we are spending money effectively, the government is still willing to invest more than five billion dollars at Fermilab over the next five to ten years. This feels almost contrarian to what you hear in the press. Yes, science is under pressure. But the commitment is there, for the very same reason Bob Wilson stated back then.

That said, I believe we carry a genuine responsibility to deliver to society. That has been the basis of the social contract since Vannevar Bush wrote Science, the Endless Frontier in 1945; the document that helped create the national laboratory system and agencies such as DOE, the National Science Foundation and NASA. I don’t expect every citizen to understand exactly what a neutrino does or why it matters. But the outcomes of science, and the technology we develop on the way, whether that’s AI, quantum information tools, electronics, those are things we have to deliver. It’s part of the social contract.

Then, under Leon Lederman, and driven forwards by figures like Helen Edwards, Fermilab expanded the world’s energy frontier with the Tevatron…

Helen Edwards is actually directly responsible for the fact that I’m in this country. It’s her fault, really. When I was a group leader at DESY in 1998, 37 years old, with two small kids and having just built a house in Germany, Helen walked into my office. She asked, “Norbert, what do you want to do with your future?” She was very direct and wouldn’t take no for an answer. I hesitated, and she said, “You need to think about this. You should go to the United States.” Six months later, I was at Fermilab.

She was undeterrable. If she had a mission, a North Star, there was no lab director, no government official, no one who could deflect her from it. She and Alvin Tollestrup, a name that doesn’t get talked about enough, developed the superconducting magnet technology under Leon Lederman’s leadership that made the Tevatron what it was. That technology later allowed DESY to build HERA and ultimately landed in the LHC at CERN.

Alvin could explain superconductor physics on first principles and very quickly come to how you wind a magnet and what fundamentally limits its performance. A physicist and a technologist at the same time. They were both giants. There’s no question about it.

You mentioned moving from Europe to the United States. How different were the two scientific cultures, in the late 1990s?

You sure you want to write about this? [chuckles] Before I left DESY, I went to the director, Björn Wiik. He was himself a visionary leader, the person behind the TESLA concept for superconducting RF. When he asked where I saw myself in five or ten years, I answered, “I want your job. I want to be a director.” He was very direct too. “You are only 35 years old,” he said. “To become a director in Europe, you have to look like me. You have to have grey hair and a beard.” I found that frustrating. But I think it was largely true at the time.

In the United States, age didn’t matter. Nationality didn’t matter. What mattered was: could I do it? A 39-year-old German, alongside a Canadian, Thom Mason, and the son of Croatian immigrants, Anthony Chargin, suddenly found themselves in charge of building one of the biggest science projects in the United States: the Spallation Neutron Source, inspired by a former South Korean accelerator physicist, Yanglai Cho. That’s a story you can’t make up. That is where my career really started.

The transition from Lederman to John Peoples coincided with both the golden age of the Tevatron and the era of the Superconducting Super Collider (SSC). What do those two directors, and that moment, tell us about leadership in big science?

I knew Leon well because I actually lived in his house. He had a place off-site, and when my family first arrived we had very little money, so he said: “You need a house. I have one.” And we moved in. He came by regularly, stored his Porsche in the garage, and we talked a great deal. I learned a lot from him.

He was the kind of person you simply liked. Everybody at Fermilab loved Leon. He was funny, extraordinarily smart and he had a vision for the laboratory. I asked him once why he stepped down after nine years as director. He told me, “If you are a lab director, you have to make important decisions, and with every decision you make, you lose 10 percent of your friends. After 10 decisions, they are all gone. That is when you step down.” That was a true Leon answer. But it reflected his deep understanding of what leadership really costs.

I deeply believe high-energy physics can again be a launchpad for open international collaboration

John Peoples was very different. He was hands-on, deeply involved in building the complex and the Antiproton Source. Where Leon was the beloved visionary, John was the builder who wanted to be involved. And he had two extraordinarily difficult jobs at the same time: managing the closure of the SSC in Texas, which you could see drain him, and running a programme that ultimately delivered the discovery of the top quark.

These were very different people, very different characters. I think every character has its time. That is as true at Fermilab as it is at CERN. You can tell the same story through CERN’s directors. We just lost one, Herwig Schopper, who was a phenomenal leader. He spoke openly about the sacrifices he and the laboratory had to make to get CERN going. And when you look at CERN 50 years later, that is still a defining legacy, with the 27-kilometre tunnel and the science that continues to come out of it.

What lessons does the abandonment of the SSC hold for the large-scale projects being discussed today?

The real lesson of the SSC isn’t the failure itself. It is about implementation. The days when you could go to a government and say your project costs this much, then come back the next year and ask for 20 percent more, and the year after that another 20 percent – those days are gone. That is not the world we live in, and at the scale of projects we are talking about today, it would not be responsible.

John understood that deeply. I have tried to carry it through my own career. On my watch, I will always be direct with our funding agencies about what I see as risks and what things actually cost. That is non-negotiable for me.

Fermilab then repositioned itself at the intensity frontier. How do you keep the laboratory aligned behind the Long-Baseline Neutrino Facility (LBNF) and the DUNE experiment?

You form a team, you focus the team and you execute. That sounds pretty mundane and simple. It is not. It is really hard. CERN went through something very similar under Robert Aymar with the LHC: the necessity to focus every resource and every engineering capability on one thing to make it happen.

I am a scientist, but also a project guy. I wake up every morning thinking about those five billion dollars. That is roughly eight hundred million a year. Three million dollars a day. My job is to organise a team that can responsibly and effectively deploy that every single day to build LBNF/DUNE.

When I spoke at my first all-hands meeting here, I laid out three bullet points, because nobody remembers more than three. First: beam at the DUNE far detector by 2031. Second: science at the High-Luminosity LHC and delivering on our commitments there. Third: develop science, technology and innovation for the benefit of society. Those are the three and everything flows from them.

I use the story of JFK visiting NASA and asking the janitor why he is there. The janitor says: “To put a man on the Moon.” That is the answer I want from everyone here. So I go around and ask people why they are here. And if I don’t get the answer I want, I ask again.

Neutrino physics is also receiving major investments in China and Japan, with JUNO already closing in on the neutrino mass hierarchy and Hyper-Kamiokande equipped to measure leptonic CP violation when it comes online. How does DUNE fit in that landscape?

We live in a world that is not the world of 20 or 30 years ago. We have to recognise that. But I deeply believe high-energy physics can again be a launchpad for open international collaboration.

The neutrino story is phenomenal for the US with the DOE’s support of the DUNE project. It is also great for CERN. The most significant large-scale investment CERN has made in an external experiment is in DUNE. And it goes both ways: Fermilab contributes significantly to the HL-LHC programme. That is one of the healthiest collaborations in the field, both at the personal level and at the level of laboratories and programmes.

In my world, it is better to make the wrong decision and correct it than to make no decision at all

As for competition among neutrino facilities, it’s healthy. It is all about what I call the three C’s: collaboration, cooperation, competition. Every scientific relationship works better when you are clear about which is which. There is competition with other neutrino experiments, of course, in the sense that whoever reaches an answer first gets the golden nugget. But there is also technology exchange, open science and the free sharing of knowledge. Both things are true.

When you look at the DUNE detector and the beam we are building, it will be, hopefully sooner than later, the most effective research instrument for this kind of science. It is nice to be number one. You never stay number one forever, but it is nice. CERN is number one in collider physics right now – a pretty good feeling. But you also have to deliver results.

How would you describe Fermilab’s culture right now?

Scientists are driven by curiosity. That hasn’t changed and it won’t. But when a large institution commits to building a major instrument, there is real tension between the broad research culture that develops over time and the laser focus that construction demands. Is there stress in the system? Yes, honestly, there is. The best thing you can do is recognise that, talk about it openly and make sure people can see the light at the end of the tunnel.

The people who love construction have a clear finish line. The researchers have an extraordinary instrument coming, and the conceptual and technical work they do now is their investment in what comes after. The two groups are not perpendicular to each other. A good instrument requires constant feedback from the science side on what it actually needs to deliver, but you also can’t have an infinite conversation about what to build while you are trying to finish building it. Finding that line is delicate, and I spent my life basically walking it. At the SNS, at LCLS-II, at ITER. You pick.

There is a saying I keep coming back to: culture eats strategy for breakfast. Getting the culture right will take time and requires healthy tension. But it also requires the willingness to make decisions. I am not afraid to make a decision. Sometimes the wrong one, and that’s fine, it needs to be corrected. But in my world, it is better to make the wrong decision and correct it than to make no decision at all.

Where should Fermilab position itself in the next chapter of global high-energy physics?

I wanna stretch my hand to Europe, and to CERN in particular. I am very proud of the connection between our two institutions, at the programmatic level and at the personal level. I think we need to continue discussing how to keep the world open for those that want to share our values and share our way of doing science. People like me should be able to come to the United States. People from here should be able to go to CERN. That’s the foundation of everything we do.

The post Execution mode appeared first on CERN Courier.

Jos, FORM has been at the heart of precision calculations for decades. But the story starts earlier, with Martinus Veltman (see “The pioneer” image). What was he trying to do?

Jos Vermaseren In 1963, Veltman was interested in the renormalisation of Yang–Mills theories. He wanted to check whether certain models produced unphysical infinities that could not be removed. These calculations are a lot of work: you don’t do that by hand. So he built himself a program, which he called Schoonschip, to do that calculation.

What was computing like in those days?

Vermaseren Very primitive by current standards. When Veltman started at CERN, they had a CDC 6600, which was for a while the biggest computer in the world. But you had to share it with maybe a few thousand people, so you had to wait for your program to come out (see “The first supercomputer” image). At Nijmegen University in the early 1970s, we had an IBM computer where you had to hand in your computer cards, then wait a few hours for output. If your program was big, it would only run during the night. Make a typo, and you’d find out the next day that nothing had happened. That kind of primitive computing was left behind when personal computers came in the 1980s. I bought an Atari ST in late 1985, and the fun part was that at Nikhef, the Dutch National Institute for Subatomic Physics, we had a CDC 173, but my Atari had more memory! That was quite amazing. Every decade, the computers became more powerful, and with that the calculations became larger. I’ve been involved in calculations where the intermediate formulas were terabytes big. That is kind of hard to imagine. But if you put in enough effort and enough checking, you still get the correct answer. There is simply no way you could ever do that by hand. No way. That’s why we absolutely need these algebra programs.

Where did Schoonschip – I apologise for my pronunciation – fit in the landscape of early computer algebra?

Vermaseren Ah, Veltman did that intentionally to tease all the foreigners. [chuckles] There were already ideas about algebraic software in the 1960s – Feynman was suggesting something in the 1950s – but nothing really usable for physics calculations when Veltman started. Around the same time, Tony Hearn started with the REDUCE program, which was formally more elegant but less powerful. Those were the main players for a while, but they all had limitations. REDUCE wasn’t nearly as fast as Schoonschip and couldn’t handle very big expressions. Schoonschip’s limitation was that Veltman had written it in assembly, so you could only use it if you had the correct computer.

How did you enter this story?

Vermaseren I was very much used to Schoonschip and was quite a good programmer with it, but CDC computers were expensive and being phased out. So there I was, faced with the idea that I wouldn’t have Schoonschip any longer. I also wanted to make a giant system for doing automated calculations that would need computer algebra in a more flexible way than Schoonschip provided. If I needed new features, I’d have to go to Veltman and wait probably a year. Veltman had built in what he needed and was so nice to provide other people with his program. But if you get a free program, you shouldn’t come up with too many demands. The conclusion was that if I really wanted to make what I needed, I would need my own program.

Schoonschip had a couple of weak points. One was the sorting mechanism, which meant that with very large expressions, the program became outrageously slow. The handling of functions and function arguments was not flexible at all. And then there was the whole business of computer availability. I asked Nikhef management if they would allow me to take some time out to work on it, and they thought it was a good idea, so my back was covered.

This may resonate with early-career researchers who want to build long-lived tools today. What would you tell them?

Vermaseren You have to put in an enormous amount of time, and if you want to get a job in physics, you can only get credit for that if at the same time you use what you make for good calculations that draw attention. You need physics publications. If you go in as a postdoc to just write useful software, you have a problem, unless somebody has already promised you a decent job.

People like to count citations, and organisations usually look at citations in the first two years. But when you have a paper about a calculation, the opposite usually occurs. In the beginning you don’t get very many citations because people aren’t using it yet. I have a lot of papers that started with hardly anything, and then after a few years they pick up and keep growing. But for a postdoc, that is a disaster.

Thomas Gehrmann I’d add to this that recognition for contributions to scientific software is usually underrated when evaluating a researcher’s performance. It’s not recognised at the same level as publications or plenary talks. We should really try to communicate to senior people making funding decisions the importance of the whole body of scientific output. Scientific software development is very useful to the community but much less easily quantifiable than citations.

Vermaseren Although, for universities it is very nice to eventually have somebody there who is generating a lot of citations and educating people to do big calculations, they just don’t recognise it. The world of theory software development needs more institutional support.

Thomas, can you describe FORM’s impact on particle physics?

Gehrmann FORM enabled calculations that would never have been possible with any other tool. At each given moment in time, ever since the inception of FORM version one in the late 1980s, early 1990s, the cutting-edge calculations were usually done with FORM. Many of these calculations were redone a few years later with other tools, but what had changed was that computers became more powerful, had more memory, more storage space and were faster, so you could also do similar calculations in Mathematica or Maple. However, FORM was always at the avant-garde of the calculations.

In groups that are performing multi-loop calculations, the first-week’s task for a new student is usually: learn FORM on a simple example, compute the scattering matrix elements in FORM to get you used to its environment. For students working on cutting-edge projects – the next loop on a scattering amplitude, the next order on a benchmark cross-section – it’s made clear from the very onset that FORM is the tool to be used, because it’s only with this tool that there’s a realistic chance to get through the project in a finite amount of time.

Can you give an example of a particularly important calculation?

Gehrmann The LHC is a proton–proton collider, but the hard scattering processes underlying the collisions are not proton–proton but collisions of quarks and gluons. To make precise predictions for anything you observe at the LHC, you need to know how quarks and gluons are distributed inside the proton. These parton distribution functions are extracted from combined fits to huge sets of data from different experiments at vastly different energy scales. I mean, from a 35 GeV electron beam at SLAC up to multi-TeV collisions at the LHC. That’s almost three orders of magnitude.

Parton distributions evolve with energy scales via the Altarelli–Parisi evolution equations: knowing the Altarelli–Parisi splitting functions to sufficient theoretical precision is one of the cornerstones enabling these fits. The calculation that enabled the current level of precision was done in the early 2000s by Jos and his collaborators Sven Moch and Andreas Vogt. It went alongside the development of FORM version three, and was a crucial result for the entire LHC physics programme.

Looking ahead to the High-Luminosity LHC and a potential FCC, how important is FORM’s continued development?

Gehrmann Both are extremely high-statistics, high-luminosity machines. They’ll give us measurements at a statistical precision never achieved before in a collider experiment. Researchers need to be empowered with proper tools to make the most of the physics, with a whole new generation of precision calculations. FORM has grown with the field, due to both the ingenious design choices Jos made at inception, when a lot was already conceived in a scalable fashion, and through continuous development addressing bottlenecks. It’s very hard to predict what will be the bottlenecks for High-Luminosity LHC calculations, and it’s even harder for the FCC. But they will require adaptations to how we do computer algebra. And, of course, committed developers.

Josh, you’ve been working on FORM 5. Why is a major release necessary now?

Joshua Davies Being able to release new versions helps convey to the community that there’s progress. Most users stick to a released version rather than rebuilding from GitHub. Being able to say “this is a new version with well-tested new features” is important for users to trust it for their work.

What are the major new features?

Davies The first is a Feynman graph generator built into FORM, from a collaborator of Jos, Toshiaki Kaneko. FORM now has an interface to this generator that lets you produce graphs from within the code without relying on external tools. It’s written in a more flexible way, which lets you add features or modify it much more easily than other tools. I also put in an interface that improved polynomial arithmetic performance. This is increasingly necessary now that people study processes with higher multiplicities or more mass scales. You end up with computations depending on many more variables than in the past.

Vermaseren The third main feature is the ability to have floating-point coefficients as opposed to rational numbers. Modern algorithms still can’t determine everything through normal calculations. You’re restricted to doing certain parts in arbitrary-precision floating point. But these capabilities have other good features. If you want to do a calculation for the LHC, in the end these run in Monte Carlo integration programs: you take a very big formula and sample it billions of times. But how numerically stable is that formula? If I have floating-point capability, I can figure out the numerical stability before I evaluate it billions of times in another program. I can determine whether I’ll run into disasters.

What does the future hold for FORM’s development?

Davies It seems unlikely that anyone is suddenly going to fund a permanent job where the main role is looking after FORM. But if we can foster an environment where postdocs or PhD students feel they can contribute and be recognised for it, and it helps them apply for their next position, this needs to be the way packages like FORM are developed. I’m a postdoc trying to apply for longer-term positions, but the future of FORM isn’t secure. I’ve put in a lot of effort, alongside Coenraad Marinissen and Takahiro Ueda, to get FORM to version five, but it’s not guaranteed people working on FORM will be able to continue.

Do we need a different institutional framework to support this kind of development?

Davies We need more recognition from the people who decide where funding goes for contributions to software work. On the experimental side, there are people whose job is the LHC software that goes into the analysis chain. We don’t really have this equivalent on the theory side. People work on software alongside their physics projects, and you always have to have physics results coming out if you want to continue to get jobs. No one can truly focus one hundred percent on the tools. What would really help is if contributing to a project like FORM was clearly recognised as a valuable scientific output in its own right, alongside physics papers. If young researchers felt that contributing to core tools genuinely strengthened their career prospects rather than putting them at risk, it would completely change how sustainable projects like this are.

Gehrmann This is exactly right. Over the years, it was crucial to have Jos as a developer in the background regularly talking to the community, getting feedback: “This is the current bottleneck we’re up against.” But that only worked because Jos could actually focus on it. We’ve been trying to improve community involvement over the past five years with dedicated workshops, bringing together developers with users pushing FORM to their limits and students coming into the field. This format has started to take off successfully. At these workshops, in the mornings the senior developers explain the internal structure of the code. And then in the afternoons people work on concrete exercises like bug fixes or small features, almost like a hackathon. But this is a bottom-up initiative. It needs a top-down approach to make the project sustainable and create career perspectives for FORM developers like Josh. I can only hail the visionary decisions Nikhef management made 40 years ago when they decided to leave Jos alone for a few years to develop version one. Without institutional recognition that creates actual career paths for theory software developers, we risk losing the very people who can secure FORM’s future – and with it, our ability to make the most of the next generation of colliders. 

The post The most important tool you’ve never heard of appeared first on CERN Courier.

These unexpected objects came into view in JWST’s first data release in 2022 thanks to its sharp images and sensitivity in the near infrared. By summer of 2023, a number of discovery papers had been written about them, identifying three traits in common: they were compact in size, had unusual “V-shaped” spectra and they showed emission from high-velocity hydrogen gas. Due to their compact size and red colour in the rest frame, they were dubbed little red dots. A few appeared in every pointing of the JWST imaging camera NIRCam, accounting for a few percent of all known galaxies in the first billion years of cosmic time. The race was on to determine their nature.

Two options initially appeared possible, but both were extraordinary and required a very precise tuning of parameters to fit the observations: too-dense galaxies or too-massive supermassive black holes. In either case, the objects had to be enshrouded in a cocoon of dust.

Galaxies or black holes?

The first paper assumed they were very massive galaxies, with their stars all assembled less than a billion years after the Big Bang. In favour of the galactic hypothesis were the V-shaped spectra, which are difficult to model without invoking massive stars. The vertex of the V-shape resembles a “Balmer break”, which is produced by the absorption of hydrogen atoms in the n = 2 level. Longward of the break, the optical continuum rises steeply toward the red, which this model attributed to the reddening of these stars by dust, with the UV being produced by starlight that was scattered out of the dust screen. However, the very high masses and early-universe star formation rates required for these models were difficult to reconcile with our understanding of the rate at which galaxies and their dark-matter halos assemble.

The first paper assumed they were very massive galaxies, with their stars all assembled less than a billion years after the Big Bang

The black-hole hypothesis was supported by evidence for very dense gas clouds moving at thousands of kilometres per second in the potential of a massive black hole. In this picture, surrounding dust would preferentially absorb ultraviolet light and re-emit it at longer wavelengths, producing the observed red colour. Though this explanation promised to alleviate the tension arising from the implied galaxy masses, it quickly became clear that these objects were not typical growing black holes. They were not detected in X-rays, nor did they show the characteristic 1000 K dust signature that is ubiquitous in actively accreting black holes. However, the most concerning piece of the black-hole interpretation was the implied black-hole masses. Applying local calibrations to the observed motion of gas in the little red dots implied black-hole masses of ten million to a billion suns, compared with galaxy masses of the same order – a stark contrast with local black holes, which have masses roughly a thousandth of their host galaxies. These overly massive black holes are hard to grow so far in advance of the galaxies, and also overproduce the total amount of black-hole mass created at such an early time.

Explaining their redness

Two major breakthroughs occurred in 2024 that clarified the nature of the little red dots. All the aforementioned models invoked heavy amounts of dust to suppress ultraviolet emission and produce the observed red colours. The conservation of energy implies that all the absorbed radiation should be re-emitted by the dust. However, multiple studies of populations and of luminous individual sources turned up non-detections of dust emission. These stringent limits on the far-infrared energy output were enough to conclusively rule out these entire classes of models, invoking reddening by dust to explain the observed red colours.

At the same time, campaigns to observe the broad population of little red dots discovered a remarkable class of sources with very little ultraviolet emission and extreme Balmer breaks. These breaks could not be produced by anything resembling a stellar population we have observed before, and served as conclusive evidence that normal stars cannot be responsible for producing the optical emission in little red dots; the photoabsorption by hydrogen in the n = 2 energy state must nevertheless be a crucial physical aspect of the little red dots, even if it wasn’t happening in the atmospheres of massive stars.

Plausible scenarios

The challenge is therefore to explain the characteristic red colour of the little red dots without dust obscuration. Any successful model would also need a substantial reservoir of hydrogen around to cause the hydrogen absorption that looked like starlight, but wasn’t. One plausible scenario that could satisfy these requirements is very dense gas arranged quasi-spherically around the black hole. In this scenario, the black holes powering the little red dots could be significantly less massive than we had originally thought, when we had assumed that dust was obscuring most of the light from the growing black hole.

The task is to explain the characteristic red colour of the little red dots without dust obscuration

In this new picture, the little red dots are powered by black holes that are accreting at much higher rates than are typically seen at later times. A higher accretion rate implies greater luminosity for a given black-hole mass, and therefore we infer much lower black-hole masses, perhaps closer to a million suns, and much more aligned with the measured galaxy masses. As a side benefit, lower black-hole masses are much more natural for objects that are so prevalent, because the number of low-mass dark-matter halos and low-mass galaxies is much higher than the number of high-mass systems.

Astronomers are still arguing about how this dense gas is configured and accretes onto the black hole, and everyone has their favourite model. We do not know if the geometry of the system is completely spherical, or if we are seeing a mixed-phase medium where the viewing angle is an important parameter. These details matter, because if we can pin down the characteristic size and density of these gas envelopes, we may be able to infer more robust black-hole masses for the population. There has been some recent speculation that the little red dots may be marking the end stages of black-hole seed growth, in which case they could be a critical missing link in our understanding of the formation of the first black holes. However, without more concrete constraints on black-hole mass, we cannot know for sure. At the same time, we need a much better theoretical understanding of what makes little red dots so distinct from the more typical growing black holes we have studied for decades, and why that mode of growth becomes so much less common as the universe ages.

One thing we do know for sure: the more we learn about the little red dots, the more complex and unexpected they become. We are excited to see what new wrinkles arise as we enter our fifth year of JWST operations.

The post The mystery of the little red dots appeared first on CERN Courier.

Even more strikingly, genuinely enormous magnetic fields also arise in a more subtle way. At LHC energies, the electric fields between crystal planes are Lorentz-boosted into effective magnetic fields of hundreds to thousands of tesla in the rest frame of passing particles. This opens up some unique possibilities for particle physics: probes of new physics once limited to long-lived particles in conventional, orders-of-magnitude weaker magnets may now come within reach for short-lived baryons.

A bobsleigh on a track

Energy loss and multiple scattering are the fate of most charged particles in matter. If carefully aligned to particle trajectories, crystals can be an exception: as positively charged particles fly past nuclei in the planes of the crystal lattice, they experience an averaged electrostatic potential that channels them between the crystal planes. Provided they don’t have enough transverse energy to cross the potential barrier to a neighbouring crystal plane, the particles oscillate between the atomic planes like a bobsleigh on a track (see “Guided paths” figure). If the crystal is mechanically bent, the entire track curves, steering the particles along with it.

Crystal channelling was predicted in simulations by Robinson and Oen in 1963, experimentally confirmed the same year by Piercy, and given its theoretical foundation by Lindhard in 1965. The idea of using bent crystals for beam control was first proposed in 1976 by Tsyganov. Proof-of-principle experiments at JINR Dubna in 1979 demonstrated the channelling of 8.4 GeV protons, achieving deflections equivalent to an 81-tesla magnetic field, and practical applications followed soon after.

A key modern application of bent crystals is the selective extraction of particles from the beam halo rather than the beam core, to produce a secondary beam. Crystal-based beam extraction was demonstrated up to 8.4 GeV at JINR in Dubna in 1984, then with higher energy protons at IHEP Protvino in 1989, and at CERN’s Super Proton Synchrotron (SPS) in 1993. Later in that decade, Fermilab’s Tevatron extracted beam particles using crystals at a record energy of 900 GeV.

Bent crystals are also used in modern accelerators’ collimation systems to deflect stray particles in the beam halo into shielding blocks that safely absorb them. The exploration of bent crystals for beam collimation began in the 1990s at Brookhaven National Laboratory and Fermilab, but the field underwent a step change in 2006 with the experimental observation of volume reflection at Petersburg Nuclear Physics Institute. This advance was enabled by new manufacturing techniques for high-quality bent silicon crystals. Predicted in the mid-1980s by Taratin and Vorobiev, volume reflection occurs when a particle is coherently deflected by the collective field of bent crystal planes without becoming trapped in a channel, effectively rebounding from the planar potential barrier.

Crystal clear

These breakthroughs motivated the UA9 Collaboration and experts in beam collimation to undertake a systematic programme of crystal-based beam manipulation at the SPS. This effort culminated in 2023, when crystal collimation became an operational reality at the LHC (see “Heavy-ion collimation” figure).

This technique addressed a critical limitation of heavy-ion operation: conventional amorphous collimators fragment heavy nuclei into lighter ions, some of which escape the collimation system and can quench downstream superconducting magnets. Bent crystals, by contrast, coherently and deterministically steer beam halo particles onto dedicated absorbers. As a result, crystal collimation was demonstrated to reduce heavy-ion beam losses at LHC magnets by factors of 5 to 13 compared with standard collimation.

New frontiers

The success of TeV-scale beam collimation at the LHC laid the groundwork for another ambitious goal: using bent crystals in the LHC not just to steer beams, but also to probe the spin of short-lived particles. In the intense internal fields between crystal atomic planes, a particle’s spin behaves much like a spinning top in a gravitational field. Rather than simply tipping over, the top’s angular momentum rotates slowly – precesses – under the action of a torque. In close analogy, the magnetic moment of a relativistic particle traversing a bent crystal precesses under the torque generated by the effective magnetic field experienced in its rest frame.

In 1992, the E761 collaboration used the fixed-target proton beam from the Tevatron to perform the first experimental demonstration of the effect by measuring the magnetic moment of the Σ⁺ hyperon (uus). This pioneering work used two 4.5 cm-long bent silicon crystals to induce spin precession, proving that the technique could effectively substitute for massive conventional magnets.

Bent crystals could open new frontiers in particle physics at the LHC

Bent crystals could open new frontiers in particle physics at the LHC. The TWOCRYST collaboration is exploring whether the technique can be extended to study the spin of short-lived charm baryons. The idea dates back to 1996, when Samsonov extended the E761 findings to charm baryons and demonstrated that despite their extremely short lifetimes, the intense effective fields of bent crystals could induce measurable spin precession. In 2016, Scandale and Stocchi proposed to use this technique to measure the magnetic dipole moments of charm baryons at the LHC.

The lightest charm baryon, the Λ_c⁺ (udc), has an extremely short lifetime of roughly 200 femtoseconds. Even at 1 TeV, it only travels a few centimetres before decaying. The magnetic fields needed to study its spin precession cannot be provided by conventional magnets, but are well within reach if bent crystals are used. If produced at a fixed target, a clean sample of its decays to a proton, a kaon and a pion can be obtained via tracking and invariant-mass reconstruction, with decay angles yielding spin information.

Such measurements promise a unique opportunity to explore QCD at the interface between heavy and light quarks. Measurements of its spin precession would also provide exceptional sensitivity to a possible electric dipole moment – a potential signature of physics beyond the Standard Model. The ALADDIN (An LHC Apparatus for Direct Dipole moments INvestigation) experimental proposal aims to measure the electromagnetic dipole moments of charm baryons, the Λ_c⁺ and the Ξ_c⁺ (usc), using a double-crystal scheme in the LHC. In this concept, a first bent crystal extracts a small fraction of the LHC beam halo and guides 7 TeV protons onto a fixed target located inside the LHC vacuum pipe, producing, amongst other particles, the charm baryons of interest. The particles would then impinge on a second bent crystal, whose intense inter-planar fields would induce a measurable spin precession.

Such an experiment must deal with challenging demands on the crystal alignment. Channelling only occurs if particles enter a crystal within a narrow angular range, known as the Lindhard angle, which decreases with increasing beam energy. At TeV energies in the LHC, this angle is only a few microradians, meaning that misalignments far smaller than the width of a human hair over a metre are sufficient to suppress channelling entirely. This alignment will be particularly challenging for ALADDIN, which will rely on protons that have scattered off the primary collimators.

TWOCRYST was installed at Insertion Region 3 (IR3) in early 2025 (see “Halo extraction” figure). The experiment marks a significant leap in complexity compared to previous LHC crystal tests. Last year, the experiment successfully channelled LHC protons through two crystals (see “Double channelling” figure). These measurements marked the first controlled deployment of a double-crystal setup in the LHC, demonstrating the technique at 450 GeV, 1 and 2 TeV – a new world record, surpassing the 270 GeV achieved by the UA9 collaboration at the SPS and corresponding to an equivalent magnetic field of 600 tesla. Preliminary analyses of the recorded data indicate that more than 20% of protons were channelled successfully at 1 TeV.

Bent crystals have come a long way since the pioneering experiments at JINR Dubna in 1979. TWOCRYST’s demonstration of double-channelling at a record energy of 2 TeV represents an important step toward using the technique for precision particle-physics measurements with bent crystals at the LHC.

Measurements of spin precession have long played a central role in particle physics, providing deep insights into fundamental interactions and symmetries. The anomalous magnetic moments of the proton and neutron – measured in the 1930s and 1940s – remained unexplained for decades until the emergence of the quark model in the 1960s. While conventional magnet-based techniques remain highly effective for relatively long-lived particles such as the muon (CERN Courier March/April 2025 p21), particles as short-lived as charm baryons have so far remained experimentally inaccessible. The results from TWOCRYST suggest that bent crystals may allow the first direct experimental probe of electromagnetic dipole moments in charm baryons, opening a new window on QCD dynamics and offering a sensitive test for physics beyond the Standard Model.

The post New directions for bent crystals appeared first on CERN Courier.

The challenge of identifying rare signals amid overwhelming backgrounds will resonate with CERN Courier readers. At the LHC, experiments increasingly deploy anomaly detection methods to search for new physics beyond the Standard Model without fully specifying the signal in advance. Both fields face a shared problem: isolating rare events from billions of observations with minimal prior assumptions about the target. “Semi-supervised” approaches that marry sparse expert knowledge with vast unlabelled datasets may prove as valuable for collider data as they have for astronomical archives.

A new semi-supervised machine-learning framework developed at the European Space Agency in December 2025 has identified 1339 unique astrophysical anomalies spanning 19 distinct morphological classes (see “Six out of 1339” figure). Approximately 65% of these – some 811 objects – had no prior reference in the scientific literature, despite residing in data that has been publicly available for years. Some of these newly discovered objects were excellent additions to existing catalogues of which examples are limited. These included collisional ring galaxies, galaxy mergers, jellyfish galaxies and gravitational lenses. Forty-three of the objects completely defied classification and remain unknown objects to this day.

Semi-supervised learning

At the heart of this work lies a fundamental tension in modern astronomy: datasets are growing far faster than our ability to label them. Traditional supervised machine learning requires large, annotated training sets, but expert labelling of millions of images is prohibitively expensive. Semi-supervised learning offers a way forward. In this approach, a model learns simultaneously from a small set of human-labelled examples and a vastly larger pool of unlabelled data, extracting patterns from the abundant unlabelled images to compensate for the scarcity of annotations.

The challenge of identifying rare signals amid overwhelming backgrounds will resonate with CERN Courier readers

The new code we have developed generates provisional “pseudo-labels” when the model’s confidence exceeds a threshold, then enforces consistent predictions with augmented versions of the same images. These augmentations take the form of cropping of the images, flipping them, inverting the pixel values, and so forth. This allows the model to leverage the statistical structure of millions of unlabelled cutouts without requiring a human to inspect each one. The algorithm then couples this semi-supervised backbone with human expertise. After each training cycle, the model ranks all images by anomaly score and a domain expert reviews the highest-ranked candidates, correcting misclassifications and confirming genuine anomalies. These newly labelled images feed the next training cycle. This human-in-the-loop design combines the pattern recognition capabilities of deep learning with the domain knowledge of an astronomer, achieving an efficiency that neither could match alone.

In our study, the entire process began with 128 standard astrophysical phenomena and three labelled anomalies where finding further examples would be valuable. The chosen examples were edge-on protoplanetary disks – young stellar objects with a proto-planetary disk around a host star that exhibits strong emission with a direct high-energy jet and secondary emission in a striking butterfly shape. Through successive iterations, the training set grew to 1400 images, at which point the model could flag anomaly types it had never been shown.

Community access

A search of this scale was made possible by ESA Datalabs, a collaborative science platform that provides researchers with direct access to ESA’s mission archives alongside computational resources – including GPU acceleration – through a browser-based environment. Rather than downloading terabytes of Hubble data, we brought our analysis code to where the data already resides. The full inference run across 99.6 million images completed in just 2.5 days on a single GPU, demonstrating that large-scale anomaly detection does not require vast computational resources, a consideration that matters as the community increasingly weighs the sustainability of data-intensive research.

The most abundant anomalies were galaxy mergers: 629 systems hosting tidal tails, bridges and other signatures of gravitational interactions that exist at the very limit of our detection power. We also found 140 candidate gravitational lenses and 39 gravitational arcs, where the warping of spacetime distorts background sources into characteristic rings. Mergers give us snapshots of hierarchical structure formation, while spacetime distortions provide direct tests of general relativity and enable dark-matter mapping on cosmological scales.

Even decades-old data can yield hundreds of new discoveries when the right tools are brought to bear

The model also independently recovered five previously catalogued quadruply lensed quasars in the Einstein cross configuration – a fourfold splitting of a distant quasar’s light by a foreground galaxy. That the model identified these without any lensed quasars in its training set validates its ability to generalise beyond the anomaly types it was explicitly taught. Fewer than 50 such systems are known, and each enables an independent “late universe” measurement of the Hubble constant; such measurements are invaluable given the persistent tension between values derived from the cosmic microwave background and the local distance ladder (CERN Courier March/April 2025 p28).

Among the genuinely new discoveries were two collisional ring galaxies – extreme systems that have undergone such an extreme galaxy interaction that a shockwave is moving through the galaxy, causing a burst of star formation through the galaxy. Thirty-five jellyfish galaxies shaped by ram pressure stripping in the intracluster medium also provide an excellent laboratory to understand the relationship between the galactic environment and the internal gas of the galaxy. Finally, 43 sources had morphologies that defied classification entirely – curved, distorted objects that fit none of the established categories and have been released to the community for further investigation.

With the Euclid space telescope now operational, and the Vera C. Rubin Observatory and Square Kilometre Array soon to follow, data volumes will dwarf Hubble’s archive by orders of magnitude. Our work shows that even decades-old data can yield hundreds of new discoveries when the right tools are brought to bear – and that AI-assisted discovery, guided by human expertise, is only just getting started.

The post A thousand anomalies hiding in plain sight appeared first on CERN Courier.

In physics, as in life, it’s important to persevere in the face of setbacks. When James Robinson joined the ATLAS experiment at CERN in 2008, the Large Hadron Collider had just sputtered into life. “I remember the excitement of the initial startup and the disappointment when data taking was delayed for a year,” recalls Robinson.” Over the next decade, Robinson built a career in experimental particle physics, analysing jets and soft-QCD events, convening subgroups, tuning Monte Carlo generators and helping measure luminosity.

By 2018, Robinson was beginning to ponder his professional priorities. “I didn’t really want to spend another three years writing grants and not having much time to do physics,” he says. Constant relocation was another strain. “It was really nice having the freedom to travel, but in your mid-thirties you start thinking maybe it’s time to settle in one location.”

Real-world research

That’s when he spotted an opening at the Alan Turing Institute, the UK’s national centre for data science and AI. The Institute is a research-led organisation who hire experts and academics to find solutions to real-world challenges and to advise UK public policy. The role Robinson initially applied for focused on advanced computing and AI strategy, one that would apply his academic skills, and help develop his practical ones. “The Institute has a lot in common with CERN,” he says. “But I applied because of its larger focus on applications of research, rather than pure blue-sky work.”

Today, Robinson is the software engineering research lead in the Turing’s Environment and Sustainability programme, where teams of researchers, data scientists and engineers tackle urgent global challenges. “Right now we’re working with the Met Office on using AI to get faster and better weather predictions in the UK,” he explains. “For other projects, we also partner with African countries to improve forecasts in the global South, and model changes in Arctic and Antarctic sea ice, which is useful for everything from animal migrations to navigation.”

One of Robinson’s first projects was to model London’s air quality to inform the mayor’s office on pollution hot spots. “Traffic turned out to be the most important factor,” he says. “We could point to areas where we thought air quality was bad but under-measured, and the mayor’s office deployed mobile sensors to check. During COVID we even repurposed the project to monitor how busy London was coming out of lockdown. It felt really nice to see a project pivot quickly and directly feed into policy.”

Although the Turing Institute engages with government and public-sector partners, it isn’t a commercial consultancy. Each team decides which areas they would like to work in, and the problems they focus on improving. Once they identify a problem, the next stage is to find the best partner who will allow their models to make the most impact. “We’re not here to build a slightly better algorithm for its own sake,” says Robinson. “We want to apply AI to make change in the real world.”

The Institute’s mission echoes the one that first drew Robinson to physics. “One of the big similarities with CERN is the sense that what you’re doing is worthwhile and good for the world,” he says. “It’s still research, but more applied. Improving the weather forecast that everyone sees on their phone – that’s easy to explain to your grandparents.”

Robinson, who had previously been part of decades-long, large-scale research projects at ATLAS, felt it extremely satisfying to see the direct impact of his work. “At CERN you contribute a tiny part to a huge experiment,” he says. “Here I get to see a project from start to finish, and sometimes adapted straight into real-world decision making.”

Transferable skills

But was high-energy physics a good preparation for Robinson’s current career?

The answer is a resounding yes. Having done a PhD and two post docs, he was used to flexible and adaptable timelines. “I was often handed a problem without a clear solution,” he recalls. “Sometimes we have to pivot quickly away from one idea or plan and dive straight into another. That ability to rethink and improve has transferred directly to Turing.”

A lack of formal technical qualifications also need not be a problem. “Many of us were self-taught programmers at CERN,” he says. “The fact you’ve done research, adapted and developed those skills is what matters.”

Collaboration is another common thread. “Like CERN, Turing is a meeting place for people from many different institutions,” he says. “No one can just order work to happen. You negotiate, you build consensus.”

But Robinson notes that applying for non-academic roles requires a shift in mindset. While academic CVs and cover letters are often long and detailed, applications for industry, consultancy or somewhere in between like the Institute, may look different.

“Don’t go into the specifics of your ATLAS analysis because it won’t be directly relevant in industry,” says Robinson. “Show your research experience, but focus on the skills: problem-solving, collaboration, adaptability.”

But most importantly, make sure the values of the company you’re applying to align with your own. For Robinson, the Turning Institute was an obvious choice.

“I’m taking the same mindset I had at CERN and using it to make a difference you can see,” says Robinson. “That’s the rewarding part: turning data into something that genuinely helps people.”

The post Policymaking with data appeared first on CERN Courier.

Particle physics is the modern manifestation of the two-thousand-year quest to understand nature at the most fundamental level possible. That journey has not only deepened our understanding of the physical world but has also reaped enormous benefits for humanity, and is continuing to do so.

I have experienced two revolutions in this quest – the 1974 revolution in particle physics and the 1998 ΛCDM revolution that cemented the relationship between particle physics and cosmology. I am now anxiously awaiting a third. This one will deepen the connections between the quantum world of elementary particles and Einstein’s expanding universe by answering big questions about the origin of space, time and the universe as well as the unity of the particles and forces.

Powerful ideas, big surprises

In the early 1970s I was a graduate student at SLAC; it was an exciting and confusing time. Deep-inelastic scattering experiments at SLAC revealed free partons inside neutrons and protons, but they could not be knocked out. The SU(3) quark model successfully classified the elementary particles and predicted mass relations, but without any dynamics. There were powerful theoretical ideas – quantum field theory, the bootstrap, Regge trajectories, the eightfold way and scattering amplitudes – but no unifying picture.

In November 1974, the discovery of the J/ψ particle was announced. It seemed like overnight the Standard Model of particle physics, with its SU(3) of colour (not flavour) and the SU(2) × U(1) electroweak unification, was in place. All the pieces had been on the table earlier – Weinberg’s broken symmetry model of the weak and electromagnetic interactions, Gross–Wilczek–Politzer’s asymptotic freedom, the GIM mechanism, and evidence for quarks, but it was the discovery of the J/ψ that was needed to make it gel.

The 1980s and 1990s were exciting as new connections between the inner space of elementary particles and the outer space of cosmology were identified – some involving my own research. Inflation and particle dark matter in the form of slowly-moving particles – cold dark matter – led to an expansive theory about the early evolution of the universe along with strong predictions, including a flat, critical density universe, formation of structure from the bottom up, and scale-invariant density perturbations that arose from quantum fluctuations.

But, measurements of the matter density were coming up far short of the critical density, predictions for the large-scale distribution of matter didn’t fit the observations, and the age of the universe and Hubble constant measurements conflicted with a flat universe and possibly each other. Amidst all the confusion, some thought the bubble of enthusiasm would burst.

We are ready for another revolution that transforms our view of matter, energy, space and time, but when?

Then, in early 1998, two supernovae teams announced that the expansion of the universe is speeding up, not slowing down, and the missing piece of the puzzle had been found. ΛCDM quickly fell into place: a flat universe with cold dark matter accounting for a third of the critical density and the other two thirds in dark energy – something like a cosmological constant.

A bittersweet memory reminds me how fast things changed. My close friend and mentor, cosmologist David Schramm, was slated to debate whether the universe was flat with Jim Peebles in April 1998. David, who had the seemingly indefensible “flat” side of the debate, died tragically in a plane crash just weeks before the discovery of cosmic acceleration. When the debate took place and I subbed for David, the title had been changed to, “Cosmology solved?”

Here we are today. Two highly successful standard models which also raise profound questions about the fundamental nature of matter, energy, space and time. There are an abundance of powerful theoretical ideas not yet fully exploited or even completely understood.

There are plenty of clues. The 125 GeV Higgs – who ordered that? The dark-matter particle, dark energy and neutrino mass are not part of the Standard Model and hint at deeper connections between inner and outer space. Recent results from DESI indicate that dark energy may be evolving and is not a cosmological constant. And there is the Hubble tension, which could be telling us something is missing, both in cosmology and particle physics.

On the hunt

But sensitive searches for the dark-matter particle, at the LHC and other colliders, in deep underground experiments and space observatories, have come up short. The Higgs has yet to reveal its secrets. And there has yet to be experimental evidence for the predictions of the powerful theor­etical ideas of supersymmetry, grand unification and string theory, which must play a role in moving forward.

We are ready for another revolution that transforms our view of matter, energy, space and time, but when? Take it from a cosmologist: predicting the past is hard and predicting the future is even harder. Nonetheless, just to illustrate, I mention two possibilities, based upon two speculative papers I have written.

The first, is the detection of gravitational waves from an unexpected cosmological phase transition at a temperature of 100 TeV or so by LIGO, and the second is the discovery that the observed CMB dipole is misaligned with that expected from large-scale structure and arises instead as a revealing relic of cosmic inflation. Either would shake things up, and lead to additions, discoveries and connections. Moreover, I am confident that the real triggering event will be even more impactful and exciting.

The discovery frontier today is very broad, from table-top experiments to colliders to telescopes on the ground and in space, and big ideas abound. The world is waiting and watching. Now is the time to double down and to believe that the next result will be the one that ushers in the coming revolution in our understanding of matter, energy, space and time.

The post The revolution ahead appeared first on CERN Courier.

The list spans nearly 250 years and multiple disciplinary domains. Many made important contributions to nuclear and particle physics, and several had close associations with strong partners to CERN such as the Centre national de la recherche scientifique (CNRS) and the Commissariat à l’énergie atomique et aux énergies alternatives (CEA).

Foremost among the women to be honoured is Polish–French physicist Marie Skłodowska Curie (1867–1934), who discovered polonium and radium, helping to establish radioactivity as an intrinsic property of atoms. She carried out systematic measurements of radioactive substances, determined radium’s atomic weight and developed methods to isolate radioactive elements from pitchblende. She shared the 1903 Nobel Prize in Physics and later won the 1911 Nobel Prize in Chemistry, becoming the first woman laureate and the only person to receive Nobel prizes in two different scientific fields.

A pioneer in X-ray spectroscopy, Yvette Cauchois (1908–1999) invented the Cauchois spectrometer, a curved-crystal spectrometer widely used for the analysis of X-rays and gamma rays. She introduced X-ray spectroscopy using synchrotron radiation to Europe and later studied the X-ray spectrum of the Sun.

A trailblazer for women physicists in Japan, nuclear physicist Toshiko Yuasa (1909–1980) studied the continuous spectrum of beta radiation emitted by artificial radioactive substances and developed her own double-focusing spectrometer. In 1955 she warned of the dangers of nuclear tests at Bikini Atoll. In the 1960s, promoted to senior research fellow at CNRS, she studied nuclear reactions using a synchrocyclotron.

Marie-Antoinette Tonnelat (1912–1980) worked on early unified theories that sought to connect gravity and electromagnetism. She served as director of research at CNRS.

Henriette Faraggi (1915–1985) introduced new techniques with photographic emulsions and directed the CEA Department of Nuclear Physics from 1972 to 1978. She also served as chair of the Nuclear Physics Commission of IUPAP and became the first woman elected president of the French Physical Society. Convinced early on of the importance of high-energy heavy-ion physics for studying quark–gluon plasma, she played a key role in the decision to build GANIL in Caen.

Cécile DeWitt-Morette (1922–2017) worked in quantum field theory and gravitation, and founded the Les Houches Summer School in 1951, which became a major international centre for theoretical physics training. She later contributed to path-integral methods in quantum theory.

Yvonne Choquet-Bruhat (1923–2025) placed Einstein’s field equations of general relativity on a firmer mathematical ground, showing how their behaviour follows from appropriate initial conditions. In 1979 she became the first woman elected as a full member of the Académie des Sciences.

A specialist in cosmic radiation, Lydie Koch (1931–2023) led stratospheric-balloon experiments to detect cosmic rays, contributed to the development of innovative germanium and silicon detectors for the HEAO-3 and COS-B satellites, and advanced X-ray and gamma-ray astronomy. She played a central role in the development of astrophysics at the CEA and was head of the Astrophysics Section from 1967 to 1979.

“It is time for this highly symbolic landmark to embrace the cause of equality between women and men, and to restore women to their rightful place on this monument dedicated to the glory of science and scientists,” said Hidalgo.

The post Eiffel honour for women physicists appeared first on CERN Courier.

Applying the Standard Model (SM) to early cosmological times leads to an uninhabitable universe, with tiny and equal amounts of matter and antimatter. Yet the universe is habitable and the local universe strongly matter-dominated. Observations of the diffuse gamma-ray background and cosmic microwave background show no evidence for the presence of antimatter on large scales and rule out a matter–antimatter symmetric universe.

From 19 to 22 January, 80 particle physicists, astronomers and cosmologists gathered at CERN for the first “All that Antimatters in the Universe” workshop to explore the frontier between the laboratory and astrophysical perspectives on the matter–antimatter asymmetry of the universe.

Broad panorama

Julia Harz (Mainz University) reviewed a broad panorama of baryogenesis models in which physics beyond the SM produces a homogeneous matter excess within the first seconds after the Big Bang, before light elements are synthesised. She highlighted their features and potential tests and constraints, including searches at colliders like the LHC and indirectly with experiments such as those looking for neutrinoless double-beta decays.

Questioning our assumptions about antimatter was a central thread of the workshop, with several presentations highlighting non-standard baryogenesis models that allow domains of antimatter to survive the Big Bang, as well as others in which antimatter is hidden in compact nuggets that could also constitute dark matter. A lively discussion explored how to hunt for these scenarios using astrophysical and cosmological observables. For example, spectral distortions of the cosmic microwave background could indicate energy injections from matter–antimatter annihilation in the early universe. Observations at 21 cm-wavelengths offer another probe: these signals trace neutral hydrogen during the cosmic-dawn epoch, when the first stars and galaxies formed, and could reveal anomalous heating or ionisation patterns characteristic of antimatter annihilation.

Questioning assumptions about antimatter was a central thread of the workshop

The discrete symmetries of charge conjugation (C), parity (P) and time reversal (T) have been central to particle physics since the discovery that nature violates them individually, yet their combined action (CPT) appears to be preserved in all standard interactions. In a particularly sharp presentation, Gabriela Barenboim (University of Valencia) stressed that while much attention is devoted to the search for differences in the interactions between particles and antiparticles through CP-symmetry violation, the more fundamental possibility of CPT violation remains largely unexplored. Unlike CP violation, which can occur within the Standard Model, any breakdown of CPT symmetry would signal new physics and could manifest as differences in the intrinsic properties of particles and antiparticles, including their masses and lifetimes.

Leading stress-tests of CPT symmetry are now carried out at CERN’s Antimatter Factory (AF), whose experiments presented an array of impressive results at the workshop. Eric Hunter (CERN) highlighted the potential of boosting the yield of antihydrogen formation at the AF experiments, showing how this could improve our knowledge of antimatter physics enormously. Improved yields of antimatter replicas of naturally occurring matter-based atoms would enable higher precision tests of key electromagnetic transitions and gravitational interactions of antimatter.

Much attention went to antimatter in cosmic rays. Primary cosmic rays are particles accelerated at astrophysical sources such as supernova remnants and injected into the galaxy, whereas secondary cosmic rays are produced when those primaries collide with gas and dust in the interstellar medium. In standard galactic cosmic-ray models, antimatter is purely a secondary product of the interactions of primary cosmic rays with the interstellar medium. However, the AMS-02 experiment operating on the International Space Station has firmly established a positron excess requiring a primary source, possibly pulsars. AMS-02 antiproton data also show some anomalies, but uncertainties in the propagation models and interaction cross-sections remain large.

Mind the GAPS

Complementary searches for cosmic-ray antimatter are also carried out by balloon-borne experiments. Principal investigator Chuck Hailey (Columbia University) described how the GAPS balloon experiment, uniquely suited to probe low-energy antiprotons, antideuterons and antihelium, reported its first data from a 25-day flight completed in early 2026. The specificity of GAPS is the exploitation of the characteristic X-ray emission produced by short-lived bound states between antimatter nuclei and ordinary atoms, which results in excellent particle-identification and background-rejection capabilities.

The atmosphere at the workshop was excellent, with participants curious to learn from other communities and expand their horizons everywhere that antimatter matters in the universe, from the cosmos to the lab, via astrophysical systems. While antimatter still holds many mysteries, All that Antimatters in the Universe brought us one step closer to answering them.

The post All that antimatters in the universe appeared first on CERN Courier.

Antonino Zichichi, one of the most influential figures in high-energy physics and a towering presence in Italian scientific culture, passed away in Rome on 9 February 2026, at the age of 96.

Born in Trapani, Sicily, in 1929, into an ancient family from Erice, Zichichi graduated from the University of Palermo in the early 1950s. In 1955 he joined CERN, at the dawn of its experimental programme, and in 1965 he led the experiment at the Proton Synchrotron that culminated in the discovery of the antideuteron – an antinucleus composed of an antiproton and an antineutron that provided decisive confirmation of the existence of nuclear antimatter.

A professor of physics at the University of Bologna since 1960, he led the Bologna–CERN–Frascati collaboration, which carried out the first search for the tau lepton and established the experimental method through which its discovery would later be achieved at SLAC National Accelerator Laboratory. Beyond these early milestones, his results and discoveries were numerous and fundamental, including significant limits on free quark production in strong and weak interactions, the discovery of the effective energy in QCD and evidence for the first beauty baryon.

A master of invention

Equally important were his early inventions, among them the electronic circuit for time-of-flight measurements, the preshower for calorimetry and a new technology for high-precision polynomial magnetic fields. Later, by securing Italian funding for the LAA project at CERN, he launched an extensive R&D programme on innovative detection technologies. This notably allowed the development of microelectronics, which together with the design of silicon strip and pixel detectors, would become crucial for the LHC experiments and the development of the Multigap Resistive Plate Chamber (MRPC), a detector with record time resolution. The first large-scale implementation of MRPC technology was the ALICE experiment’s Time-of-Flight (TOF) system that Zichichi led for over two decades.

His scientific legacy cannot be separated from his profound and lasting contribution to the Italian National Institute for Nuclear Physics (INFN). Serving as its president from 1977 to 1982, he played a decisive role in strengthening the institute at a crucial stage of its development, consolidating its international standing and reinforcing Italy’s participation in the great global enterprises of particle physics. Under his leadership, INFN expanded its experimental commitments at CERN and in the US, while investing strategically in detector development and advanced technologies.

Zichichi was instrumental in establishing major research facilities and many large projects are tied to his name: from the LEP and LHC projects at CERN to the HERA project at DESY, and the Gran Sasso National Laboratories at INFN, that he conceived and strategically designed with its experimental halls pointing towards CERN. Today recognised as the world’s foremost underground laboratories for astroparticle physics, attracting thousands of scientists from leading institutions across the globe, the Gran Sasso National Laboratories stand as a monumental testament to Zichichi’s foresight. The idea that an international research centre such as the Gran Sasso Laboratories can serve as a crossroads for scientists from different backgrounds, cultures and institutions, collaborating in fundamental research, reflects the vision that Zichichi consistently pursued. A vision that sees science as a means of diplomacy, enabling dialogue among nations around a common goal.

Strongly convinced that scientific cooperation could be a concrete tool for diplomacy and peacebuilding, Zichichi founded the Ettore Majorana Foundation and Center for Scientific Culture in Erice, Sicily, in 1963, which became a hub for international scientific collaboration and a forum for discussion among researchers from around the world. From there, in 1982, he promoted the Erice Statement for Peace, an urgent appeal to the international scientific community to place its work in the service of peace rather than war, at a time of heightened risk of global nuclear conflict.

That same conviction informed his engagement in European and international scientific governance. Zichichi was among the founders of the European Physical Society (where he served as its president from 1978 to 1980), chaired the NATO Committee on Disarmament Technologies and represented the European Economic Community on the scientific committee of the International Science and Technology Center in Moscow. From 1986 onwards, as president of the World Lab and the World Federation of Scientists, he supported scientific development in emerging countries and focused attention on planetary emergencies.

He did not limit himself to building bridges between scientists, but also between science, culture and society. A highly skilled communicator and educator, he published widely read books and essays aimed at the broader public, and appeared frequently in the Italian media, inspiring young people across Italy and conveying to them his passion for, and belief in, the importance of scientific research. He helped shape scientific culture in Italy in the latter half of the 20th century, insisting that fundamental research is not merely a technical endeavour but a cornerstone of human progress.

Multiple honours

Over the course of his long career, Zichichi received more than 60 awards and honours in Italy and abroad, including the Knight Grand Cross of the Order of Merit of the Italian Republic and the Enrico Fermi Prize of the Italian Physical Society. He was also president of the Enrico Fermi Historical Museum and Research Centre, further testifying to his dedication to preserving and promoting Italy’s scientific heritage.

With his death, the global scientific community loses a visionary researcher, a formidable architect of international scientific collaborations, and a tireless advocate for science as a vehicle of dialogue and peace. What always struck those who shared with him the demanding and inspiring journey of research was his unfailing enthusiasm and deep passion for science, which he cultivated tirelessly until his final days. That same passion lives on not only in his discoveries and in the institutions he helped to create, but also in the generations of scientists who continue to build bridges across borders in the name of knowledge.

The post Antonino Zichichi 1929–2026 appeared first on CERN Courier.

One hundred researchers gathered in Santiago de Compostela from 21 to 23 January for Iberian Strings, the annual meeting of the vibrant Spanish and Portuguese string theory community. From the idea that black holes may test quantum gravity to the new, string-inspired ways of organising quantum field theories using symmetries and defects, the programme offered a broad overview of where string theory and holography currently sit. What stood out was the extent to which very different problems are now being tackled with a shared set of theoretical tools.

Black holes remain a clean laboratory for probing ideas about quantum gravity. Decades of work have shown they behave much like ordinary thermodynamic systems, with quantities such as temperature and entropy. A central question is how this simple large-scale behaviour arises from an underlying quantum description. Vijay Balasubramanian (University of Pennsylvania) emphasised that the challenge is not only reproducing the familiar area law – which links entropy to the area of the event horizon – but also understanding what different semiclassical calculations are really describing.

Calculations under control

One way to address this problem is to count the quantum states that give rise to a black hole’s entropy. To make progress, researchers often focus on settings where calculations are under better control. Gabriel Cardoso (IST Lisbon) discussed BPS black holes, highly symmetric solutions that allow precise calculations using holography. Stefano Trezzi (University of Barcelona) showed that near-extremal black holes, systems close to a zero-temperature limit, exhibit a universal near-horizon behaviour that provides a clean setting to study how quantum effects modify the semiclassical picture.

So much for static black holes; what about their evolution in time? Marija Tomašević (CERN) suggested that quantum effects can form a horizon where classical gravity would predict a naked singularity. Pablo A Cano (University of Murcia) and Marina David (KU Leuven) explored instead how black holes react when they are perturbed, emitting gravitational waves as they settle back to equilibrium through a process known as ringdown. Across these contributions, the focus was on separating what can be understood within controlled semi­classical calculations from what requires genuinely microscopic, quantum-gravitational input.

Some particle theories may have been gravity all along. And vice versa. These seemingly disparate worlds, with particle beams and colour confinement in one (particle physics) and curved spacetime in the other (gravity), may simply be two languages for the same physics. To translate between them, the particle side must live in one fewer dimension. Just as a hologram stores a 3D image on a 2D plate, a gravitational theory in D dimensions may be exactly equivalent to a non-gravitational quantum field theory in D–1 dimensions. This holographic correspondence is central to modern approaches to quantum gravity. The focus at the workshop was on its more applied uses, as a controlled way to learn about dynamics at strong coupling.

Elias Kiritsis (University of Crete) provided a concrete example. Using familiar spacetime physics, he studied how strongly interacting quantum systems respond to gentle deformations at low temperature, a standard probe of transport. In this setting, quantum effects can modify quantities such as the ratio of viscosity to entropy density beyond the semiclassical value.

To round the picture, Francesco Nitti (APC Paris), explored holographic models in which varying the curvature of spacetime can affect confinement, while Shota Komatsu (CERN) presented an overview of matrix-model methods in holography, emphasising how they can provide tractable descriptions of strong-coupling dynamics in specific regimes, such as large-N limits. Following ’t Hooft, theor­ists often treat the number of colours in an SU(N) gauge theory as a tunable parameter, providing a controlled simplification of strongly coupled dynamics.

Black holes remain a clean laboratory for probing ideas about quantum gravity

Working in simplified settings can be an effective way to make progress. In holography, a quantum field theory in two dimensions can map to a three-dimensional spacetime with a negative cosmological constant. Symmetries then constrain the gravity side, allowing us to pose – and sometimes answer – questions that would be far harder to tackle in higher dimensions or less symmetric settings. In this spirit, Stéphane Detournay (Université Libre de Bruxelles) showed how near-extremal black holes themselves can behave like two-dimensional systems, where effects due to thermodynamics, symmetry and quantum corrections can often be disentangled cleanly.

Rapid progress in understanding generalised symmetries and defects was a hot topic. Guillermo Arias-Tamargo (Imperial College London) described how recent work on non-invertible symmetries in non-linear sigma models pushes beyond the traditional picture of symmetries as simple group actions on local fields. In this modern framework, symmetries are realised through extended objects, such as defects or interfaces. Tracking how observables transform across these structures provides concrete constraints on the dynamics and phases of the theory.

A particularly sharp application came from José Calderón Infante (Caltech), who used defect-based arguments to rule out global shift symmetries in quantum gravity. Interfaces also featured prominently as physically meaningful probes, naturally connecting abstract symmetry ideas to concrete quantities such as boundary degrees of freedom and entropy-like measures – as discussed by Carlos Hoyos (Universidad de Oviedo).

The meeting covered a wide range of active topics, but controlled semiclassical arguments, low-dimensional holographic models and defect-based symmetry arguments resurfaced throughout the programme. In that sense, Iberian Strings provided an overview not only of open questions but also of modern methods.

The post String pilgrimage to Santiago appeared first on CERN Courier.

This summer will mark the start of a four-year-long intensive work period to transform the LHC into the HiLumi LHC – a groundbreaking accelerator that will usher in a new era for high-energy physics. The HiLumi LHC will increase the number of particle collisions by a factor of 10, increasing the volume of physics data available for researchers. This leap forward will allow physicists to explore the behaviour of the Higgs boson and other elementary particles with unprecedented precision and to uncover rare new phenomena that might reveal themselves.

Exploring the unknown

“I don’t think it is possible to overstate the importance and excitement of the High-Luminosity LHC, which is the largest project undertaken by CERN for the past 20 years,” explains Mark Thomson, CERN Director-General. “Coupled with advanced new data tools and upgraded detectors, it will allow us to understand, for the first time, how the Higgs boson interacts with itself – a key measurement that will shed light on the first instants and possible fate of the universe. The HiLumi LHC will also explore uncharted territory and could reveal something completely new and unexpected. That’s the whole point of exploring the unknown: you don’t know what’s out there.”

Many of the technologies developed for the HiLumi LHC – such as super­conducting crab cavities that tilt the particle beams before they collide, crystal collimators designed to remove errant particles and high-temperature superconducting electrical transfer lines to power the HiLumi magnets as efficiently as possible – have never been used in a proton accelerator before. Among these new key technologies, the inner triplet beam-focusing magnets are made of a superconducting compound based on niobium and tin (Nb₃Sn), enabling magnetic fields higher than those achieved with the current LHC niobium–titanium (NbTi) magnets (see “Superconductors for the energy frontier”). These new magnets will be deployed on both sides of the ATLAS and CMS experiments, alongside new cryogenic, powering, protection and alignment systems, and will operate at a temperature of 1.9 K, just like the LHC magnets.

The entire accelerator complex and associated experiments will benefit from the improvements

To ensure seamless integration, CERN has built, in an above-ground test hall, a full-scale test stand called the Inner Triplet String (IT String), which mirrors the underground configuration (CERN Courier March/April 2025 p8).

“All the systems have already been tested individually. The goal of the IT String is to validate their integration and their collective performance under operational conditions,” explains Oliver Brüning, CERN Director for Accelerators and Technology. “The connection and operation of all the equipment in the IT String give us a chance to optimise our procedures before the actual installation in the tunnel, so that we will be prepared and ready for an efficient and smooth installation.”

Harnessing potential

The large LHC experiments ATLAS and CMS will also undergo a major upgrade to enable them to harness the full scientific potential of the HiLumi LHC collisions – work that is being carried out in close coordination with hundreds of institutes worldwide. Additionally, the entire accelerator complex and associated experiments will benefit from improvements, says the lab, solidifying CERN’s leadership in high-energy physics.

The cooldown of the HiLumi LHC test string, which is achieved using a liquid-helium refrigeration and distribution system, is expected to take several weeks to complete.

The post HiLumi magnets face full-scale test appeared first on CERN Courier.

The winning image of the 2025 Global Physics Photowalk, by photographer Marco Donghia, shows INFN National Laboratories of Frascati researcher Raffaella Donghia seated beside an open cryostat during installation of an ultracold experiment at COLD, the CryOgenic Laboratory for Detectors (see “First place” image). The apparatus houses an axion haloscope – a cryogenic antenna consisting of a microwave cavity resonating at about 9 GHz, immersed in a powerful 9 tesla magnetic field and connected to an ultra-low-noise amplification system designed to search for ultralight dark-matter candidates such as axions or dark photons (CERN Courier January/February 2026 p21). If ultralight dark matter circulates in a galactic halo, it could excite the resonant cavity at a frequency corresponding to the particle’s mass, appearing as a minute increase in electromagnetic power at that frequency. Cooling the system to 10 mK suppresses thermal noise to the point that quantum noise dominates.

“The image stood out for its clear visual storytelling and masterful use of light, which leads the eye through the scene and emphasises the moment of discovery,” said judge Tabea Rauscher, then creative lead at the European Molecular Biology Laboratory. “The researcher appears small in relation to the cryostat, highlighting the scale of the technology while keeping the human presence at the centre. The lighting creates a quiet, almost cinematic atmosphere that captures both the intensity and the solitude of scientific work.”

The photographs move between abstraction and lived experience

Fellow judge Dmitri Denisov, deputy associate laboratory director for high-energy physics at Brookhaven National Laboratory in the US, noted that while the judges chose Donghia’s photograph for its ability to convey the “deep connection between the apparatuses used in particle physics and the human developing them,” the second- and third-place photographs were chosen for their “deep looks into the inner workings of experiments and impressive display of colours.”

The judges awarded second place to Matteo Monzali for his photograph of a nuclear-physics experiment at INFN National Laboratories of Legnaro in Italy (see “Runner up” image) and third place to Hugo Pardinilla for a close-up image of a photomultiplier from the KM3NeT/ORCA experiment, a neutrino telescope currently being installed in the Mediterranean Sea at a depth of 2500 metres off the coast of Provence, France (see “Third place” image). Members of the public awarded first and second place to Yannig Van De Wouwer’s photographs of GANIL, the heavy-ion accelerator in Caen, France, featuring pipes and cables serving the SPIRAL2 linear accelerator and iridescent patterns in a beam pipe (see “Public preference” image). The public’s third choice went to Monzali’s snap of the AGATA–PRISMA setup in INFN Legnaro.

Deeply human

“Serving as a judge for the 2025 Global Physics Photowalk, I was struck by the range and sensitivity of the submissions,” concludes judge Will Warasila, a freelance photographer for the New York Times. “The photographs move between abstraction and lived experience – finding form, rhythm and quiet beauty in scientific spaces, while foregrounding the people whose labour and curiosity make this work possible. Across geographies and institutions, these images show how photography can slow us down, make complex systems legible and remind us that science is not only technical, but deeply human.”

The Global Physics Photowalk is organised by the Interactions Collaboration (interactions.org), an international network of particle-physics institutions including CERN and over 20 partner laboratories and research infrastructures around the world.

The post Physics labs under the lens appeared first on CERN Courier.

What happens when an artist enters a particle-physics laboratory, not to explain its discoveries or visualise its equations, but simply to remain, observe and respond? In the Spaces Between, a sustained reflection on the long-running Arts at CERN programme, argues that what emerges is not illustration or explanation, but a shared space of inquiry – one that works with uncertainty rather than resolving it, echoing the statistical, instrument-mediated nature of con­temporary physics.

Both art and particle physics push at the edges of what can be known, imagined and expressed. Through its programmes, Arts at CERN hosts artists for extended residencies at the laboratory, where they meet physicists and engineers, attend seminars, visit experimental sites and engage directly with ongoing research. The artists are not tasked with illustrating experiments or communicating results. Instead, they develop independent works – installations, performances, films, sculptures – shaped by sustained dialogue with the scientific community.

Creating coalitions

Edited by Mónica Bello, former head of Arts at CERN (CERN Courier March/April 2025 p41), the book brings together essays, images and reflective texts by artists, scientists and collaborators involved in the artist residency programme. Rather than presenting a catalogue of finished works, it focuses on the conditions that make exchange possible: how artists encounter scientific infrastructures, and how meaning begins to form in spaces where neither discipline fully sets the rules.

The book is organised around four broad themes: “quantum”, cosmology, experimentation and the unknown. These function less as explanatory frameworks than as loose points of orientation, allowing contributions to remain fragmentary and open-ended. The structure mirrors the reality of interdisciplinary work, which rarely unfolds in clean, linear ways, but instead through moments of partial understanding, misalignment and return.

For readers trained in physics, this approach may feel unexpectedly familiar. Scientific knowledge rarely emerges fully formed; it develops through iteration, uncertainty and interpretation. In a similar spirit, the contributions resist tidy conclusions and treat concepts not as definitions to be settled, but as materials for creative reworking. What matters is less resolution than the act of thinking itself, an openness that mirrors the exploratory character of research. At times this displacement can feel destabilising, yet it is precisely this imaginative expansion that gives the book much of its intellectual force.

This sensibility is vividly captured in Rohini Devasher’s Beyond the Standard Model. Spread across a dark, planetary surface, words such as “uncertainty”, “duality”, “observer”, “wonder” and “serendipity” – form a dense, drifting constellation. Some terms carry clear scientific weight; others belong to the emotional and imaginative registers that accompany research but rarely appear in formal papers. For Devasher, the interest lies precisely in language. By placing these words on the same visual plane, the piece loosens disciplinary hierarchies and allows concepts to float, cluster and collide. As the artist notes, the words are intended to read as a web. Rather than explaining physics, it evokes the conceptual environment in which physics thinking takes place.

Places and perspectives

On another page, language again becomes material in Cecilia Vicuña’s Ceque. The work draws on the ceq’e system of the Inca civilisation: a network of conceptual and ceremonial lines radiating outward from the city of Cusco, that are used to organise ritual practice, social relations and cosmological understanding. Rather than functioning as fixed geometrical paths, ceq’es describe relationships between places, perspectives and moments in time.

The page opens with the line “The ceq’e is not a line, it is an instant, a gaze.” Around it, words tilt, scatter and spiral – “a thought, radiating”, “another meridian”, “seen from above or from below”. Reading becomes a spatial act rather than a linear one. Meaning is not extracted or fixed; it unfolds uneasily alongside the order, diagrammatic structures through which Western science typically organises knowledge. The book offers little explicit explanation of the concept, allowing the work instead to function as an alternative way of organising knowledge: relational, situated and resistant to a single point of view.

Visual thinking also surfaces in drawings from Suzanne Treister’s project The Holographic Universe Theory of Art History (THUTOAH), including Alessandra Gnecchi’s Holographic Universe Principle. The work resembles a hand-drawn cosmology sketched in coloured pencil: strings, branes and horizons coexist with handwritten annotations and looping arrows. The emphasis is not on polished representation, but on the labour of thinking – the scribbles, approximations and half-formed connections that precede formalisation. Theory appears not as a final statement, but as something constantly under construction.

One of the more quietly striking works in the book is Julijonas Urbonas’s When Accelerators Turn into Sweaters: a translucent garment constructed from fine copper-stabilised superconducting fibres (see “Accelerator materiality” image). The title collapses the scale of accelerator infrastructure into a wearable object, shifting attention from machines as abstract systems to the materials from which they are built. As Urbonas puts it, the work aims to “bring a monumental, sealed infrastructure into the scale of the body, not just visually, but physically and imaginatively… a translation from the remote language of high-energy physics into something you can almost inhabit.” 

In doing so, it foregrounds the mat­erial reality of high-energy physics – copper as thread and cable at once. Though made of copper, the sweater evokes the magnetic levitation of the Meissner effect, a reference to the cryogenic superconductivity of the LHC. As Urbonas observes, “the accelerator needs extreme cold to do its job, while a sweater’s whole purpose is warmth.” By keeping that gap open, the piece operates less as demonstration than as speculation: a domestic object positioned against an environment colder than outer space, inviting viewers to rethink how scientific infrastructure is imagined. Urbonas leaves the reader with a provocation: “What if physicists talked in the knitwork of the world instead?”

For accelerator physicists, this change of scale may register not simply as metaphor, but as a reminder that even the largest facilities depend on materials physically assembled, connected and maintained by hand. By reframing accelerator infrastructure at human scale, the piece foregrounds construction and material composition rather than the monumental image of the machine, aligning with the book’s broader emphasis on process over spectacle.

The contributions make clear that Arts at CERN is not a peripheral outreach activity, but a mature programme of sustained exchange

In the Spaces Between does not romanticise interdisciplinarity as a seamless merging of perspectives or a frictionless dialogue between equals. Several contributors openly acknowledge the asymmetries between artistic and scientific practice within a large research institution, where scientific priorities and infrastructures inevitably set the operating conditions. Rather than glossing over these tensions, the book treats them as productive constraints that actively shape how collaboration unfolds.

Taken together, the contributions make clear that Arts at CERN is a mature programme of sustained exchange. Its longevity has not led to conceptual closure; instead, the dialogue has deepened while remaining exploratory, evolving rather than resolving.

With its emphasis on process rather than outcomes, the book offers a rare window into how artistic inquiry operates inside a laboratory environment. It does not try to merge art and science, nor to reduce one to the language of the other. Instead, it traces the intellectual and imaginative terrain that lies between them, a space defined not by synthesis, but by ongoing negotiation.

Ultimately, In the Spaces Between suggests that experimentation runs deeply through both artistic and scientific practice, not only as a set of methods for testing ideas, but as a shared commitment to iteration, risk and revision. The sustained dialogue documented here does not aim at synthesis or resolution; rather, it creates conditions in which new forms of knowledge can emerge, forms that remain open-ended. The book will be of particular interest to those working at the intersections of art, science and research institutions, and to readers interested in what happens when disciplines meet without being forced into premature coherence.

The post When accelerators turn into sweaters appeared first on CERN Courier.

Professor Michele Parrinello

Known for his innovative approach to computational science, professor Parrinello’s role in the development of the Car–Parrinello method (with Roberto Car) remains one of his most influential contributions to molecular dynamics. He is similarly celebrated for his role in co-developing the Parrinello–Rahman method, alongside his recent work in metadynamics.

Testament to his global influence, professor Parrinello has received several accolades, such as the Rahman Prize, the Dirac Medal and the Erwin Schrödinger Institute for Mathematics and Physics Medal. He is also a member of several academies and learned societies, including the German Berlin-Brandenburgische Akademie der Wissenschaften, the British Royal Society and the Italian Accademia Nazionale dei Lincei.

Reflecting on his advice to young researchers, professor Parrinello says that they should not fear new ideas. He has observed that many early-career scientists hesitate to go against the mainstream, often worrying about potential consequences. Instead, he encourages them to remain confident in the value and meaning of their work, and to avoid being overly influenced by the opinions of others.

Through the Michele Parrinello Award, it is hoped that professor Parrinello’s remarkable legacy will inspire future generations to pursue excellence in their fields.

The full interview with professor Parrinello is available online.

The Michele Parrinello Award

Michele Parrinello’s work has been characterised by its interdisciplinary impact. Accordingly, the award welcomes nominees from a range of related fields, including physics, chemistry and materials science.

Nominations will close on 31 March 2026, with the winner announced on 31 July 2026. The awardee will receive a monetary prize of EUR 50,000, alongside a commemorative medal and a certificate.

For more information about the nomination process, visit the award homepage.

2025 Award Committee

The Michele Parrinello Award Committee is chaired by professor Xin-Gao Gong. Professor Gong studied with professor Parrinello in Italy during his early career. As an academician of the Chinese Academy of Sciences and a professor at Fudan University in China, he focuses his research on computational physical sciences and condensed-matter physics.

Much like how professor Parrinello inspired his early career, professor Gong hopes that “The Michele Parrinello Award will recognise scientists who have made significant contributions to the field of computational condensed-matter physics and at the same time set a benchmark for the younger generation, providing clear direction for their pursuit.”

Watch the full interview with professor Xin-Gao Gong online.

MDPI champions outstanding research

Recognising the exceptional work of academics lies at the heart of MDPI’s mission to foster open scientific exchange, and is reflected in its extensive awards programme.

The MDPI Sustainability Foundation furthers this mission through its commitment to advancing sustainable development, advocating for scientific progress and global collaboration.

Alongside the Michele Parrinello Award, the foundation oversees the World Sustainability Award, the Emerging Sustainability Leader Award and the Tu Youyou Award.

The post Michele Parrinello Award honours innovation in computational physical science appeared first on CERN Courier.

Supersymmetry has so far eluded discovery at the LHC, yet it retains strong theoretical appeal as an extension of the Standard Model (SM), and potential hiding places remain. In two recent analyses, the ATLAS collaboration sets new bounds on compressed higgsino models, where the proposed particles lie very close in mass. The collaboration used machine-learning techniques to target some of the most elusive signatures at the LHC: low-momentum decay products.

Without extreme fine tuning, quantum corrections would drive the Higgs-boson mass far above the electroweak scale. Supersymmetry prevents this by introducing fermion partners for the SM bosons (and vice versa) so that their quantum contributions naturally cancel. The result is a partner for every SM particle – including higgsinos, the fermionic counterparts of the Higgs field. Higgsinos mix with the partners of the electroweak gauge bosons to form electrically neutral and charged states known as neutralinos (χ̃⁰) and charginos (χ̃^±). The lightest neutralino (χ̃⁰₁) is stable in a wide class of models and may naturally account for the observed dark-matter abundance.

In compressed scenarios, the tiny mass-splitting between these new particles poses a distinct experimental challenge. When a heavier state decays to χ̃⁰₁, the small mass difference leaves little energy for the accompanying SM particles. The visible decay products therefore carry very low momentum and may fall below reconstruction and identification thresholds. The new analyses focus precisely on this regime using the full Run 2 dataset collected at √s = 13 TeV, with two complementary strategies optimised for different values of the mass splitting.

Firstly, a “displaced track” search targets scenarios with a mass difference between the lightest chargino χ̃^±₁ and χ̃⁰₁ of 0.3 to 1 GeV, in which the χ̃^±₁ has a non-negligible lifetime and can travel a few millimetres before decaying into an invisible χ̃⁰₁ and a low-momentum charged pion. The resulting event signature is a pion track with a large transverse impact parameter and high missing transverse momentum from the neutralinos. Significant improvement in signal sensitivity is achieved by the use of two dedicated neural networks (NNs), where one exploits the global event kinematics and the other focuses on the displaced track characteristics.

A “one-lepton-one-track (1ℓ1T)” search instead targets scenarios with a larger mass splitting of 1 to 3 GeV, in which the heavier neutralino χ̃⁰₂ promptly decays into the χ̃⁰₁ and two low-momentum leptons. Since these could elude the existing ATLAS identification techniques, dedicated low-momentum electron and muon identification algorithms have been developed using NNs that exploit track and calorimeter information. The new algorithms are applied to leptons with momentum as low as 0.5 GeV for electrons and 1 GeV for muons, below the standard reconstruction thresholds, resulting in a signature consisting of one lepton and one lepton-like track. An additional NN enhances sensitivity for event classification, exploiting kinematic features that depend strongly on the mass splitting.

The observed data are consistent with the SM predictions, with no signs of new physics emerging in the targeted phase-space. Based on this result, lower limits on the higgsino masses are set at 95% confidence level (CL) (see figure 1). The 1ℓ1T search excludes a mass-splitting region between 0.8 and 2.0 GeV, extending previous limits from the LEP experiments up to a maximum χ̃^±₁ mass of 132 GeV for a 1.8 GeV mass splitting. The displaced track search extends the exclusion limits previously set by the ATLAS experiment by about 30 GeV, reaching a χ̃^±₁ mass of 199 GeV for a 0.6 GeV mass splitting. Together, the two searches exclude χ̃^±₁ masses below 126 GeV at 95% CL over the targeted mass splitting range. Limits set by the ATLAS collaboration now supersede those from the LEP experiments in all mass-splitting ranges.

With this result, ATLAS is now able to set limits over the full range of higgsino mass splittings that are interesting for naturalness, marking a significant milestone in the search for supersymmetry. The new Run 3 dataset, along with advanced analysis techniques, will push these searches even further – perhaps towards the discovery of physics beyond the SM.

The post The most elusive higgsinos appeared first on CERN Courier.

Particle beams are becoming increasingly important to materials engineering. Yunseok Kim (Sungkyunkwan University) discussed how helium-ion irradiation can be used to manipulate hafnium oxide, a material widely employed as an insulating layer in modern micro­electronics. In very thin films, hafnium oxide can sustain a switchable electric polarisation that allows information to be stored, known as ferroelectricity. Yet, this state is normally fragile. Kim showed that controlled irradiation with low-energy helium ions can introduce and rearrange atomic-scale defects in the crystal lattice, stabilising the polarised state.

The meeting also addressed applications in nuclear medicine. A team from the Institute for Rare Isotope Science (IRIS) reported progress towards a domestic production route for the therapeutic alpha-emitter actinium-225, based on irradiation of thorium-232 targets with 50–70 MeV protons. Actinium-225 is both expensive and scarce, with current clinical use relying heavily on imports. Even an initial domestic supply would improve clinical availability and support the wider adoption of targeted alpha therapies.

Alongside applications, there was also a focus on progress in accelerator hardware itself

Alongside applications, contributions also focused on progress in accelerator hardware itself. Garam Hahn (PAL) and collaborators reported on a compact 5 T magnet system based on high-temperature superconductors (see p30). Operating without liquid cryogens, it is designed to shift the wavelength of synchrotron radiation, since stronger magnetic fields force tighter beam curvature and raise the characteristic photon energy. The system drew substantial attention from the accelerator-technology community, as it has the potential to increase high-energy photon brilliance by many orders of magnitude.

Beyond technical developments, ICABU2025 also addressed the evolving policy landscape for large-scale research infrastructure. In South Korea, the Korea Large Accelerator Act was recently established to manage, support and govern large accelerator facilities. Dongsoo Jang, deputy director of the Ministry of Science and ICT (MSIT), outlined strategies aimed at improving coordination, access and long-term planning across the country’s accelerator infrastructure.

Next year, the event will be hosted by the Korea Multi-purpose Accelerator Complex (KOMAC) and held in Gyeongju. Often described as a “museum without walls,” Gyeongju is one of Korea’s most historic cities and a symbol of cultural diplomacy, aligning well with the spirit of ICABU.

The post ICABU fishes for accelerator innovations in Pohang appeared first on CERN Courier.

When atomic nuclei collide at the LHC, they produce tiny droplets of quark–gluon plasma (QGP) and energetic partons plough through it, slowing down in the process. In a new analysis, the CMS collaboration compared high transverse momentum (p_T) particle yields in oxygen–oxygen, neon–neon, xenon–xenon and lead–lead collisions, with the nucleon numbers of the colliding particles increasing in the sequence 16 < 20 < 129 < 208. The results suggest a steady growth of parton energy loss with the size of the colliding system.

High-p_T particles come from the fragmentation of quarks and gluons produced in the earliest hard scatterings of a collision. As these partons cross the QGP, they interact with the medium and radiate, losing energy in the process. This is one of the clearest signatures of QGP formation. How much energy partons lose depends on how far they travel inside the medium, which in turn grows with the size of the colliding nuclei. Although firmly established in xenon–xenon and lead–lead collisions, the precise way this quenching depends on the path length is not yet fully understood.

Light-ion collisions provide a controlled way to vary the system size and isolate this path-length dependence. In July 2025, the LHC delivered its first ever oxygen–oxygen and neon–neon collisions (CERN Courier November/December 2025 p8). The CMS collaboration analysed the data from this dedicated one-week run to perform a systematic study of high-p_T charged-particle suppression across multiple collision systems.

The analysis combines existing measurements in oxygen–oxygen, xenon–xenon and lead–lead collisions with the first measurement of the charged-particle nuclear modification factor, R_AA, in neon–neon collisions at a centre-of-mass energy of 5.36 TeV per nucleon pair. The observable R_AA quantifies how particle yields deviate from expectations based on proton–proton collisions. The four systems were analysed using identical p_T-intervals, enabling a consistent comparison across systems.

The results should help inform the choice of ion species

For smaller nuclei, such as oxygen and neon, many experimental uncertainties shared with the proton–proton reference largely cancel, for example, those related to tracking. This leads to particularly precise measurements of R_AA across a wide p_T range, which is difficult to achieve in larger systems. Combined with the wide span of nuclear sizes, this precision enables a more direct assessment of how parton energy loss depends on in-medium path length.

For a fixed transverse momentum interval, the suppression increases smoothly with system size, from light to heavy ion collisions (see figure 1). Conversely, for a given nuclear system, the suppression is stronger at lower transverse momenta and progressively weakens as it increases. Expressed in terms of the cube root of the nucleon number, which is proportional to the nuclear radius, the results follow a simple ordering with the size of the system, offering a natural framework to test the evolution of energy loss with system size.

The data indicate that nuclear suppression develops gradually as the nuclear system grows, consistent with a picture in which partons interact with QGP droplets whose extent and density evolve smoothly across collision systems. Calculations that omit energy loss show little variation with system size and do not describe the observed suppression, whereas models that include it qualitatively reproduce the observed trend within uncertainties. The data, presented this way, offer a guide for further improvements on their A-dependence.

This study places new quantitative constraints on parton-energy-loss mechanisms and on the emergence of QGP-like behaviour in small nuclear systems. The results should help guide future theoretical developments and inform the choice of ion species in upcoming heavy-ion studies at the LHC.

The post Suppression grows with system size appeared first on CERN Courier.

Quarkonium physics dates back to the November Revolution of 1974 and the discovery of the J/ψ, a bound state of a charm quark and its antiquark; this was soon followed by the excited ψ(2S) state and its bottom–antibottom analogue ϒ(1S) (CERN Courier September/October 2025 p35). These non-relativistic systems hold a unique place in QCD, encompassing a precise hierarchy of characteristic energy scales. Some, such as heavy-quark masses, are amenable to perturbative treatment, while others, such as the confinement scale, are inherently non-perturbative. To capture this interplay systematically, effective field theories such as non-relativistic quantum chromodynamics (NRQCD) were developed from the 1990s onwards.

The quest to interpret quarkonium phenomena within this unified framework, combined with an explosion of experimental results from B factories and hadron colliders, sparked the creation of the Quarkonium Working Group (QWG) Workshop in 2002. Now organised roughly every 18 months at research institutions around the world, the workshop has become a regular meeting point for the quarkonium community. The 17th QWG brought together more than 200 researchers at CERN from 17 to 21 November.

Renaissance

The first part of the workshop naturally reflected this historical and conceptual foundation, focusing on spectroscopy and decays. In recent years, quarkonium spectroscopy has become a driver of new discoveries in QCD. A prime example is the so-called charmonium renaissance, marked by the observation of several exotic states – including the χ_c1(3872), T_cc+(3875) and charged Z_c states. These “XYZ” states can’t be interpreted as conventional charmonia and their internal structure remains under active investigation both at the experimental and theoretical levels (CERN Courier November/December 2024 p33).

Experimental talks in the opening sessions reported on searches for exotic hadrons and their decay channels. Dmytro Meleshko (Giessen University) from the Belle II collaboration reported on excited bottomonium states, placing particular emphasis on the ongoing analysis of the ϒ(10753) resonance, and the experimental signatures that can distinguish between a tetraquark, a hybrid and a S–D mixed bottomonium state. Ilya Segal (Bochum University) presented recent results by the LHCb collaboration on the radiative decay χ_c1(3872) → ψ(2S)γ. Yue Xu (University of Washington) illustrated an analysis for fully-charmed tetraquarks in the J/ψ–ψ(2S) channel by the ATLAS collaboration, confirming the X(6900) resonance with high significance.

On the theory side, Abhishek Mohapatra (TUM) described ongoing efforts to extend effective-field-theory methods originally developed for quarkonium to more complex exotic systems using the Born–Oppenheimer (BOEFT) approach, which takes lattice QCD inputs to address the QCD non-perturbative dynamics without assuming a specific internal structure for the exotic states.

The third day turned to production. Some NRQCD calculations predict negative production rates for J/ψ and χ_c mesons at high transverse momentum, a clearly unphysical result. Hee Sok Chung (Gangneung-Wonju National University) highlighted how this problem can be addressed by improving the formal treatment of emissions near the production threshold. New production measurements for J/ψ and ψ(2S) from the CMS and ALICE collaborations were presented, alongside new calculations for the production of the χ_c1(3872) and of the pentaquarks P_cc(4312) and P_cc(4457).

The field’s rapid evolution makes the time ripe for a third, comprehensive QWG document

The programme then broadened to Standard Model applications, where quarkonium observables can constrain fundamental QCD parameters such as the strong coupling constant and gluelump masses – the gluonic mass contribution in quarkonium hybrid states, as obtained from lattice QCD. Laurids Jeppe (DESY) from the CMS collaboration discussed the enhancement observed around the top–antitop threshold in the invariant mass spectrum, first measured by CMS and later confirmed by ATLAS (CERN Courier September/October 2025 p9). In a round-table discussion, participants debated the signal’s interpretation in terms of a quasi-bound top–antitop meson or a possible new-physics origin, with both scenarios allowed by the current level of experimental precision, and with the main uncertainties coming from the background modelling. The workshop closed with sessions on quarkonium in media, featuring recent  progress in calculating quarkonium transport coefficients from both lattice QCD and perturbation theory.

Progress and puzzles

The discussions across previous QWG workshops crystallised into two foundational documents named “Heavy quarkonium physics” and “Heavy quarkonium: progress, puzzles, and opportunities”, that have since trained generations of young physicists and stand as key references for the community. The field’s rapid evolution makes the time ripe for a third, comprehensive QWG document to capture the wide range of new and enduring topics that currently define it, including the BOEFT framework as a tool to achieve a unified description of all XYZ exotic states, studies of non-equilibrium quarkonium evolution in the QCD medium, informed by new data from the CBM experiment, and the recent development of new automated event generators for quarkonium production.

The next workshop will take place in spring 2027.

The post Quarkonium experts regroup at CERN appeared first on CERN Courier.

Photon detectors lie at the heart of modern experimental physics. Their ability to measure extremely faint light signals, down to the single photons, makes them indispensable in areas ranging from high-energy and nuclear physics to astroparticle physics, astronomy, medical imaging and emerging quantum technologies. In recent years, rapid progress in devices such as silicon photomultipliers (SiPMs), avalanche photodiodes (APDs) and microchannel-plate (MCP-PMT) detectors has delivered improvements in timing resolution, radiation tolerance and large-scale integration. PD2025 provided a timely snapshot of this evolving field, combining technology-driven discussions with reports from experiments already exploiting these advances.

A significant fraction of the invited talks focused on the latest developments in SiPM technology, which has become the workhorse photodetector for many contemporary experiments. Alberto Gola (FBK) and Edoardo Charbon (EPFL) highlighted progress in custom SiPM and digital SPAD devices, respectively, stressing their improvements in photon-detection efficiency and sub-100 ps timing performance, as well as ongoing efforts to mitigate correlated noise and radiation-induced degradation. These technological developments were complemented by reports from large-scale experiments – such as ALICE3, CMS, DARKSIDE, DUNE, ePIC and JUNO-TAO – in high-energy and astroparticle physics, outlining the status of ongoing developments and the anticipated role of SiPM-based systems in large-area calorimetry, precision timing and Cherenkov imaging in future detectors.

Equally prominent were contributions on vacuum photodetectors and on enabling technologies. Albert Lehmann (University of Erlangen-Nürnberg) reviewed the status and future prospects of microchannel-plate photomultiplier tubes (MCP-PMT), while Angelo Rivetti (INFN Torino) addressed the challenges of fast, low-power front-end electronics capable of handling the ever-increasing channel counts of modern detectors. Modelling of photon-detection devices was discussed by Werner Riegler (CERN), who introduced an analytic description of timing and efficiency in SPADs and SiPMs, clarifying their performance limits for single-photon and charged-particle detection.

Several contributions underlined the increasingly close relationship between academia and industry in photon-detector development, touching on technology transfer, production scalability and long-term reliability, issues that are becoming central as detectors transition from small-scale prototypes to systems comprising hundreds of thousands, or even millions, of channels, such as in the case of the use of digital SiPMs for physics experiments.

The next edition of the conference will take place in May 2027 in Beijing.

The post Photon detectors light up Bologna appeared first on CERN Courier.

2025 marked the 30th anniversary of the top quark’s discovery by the DØ and CDF experiments at Fermilab. Three decades on, and despite ever-increasing experimental precision, the top quark’s properties remain only partially understood. While its mass is now known at the sub-GeV level and its production cross sections agree well with Standard Model predictions, questions persist about its electroweak couplings, its interactions with the Higgs boson, and the detailed structure of top–antitop production at high energies. Because of its large mass and correspondingly strong coupling to the electroweak sector, many in the community continue to view the top quark as a sensitive probe of physics beyond the Standard Model.

The conference opened with an inspiring keynote address by Juan Antonio Aguilar Saavedra (IFT Madrid), who explored the connections between top-quark physics and quantum science and technology. A notable example is the recent observation of quantum entanglement in top-quark pair production by the ATLAS and CMS experiments, which has opened a promising new line of research linking collider physics with concepts more familiar to quantum information researchers. Entanglement can be measured in the top–antitop system because top quarks decay before hadronisation takes place, allowing direct access to their spin correlations.

Top physics is currently enjoying a golden era. Last year, the CMS collaboration reported an excess near the top–antitop threshold (CERN Courier May/June 2025 p7), later confirmed by ATLAS with a significance of 7.7σ above the background predicted by perturbative quantum chromodynamics (CERN Courier September/October 2025 p9). This excess is consistent with expectations from non-relativistic quantum chromodynamics, an effective theory that describes the dynamics of heavy quark pairs near threshold and with simplified models involving a pseudoscalar “quasi-bound-state”, called toponium.

During a mini-workshop dedicated to toponium, Benjamin Fuks (LPTHE) presented an intriguing scenario in which the excess could be explained by two contributions: one from a top–antitop bound state and another from a beyond-the-Standard-Model signature, although the data are also compatible with Standard-Model-only components.

The next edition of the TOP conference will take place in Antalya, Turkey, from 5 to 9 October 2026.

The post The top turns thirty appeared first on CERN Courier.

Proton–proton collisions at the LHC fling quarks and gluons out at massive energies. As they radiate and split into ever more partons, the strong force confines them into sprays of hadrons called jets. The total momentum of a jet, split among its components, approximates that of the initial quark or gluon, which cannot be accessed directly. By tracking how much of a jet’s momentum each hadron carries, the LHCb collaboration has now compared how charm, beauty and light quarks hadronise.

While the production and radiation of individual quarks and gluons can be treated perturbatively, their conversion into hadrons occurs in the non-perturbative regime and cannot be calculated from first principles. Instead, the transition is described using phenomenological probability distributions, called fragmentation functions, which encode how a quark of a given flavour produces specific hadrons. Measuring the content and structure of jets, as well as their kinematic properties, can help constrain these functions.

Previously, the LHCb collaboration measured observables sensitive to fragmentation functions in samples dominated by light-quark-initiated jets. The same measurements were recently carried out for charm- and beauty-quark-initiated jets, allowing a direct comparison of hadronisation across three different jet flavour categories at a single experiment. The light-quark sample was obtained by selecting jets produced nearly back-to-back with a Z boson. In such events, the single parton initiating the jet is typically a gluon or a light quark. In the forward kinematic region accessible to the LHCb detector, where one incoming parton often carries a large fraction of the proton momentum, the proportion of light-quark jets gets further enhanced. Samples of predominantly charm- and beauty-quark-initiated jets were instead obtained using a dedicated flavour-tagging algorithm, which makes use of LHCb’s excellent performance at heavy flavour identification and reconstruction.

The new measurements allow a direct comparison of hadronisation across three different jet flavour categories

A key observable for constraining fragmentation functions is the longitudinal momentum fraction z, defined as the share of jet momentum carried by a hadron along its axis. With respect to their light-quark analogues, heavy-quark-initiated jets appear suppressed at high z, consistent with the leading heavy-flavour hadron carrying most of the jet momentum (see figure 1).

Previous measurements of the hadron­isation of a heavy quark into a single heavy-flavour hadron showed that this hadron carries most of the parent quark’s momentum. The new LHCb analysis extends this picture to the full multi-hadron structure of heavy-quark-initiated jets and is consistent with single-hadron measurements: relatively few charged hadrons possess a large fraction of the jet momentum – a result compatible with the heavy-flavour hadron carrying most of it. This result demonstrates the complementarity of single- and multi-hadron measurements, which are both necessary to fully understand high-energy hadronisation.

The analysis also measured the transverse momentum of the hadron with respect to the jet axis, which is sensitive to transverse-momentum-dependent fragmentation functions. Experimental constraints on these functions remain limited, yet they are crucial in reconstructing a three-dimensional description of hadronisation.

The post Charm and beauty alike in fragmentation appeared first on CERN Courier.

Space Radiobiology is authored by Alessandro Bartoloni (INFN Roma) and Lidia Strigari (University Hospital of Bologna), whose combined expertise spans astroparticle physics, radiation transport and clinical radiobiology. The book explores a meeting point between two fields that have long followed separate paths but are now clearly converging around shared questions in radiation science.

At its core, the book argues for a closer integration of astroparticle physics and medical physics, demonstrating how both fields benefit from a common radiobiology perspective and a shared concern for radiation protection. At the heart of the volume is a thorough and well balanced discussion of space radiation and its implications for human spaceflight. The authors guide the reader through the complexity of the space-radiation environment – galactic cosmic rays, solar-particle events and their interactions – without losing clarity. These elements are consistently linked to real concerns for astronaut health, both for short missions and for the long-duration journeys that are becoming increasingly realistic. By connecting radiation sources, transport mechanisms and biological effects, the book builds a clear picture of where the risks lie and how they might be managed, making it especially relevant at a time when deep-space missions are moving from concept to planning.

What makes the book particularly engaging is that it never treats space research as an isolated niche. Instead, it repeatedly shows how ideas and tools developed for space can feed back into medical physics. From dosimetry and radiation monitoring to risk assessment, the authors highlight how methods refined for astroparticle experiments can be applied in clinical and research settings on Earth. Advances in detectors, modelling and data analysis developed for space missions are presented not as abstract achievements, but as practical contributions that can improve radiation therapy and diagnostic imaging.

From space to the hospital

This interdisciplinary spirit comes through especially well in the case study of the Alpha Magnetic Spectrometer group at INFN Roma Sapienza. Operating aboard the International Space Station, AMS was designed to study cosmic rays and search for signs of dark matter and antimatter. The book shows, however, that its high-precision measurements of charged-particle spectra, particle composition and energy deposition in low-Earth orbit have direct relevance for space radiobiology and radiation-protection research. In particular, AMS data helped characterise the flux, charge and energy distribution of galactic cosmic rays and solar energetic particles, key parameters for modelling dose, dose-rate and track-structure effects in biological tissue. These measurements inform risk assessments for astronaut exposure, improve shielding models, and support more realistic simulations of DNA damage and long-term health effects associated with chronic low-dose, high-energy radiation in space. Rather than serving as a standalone example, this case study acts as a concrete illustration of how cross-disciplinary collaboration actually works in practice: how shared technologies, experimental approaches and theoretical frameworks can produce insights that matter across fields.

The sections on radiobiology strike a careful balance between accessibility and depth. Topics such as DNA damage, cellular responses and long-term health effects are explained clearly, without oversimplifying issues that are inherently complex (CERN Courier November/December 2025 p27). One of the book’s strongest messages is that space radiobiology, with its extreme and unconventional exposure conditions, offers a unique lens for understanding radiation effects that are also relevant to clinical and occupational environments on Earth.

By focusing on shared biological endpoints and common dosimetric challenges, the book shows how progress in one area can meaningfully inform the other. The discussion on developing common platforms for radiation measurement and monitoring reinforces this point, arguing that integrated approaches are not only efficient but scientifically necessary in increasingly complex radiation environments.

Space Radiobiology succeeds in bringing together different scientific communities around a common language and set of challenges. It will resonate with researchers in physics, space science, radiobiology and medical physics, as well as with graduate students looking for a broader, more connected view of radiation science. At a moment when deep-space exploration is becoming a tangible goal rather than a distant idea, the book offers a thoughtful and convincing picture of how lessons learned beyond Earth can shape safer and more effective uses of radiation here at home.

The post Space radiobiology appeared first on CERN Courier.

Partons produced in heavy-ion collisions at the LHC must push their way through a hot, dense quark–gluon plasma (QGP). In doing so, they experience medium-induced energy loss that depends on the parton’s mass. In a recent analysis, the ALICE collaboration compared the yields of charged particles associated with electrons from heavy-flavour hadron decays with those of the light hadrons. Both show a suppression of high-momentum particles emitted opposite to the tagged particle, with no significant difference between the two.

After a hard scattering, high-energy partons fragment into collimated sprays of hadrons known as jets. These are well described in proton–proton (pp) collisions, where their substructures provide stringent tests of perturbative QCD. In heavy-ion collisions, instead, they propagate through the QGP and emerge modified – a phenomenon known as jet quenching. Previous measurements (CERN Courier March/April 2025 p13) suggest that jets initiated by charm and beauty quarks lose less energy than those from light quarks and gluons, owing to their larger mass. This difference is commonly attributed to the dead-cone effect, which suppresses gluon emission by heavy quarks at small angles. Jet-quenching effects can be further characterised by measuring the transverse-momentum distribution of particles within jets, providing insight into the redistribution of the quenched energy.

To study this, the ALICE collaboration employs azimuthal-correlation measurements. This technique measures the angular correlation between a heavy-flavour hadron or its decay daughter (“trigger” particle) and other associated charged particles in the same event. The resulting distribution features two correlation peaks: a near-side peak from particles produced alongside the trigger and an away-side one from the recoiling jet, particularly sensitive to jet-medium interactions. Jet quenching is then quantified by the per-trigger nuclear modification factor I_AA, which is the ratio of away-side charged-particle yield in heavy-ion collisions to pp collisions. Values of I_AA deviating from unity indicate QGP-induced modifications of the jet.

The ALICE collaboration now reports the measurements of jet-like structures in the heavy-flavour sector of lead-lead collisions at a centre-of-mass energy of 5.02 TeV per nucleon pair. The analysis, based on LHC Run 2 data, uses electrons from semi-leptonic decays of charm and beauty hadrons as trigger particles. Electron identification relies on a combination of energy-loss measurements in the time-projection chamber, energy-momentum matching in the calorimeter and selection of shower shapes. Invariant-mass tagging techniques allowed for the subtraction of the large backgrounds from photon conversions and light-meson decays to electron-positron pairs.

The measurement is challenging due to the high multiplicity of lead-lead collisions and the need to extract jet-like correlations from large combinatorial and collective-motion backgrounds. A corresponding analysis of pp collisions at the same energy provides the reference needed to compare jet evolution in the presence of the QGP.

The away-side shows a suppression for associated particles with transverse momenta between 4 and 7 GeV/c (see figure 1), indicating relevant jet quenching with a 2.5σ significance. Conversely, a hint of an enhancement is observed below 2 GeV/c, possibly signalling the redistribution of lost energy into the medium and the subsequent formation of additional low-momentum particles.

These results are consistent with corresponding measurements using light-flavour triggers across all measured intervals. While this suggests that the QGP modifies jets consistently regardless of the initiating parton’s mass, important caveats remain. Variations in parton-to-hadron momentum scaling, as well as the fact that the heavy flavour is tagged via decay electrons, could introduce kinematic differences that complicate a direct comparison. Whether QCD predicts a deviation remains an open question for future modelling of mass-dependent parton-medium interactions.

LHC Run 3 will provide an order of magnitude more heavy-ion events. This increased luminosity will enable higher-precision analyses, offering a deeper understanding of how QGP modifies heavy- and light-flavour jets.

The post The flavour dependence of jet structures appeared first on CERN Courier.

Erich Lohrmann, an experimental physicist who shaped the research programme at DESY, passed away on 10 January 2026 at the age of 94.

Lohrmann was born on 25 May 1931 in Esslingen am Neckar near Stuttgart. From 1950 to 1955 he studied at the Technische Hochschule Stuttgart (TH Stuttgart), where in his doctoral dissertation, completed in 1956, he investigated particle production by cosmic rays in nuclear emulsions and together with Martin Teucher observed the creation and annihilation of an antiproton shortly after its discovery at Berkeley. For this discovery, Owen Chamberlain and Emilio Segrè were awarded the Nobel Prize in Physics in 1959. From 1956 to 1961, Lohrmann continued his work on cosmic rays at TH Stuttgart and at the universities of Bern, Frankfurt and Chicago. In Chicago he also met Masatoshi Koshiba, with whom he shared a lifelong friendship.

Lohrmann joined DESY in 1961. He convinced the director, then Willibald Jentschke, that a liquid-hydrogen bubble chamber exposed to the photon beam from the 6 GeV electron synchrotron would be ideally suited to investigate hadronic reactions. Five million bubble-chamber photographs were analysed by a large collaboration, resulting in a rich scientific harvest that received great international recognition. To facilitate the measurement and analysis of millions of photographs, Erich worked on automated measurement methods and data analysis, and founded DESY’s IT group. In 1969, together with Peter Stähelin, he established the Institute of Informatics at Hamburg University, where he and members of the DESY IT group gave lectures on informatics and data analysis.

As research director from 1968 to 1972 and from 1979 to 1981, he played a key role in strategic decisions at DESY. He was one of the few scientists who encouraged Jentschke to build the electron–positron storage ring DORIS. In the years from 1966 to 1968, it was a risky decision to base DESY’s future on this technique, since the prevailing opinion among particle physicists was that it would only allow tests of the validity of QED. The discovery of the “new particles” in the November Revolution of 1974 (CERN Courier November/December 2024 p41) showed that this was the right decision, and it has shaped research at DESY until today.

Lohrmann was the driving force behind the conception and realisation of the PLUTO detector at the DORIS storage ring. Against considerable opposition, he insisted on a superconducting coil, which laid the foundation for DESY’s expertise in superconducting technology. This was subsequently a crucial prerequisite for the construction of HERA. The experimental programme at DORIS, in which Koshiba’s group was also heavily involved, proved to be extremely successful. The strong Japanese–German collaboration was continued at the large electron–positron storage ring PETRA and later at HERA. The PETRA experiments produced a wealth of new results, the most important of which was the discovery of gluons. After his term as research director ended, Erich then played an influential role in the TASSO experiment.

In the HERA project, he strongly supported Björn Wiik’s forward-looking proposal to build an electron–proton collider with a superconducting proton storage ring 6 km in circumference. In the ZEUS experiment, Lohrmann played a central role in setting up the collaboration, designing the interaction region and in data analysis, to name just a few examples. His important contributions to the critical analysis of publications continued until very recently.

Erich also promoted research with synchrotron radiation through the conversion of DORIS into a high-brilliance radiation source. Later, PETRA was also converted into a synchrotron radiation source. Today, DESY is a world leader in photon science.

From 1976 to 1978, Lohrmann served as CERN director responsible for research. Until his retirement in 1996, he was a professor at the University of Hamburg. With his lectures on physics, statistics and methods of data analysis, he inspired numerous students and provided them with a solid education. Based on his teaching experience, he also authored three books, one of them, Statistical and Numerical Methods of Data Analysis, with his colleague Volker Blobel. Together with Paul Söding, he described the history of DESY in detail up to 2008 in the book Von schnellen Teilchen und hellem Licht. Even after his retirement, Lohrmann was frequently at DESY and remained active in research. One example is the GRAVI experiment, which investigates Newton’s law of gravitation in weak fields.

Despite his great scientific achievements, Erich remained modest. Thanks to his sober yet humorous Swabian manner, his expertise and his commitment to scientists, he enjoyed great trust and esteem.

With the passing of Erich Lohrmann, physics loses a scientist of great foresight and an inspiring teacher. His contributions to physics and his scientific legacy will continue to inspire us in the future.

The post Erich Lohrmann 1931–2026 appeared first on CERN Courier.

Matts Roos, who promoted the international standardisation of high-energy-physics data and developed the popular statistical minimisation system, passed away on 25 November 2025 in his hometown of Helsinki at the age of 94.

Roos was born on 28 October 1931. He completed an MSc degree in technical physics at the Helsinki University of Technology in 1956 and began his career in Stockholm at AB Atomenergi, where he investigated materials for radiation safety. However, he had basic science in his genes, or at least on his mind. Encouraged by his uncle, Ragnar Granit, who in 1967 was awarded the Nobel Prize in Physiology or Medicine, Roos became a research assistant in theoretical physics at the University of Stockholm, from where, a few years later, he continued to the Nordic Institute for Theoretical Physics and the Niels Bohr Institute in Copenhagen. In 1967, he defended his doctoral thesis on CP non-invariance in neutral-kaon decays.

Together with Arthur H Rosenfeld from Berkeley, Roos laid the foundations of the Particle Data Group (PDG). Rosenfeld published the first tables of particle data in 1957 and in 1963 Roos published his own particle tables. In 1964 these two tables were merged into what is now known as the Review of Particle Physics. Sixty years later, this highly cited opus has swollen to 1400 pages.

A particularly significant phase in Roos’ career were the five years he spent at CERN in Geneva, although Finland was not yet a member of CERN in 1965. Victor Weisskopf, the Director-General of CERN, invited Roos, on the basis of his work with the PDG, to apply for a temporary position in the Theory Division, then lead by Léon Van Hove. Motivated by the work on the validation of properties of by then discovered elementary particles, CERN then invited Roos to lecture on statistical methods. This course eventually crystallised into Statistical Methods in Experimental Physics, published in 1971 in collaboration with Fred James, Daniel Drijard, Bernard Sadoulet and William Eadie.

Roos’ international reputation is also based on another CERN-period achievement that greatly benefited the scientific community: the MINUIT software developed together with James. This is a versatile statistical tool that has been used in particle-physics research throughout the decades, with reference to the original publication still increasing today.

The years abroad brought the sociable and multilingual Roos a wide circle of friends and acquaintances among researchers and made him cosmopolitan. Roos returned to Finland in 1971 after the University of Helsinki appointed him as an associate professor in the field of elementary particle physics. From 1977 until his retirement, he served as a personal professor of particle physics. Later, Roos turned to cosmology in addition to elementary particles. He devoted himself to the field by writing a textbook, Introduction to Cosmology, which went through four editions between 1994 and 2015. Roos also served as a member of the International Neutrino Commission for decades. In 1996 he organised the 17th International Conference on Neutrino Physics and Astrophysics in Helsinki.

In his spare time, Roos began to pursue visual arts in the 1980s, developing over the years from an enthusiastic amateur to a professional painter. He stated that art provides a counterbalance to research work, because “science progresses logically and art illogically”. His interest in art must have been rooted in the family, as his father and brother were well-known photographers and filmmakers, and his sister was an architect.

Roos took an active part in the debates in society, supported colleagues behind the iron curtain with forbidden scientific litera­-ture or Solzhenitsyn, and established a think tank on the civil use of nuclear power. He also helped introduce Transcendental Meditation into Finland, after having experienced it himself during a congress in California in the early 1960s.

After returning to Finland, Matts Roos settled in Helsinki with his Swiss-born wife Jacqueline, whom he met while participating in a choral music society, and with the family’s three children. In the summers, the family enjoyed their cottage in the Sipoo archipelago, where many colleagues also were invited.

We shall keep the memory of the Dear alive.

The post Matts Roos 1931–2025 appeared first on CERN Courier.

CLIC

The Compact Linear Collider (CLIC) is a staged linear collider that collides a polarised electron beam with an unpolarised positron beam at two interaction points (IPs) which share the luminosity (see figures and “Design parameters” table). It is based on a two-beam acceleration scheme where power from an intense 1 GHz drive beam is extracted and used to operate an X-band 12 GHz linac with accelerating gradients from 72 to 100 MV/m. The potential of two-beam acceleration to achieve high gradients enables a compact linear-collider footprint. Collision energies between 380 GeV and 1.5 TeV can be achieved with a total tunnel length of 12.1 or 29.4 km, respectively. The proof-of-concept work at the CLIC Test Facility 3 (CTF3) has demonstrated the principles successfully, but not yet at a scale representative of a full collider. A larger-scale demonstration with higher beam currents and more accelerating structures would be necessary to achieve full confidence in CLIC’s construction readiness.

The project has a well developed design incorporating decades of effort, and detailed start-to-end (damping ring to IP) simulations have been performed indicating that CLIC’s design luminosity is achievable. CLIC requires tight fabrication and alignment tolerances, active stabilisation, and various feedback and beam-based correction concepts. Failure to achieve all of its tight specifications could translate into a luminosity reduction in practical operation. CLIC still requires a substantial preparation phase and territorial implementation studies, which introduces some uncertainty on its proposed timeline.

FCC-ee

The electron–positron Future Circular Collider (FCC-ee) is the proposed first stage of the integrated FCC programme. This double-ring collider, with a 90.7 km circumference, enables collision centre-of-mass energies up to 365 GeV and allows for four IPs.

FCC-ee stands out for its level of detail and engineering completeness. The FCC Feasibility Study, including a cost estimate, was recently completed and has undergone scrutiny by expert committees, CERN Council and its subordinate bodies (CERN Courier May/June 2025 p9). This preparation translates into a relatively high technical-readiness level (TRL) across major subsystems, with only a few lower-level/lower-cost elements requiring targeted R&D. The layout has been chosen after a detailed placement study considering territorial, geological and environmental constraints. Dialogue with the public and host-state authorities has begun.

Performance estimates for FCC-ee are considered robust: previous experience with machines such as LEP, PEP-II, DAΦNE and SuperKEKB has provided guidance for the design and bodes well for achieving the performance targets with confidence. In terms of readiness, FCC-ee is the only project that already possesses a complete risk-management framework integrated into its construction planning.

FCC-hh

The hadron version of the Future Circular Collider (FCC-hh) would provide proton–proton collisions up to a nominal energy of 85 TeV – the maximum achievable in the 90.7 km tunnel for the target dipole field of 14 T. As a second stage of the integrated FCC programme, it would occupy the tunnel after the removal of FCC-ee, and so could potentially start operation in the mid-2070s. FCC-hh’s cost uncertainty is currently dominated by its magnets. The baseline design uses superconducting Nb₃Sn dipoles operating at 1.9 K, though high-temperature superconducting (HTS) magnets could reduce the electricity consumption or allow higher fields and beam energies for the same power consumption. Both technology approaches are active research directions of Europe’s high-field magnet programme.

The required Nb₃Sn technology is progressing steadily, but still needs 15 to 20 years of R&D before industry-ready designs could be available. HTS cables satisfying the specifications required for the magnets of a high-luminosity collider, although extremely promising, are at an even earlier stage of development. If FCC-hh were to proceed as a standalone project, operations could possibly start around 2055 from a technical perspective. In that case the magnets would need to be based on Nb₃Sn technology, as HTS accelerator-magnet technology is not expected to be available in that timeframe.

FCC-hh’s performance expectations draw strength from the LHC experience, though the achievable integrated luminosity would depend on the required “luminosity levelling” scenario that might be determined by pile-up control at the experiments. Luminosity levelling is a technique used in particle colliders such as the LHC to keep the instantaneous luminosity approximately constant at the maximum level compatible with detector readout, rather than letting it start very high and then decay rapidly.

LCF

The Linear Collider Facility (LCF) is a linear electron-positron collider, based on the design of the International Linear Collider (ILC), in a 33.5 km tunnel with two IPs sharing the pulses delivered by the collider and with double the repetition rate of ILC. The first phase aims at a centre-of-mass energy of 250 GeV, though the tunnel is sized to accommodate an upgrade to 550 GeV. LCF’s main linacs incorporate 1.3 GHz bulk-Nb superconducting radiofrequency (SRF) cavities for acceleration, operated at an average gradient of 31.5 MV/m and a cavity quality factor twice that of the ILC design at the same accelerating gradient. The quality factor of an RF cavity is a measure of how efficiently the cavity stores electromagnetic energy compared with how much it loses per cycle. LCF can deliver polarised positron and electron beams. Its engineering definition is solid and its SRF technology widely used in several operational facilities, most prominently at the European XFEL, however, the specific performance targets exceed what has been routinely achieved in operation to date. Demonstrating this combination of high gradient and high quality remains a central R&D requirement.

Several lower-TRL components – such as the polarised positron source, beam dumps and certain RF systems – also require focused development. Final-focus performance, which is more critical in linear colliders compared to circular colliders, relies on validation at KEK’s Accelerator Test Facility 2, which is being extended and upgraded. The overall schedule is credible but depends on securing the needed R&D funding and would require a preparation phase including detailed territorial implementation studies and geological investigations.

LEP3

The Large Electron Positron collider 3 (LEP3) proposal explores the reuse of the existing LEP/LHC tunnel for a new circular electron–positron (e⁺e^–) collider. LEP3 has two IPs and the potential for collision energies ranging from 91 to 230 GeV; its luminosity performance and energy range are limited by synchrotron radiation emission, which is more severe than in FCC-ee due to its smaller radius and the limited space available for the SRF installation.

The LEP3 proposal is not yet based on a conceptual or technical design report. Its optics and performance estimates depend on extrapolations from FCC-ee and earlier preliminary studies, and the design has not undergone full simulation-based validation. The current design relies on HTS combined quadrupole and sextupole focusing magnets. Though they would be central to LEP3 achieving a competitive luminosity and power efficiency, these components currently have low TRL scores.

Although tunnel reuse simplifies territorial planning, logistics such as dismantling HL-LHC components introduce non-trivial uncertainties for LEP3. In the absence of a conceptual design report, timelines, costs and risks are subject to significant uncertainty.

LHeC

The Large Hadron–Electron Collider (LHeC) proposal incorporates a novel energy-recovery linac (ERL) coupled to the LHC. High-luminosity collisions take place between a 7 TeV proton beam from the HL–LHC and a high-intensity 50 GeV electron beam accelerated in the new ERL. The LHeC ERL would consist of two linacs based on bulk-Nb SRF 800 MHz cavities, connected by recirculation arcs, resulting in a total machine circumference equal to one third that of the LHC. After acceleration, the beam will collide with the proton beam and will be successively decelerated in the same SRF cavities, “giving back” the energy to the RF system.

The LHeC’s performance depends critically on demonstrating high-current, multi-pass energy recovery at multi-GeV energies, which has not yet been demonstrated. The PERLE (Powerful Energy Recovery Linac for Experiments) demonstrator under construction at IJCLab in Orsay will test critical elements of this technology. The main LHeC performance uncertainties relate to the efficiency of energy recovery and beam-loss control of the electron beam during the deceleration process after colliding with the proton beam. Schedule, cost and performance will depend on the outcomes demonstrated at PERLE.

Muon collider

Among the large-scale collider proposals submitted to the European Strategy for Particle Physics update, a muon collider offers a potentially energy-efficient path toward high-luminosity lepton collisions at a centre-of-mass energy of 10 TeV. The larger mass of the muons, as compared with electrons and positrons, reduces the amount of synchrotron radiation emitted in a circular collider of a given energy and radius. The muons are generated from the decays of pions produced by the collision of a high-power proton beam with a target. “Ionisation cooling” of the muon beams via energy loss in absorbers made of low-atomic-number materials and acceleration by means of high-gradient RF cavities immersed in strong magnetic fields is required to reduce the energy spread and divergence of this tertiary beam. Fast acceleration is then needed to extend the muons’ lifetimes in the laboratory frame, thereby reducing the fraction that decays before collision. To achieve this, novel rapid-cycling synchrotrons (RCSs) could be installed in the existing SPS and LHC tunnels.

Neutrino-induced radiation and technological challenges such as high-field solenoids and operating radiofrequency cavities in multi-Tesla magnetic fields present major challenges that require extensive R&D. Demonstrating the required muon cooling at the required level in all six dimensions of phase space is a necessary ingredient to validate the performance, schedule and cost estimates.

WG2a’s comparison, together with the analysis conducted by the other working groups of the European Strategy Group, notably that of WG2b, which is providing an assessment of the physics reach of the various proposals, provides vital input to the recommendations that the European particle-physics community will make for securing the future of the field. 

The post Seven colliders for CERN appeared first on CERN Courier.

In the past, the Higgs field settled into this ring, where it still dwells today. Since then, the field has been permanently “switched on” – a directionless field with a nonzero “vacuum expectation value” that is ubiquitous throughout the universe. Its interactions with a number of other fundamental particles give them mass. What remains unclear is how the Higgs field behaves once pushed from this familiar minimum. Where will it go next, how did it get there in the first place and might new physics modify this picture?

The LHC alone has shed experimental light on this physics. Further progress on this compelling frontier of fundamental science requires upgrades and new colliders. The next step along this path is the High-Luminosity LHC (HL-LHC), which is scheduled to begin operations in 2030. The HL-LHC is set to outperform the LHC by far, with a total dataset of 380 million Higgs bosons created inside the ATLAS and CMS experiments – a sample more than 10 times larger than any studied so far (see “A leap in technology” panel). We still need to unlock the full reach of the HL-LHC, but three scientific questions may serve to illustrate what can be studied with 380 million Higgs bosons.

What is the fate of the universe?

The stability of our universe hangs in a delicate balance. Quantum corrections could make the Higgs potential bend downward again at high values of the Higgs field, creating a lower-energy state beneath our own (see “The Higgs potential” panel). Through quantum tunnelling, tiny regions of space could spontaneously make the transition, releasing energy as the Higgs field settles into a new minimum of the Higgs potential. Bubbles of the new vacuum would expand at the speed of light, changing the vacuum state of the regions they encounter.

Details matter. The Higgs potential is modified by the effect of virtual loops from all particles interacting with the Higgs field. Bosons push the Higgs potential upwards at high field values, and fermions pull it downwards. If the Standard Model remains valid up to high field values, perhaps as high as the Planck scale where quantum gravity is expected to become relevant, these corrections may determine the ultimate fate of the vacuum. As the most massive Standard Model particle yet discovered, the top quark makes a dominant negative contribution at high energies and field strengths. Together with a smaller effect from the mass of the Higgs boson itself, the top-quark mass defines three possible regimes. 

In the stable case, the Higgs potential remains above the current minimum up to high field values, and no deeper minimum is present.

If a second, lower minimum forms at high field values, but is shielded by a large energy barrier, the vacuum can be “metastable”. In that case, quantum tunnelling could in principle occur, but on timescales exceeding the age of the universe.

In the unstable regime, the barrier is low enough for decay to have already occurred.

Current observations place our universe safely within the metastable zone, far from any immediate change (see “A second minimum?” figure). Yet the precision of the latest LHC measurements, based on independent determinations of the top-quark mass (purple ellipses), leaves unresolved whether the universe is stable or metastable. Other uncertainties, such as that on the strength of nature’s strong coupling, also affect the distinction between the two regimes, shifting the boundary between stability and metastability (orange band).

The HL-LHC will be well placed to help resolve the question of the stability of the vacuum thanks to improvements in the measurements of the top quark and Higgs-boson masses (red ellipse). This will rely on combining the HL-LHC’s large dataset, the ingenuity of expected analysis improvements and theoretical progress in the fundamental interpretation of these measurements.

Why is there more matter than antimatter?

The Higgs potential wasn’t always a Mexican hat. If the early universe got hot enough, interactions between the Higgs field and a hot plasma of particles shaped the Higgs potential into a steep bowl with a minimum at zero field, yielding no vacuum expectation value. As the universe cooled, this potential drooped into its familiar Mexican-hat shape, with a central peak surrounded by a ring of minima, where the Higgs field sits today. But did the Higgs field pass through an intermediate stage, with a “bump” separating the inner minimum from the ring?

The answer depends on the strength of the Higgs self-coupling, λ₃, which governs the trilinear coupling where three Higgs-boson lines meet at a single vertex in a Feynman diagram. But λ₃ is not yet measured. The most recent joint ATLAS and CMS analysis excludes values outside of –0.71 to 6.1 times its expected value in the Standard Model with 95% confidence.

In the Standard Model, the vacuum smoothly rolled from zero Higgs field to its new minimum in the outer ring. But if λ₃ were at least 50% stronger than in the Standard Model, this smooth “crossover” phase transition may have been prevented by an intermediate bump. The vacuum would then have experienced a strong first-order phase transition (FOPT), like ice melting or water boiling at everyday pressures. As the universe cooled, regions of space would have tunnelled into the new vacuum, forming bubbles that expanded and merged. These bubble-wall collisions, combined with additional processes beyond the Standard Model that violate the conservation of both charge and parity together, could have contributed to the observed excess of matter over antimatter – one of the deepest mysteries of modern physics, wherein there appears to have been an excess of baryons over antibaryons in the early universe of roughly one part in a billion, resulting in the surplus we observe today after the annihilation of the others into photons.

The most direct probe of λ₃ comes from Higgs-boson pair production (HH). HH production happens most often by the fusion of gluons from the colliding protons to create a top-quark loop that emits either two Higgs bosons or one Higgs boson splitting into two, yielding sensitivity to λ₃.

HH production happens only once for every thousand Higgs bosons produced in the LHC. Searches for this process are already underway, with analyses of the Run 2 dataset by the ATLAS and CMS collaborations showing that a signal 2.5 times larger than the Standard Model expectation is already excluded. This progress far exceeds early expectations, suggesting that the HL-LHC may finally bring λ₃ within experimental reach, clarifying the shape of the Higgs potential near its current minimum (see “Constraining the Higgs potential” figure).

Measuring λ₃ at the HL-LHC would shed light on whether the Higgs potential follows the Standard Model prediction (black line) or alternative shapes (dashed lines), which may arise from physics beyond the Standard Model (BSM). The corresponding sensitivity can be illustrated through two complementary approaches: one based on HH production, assuming no effects beyond λ₃ and providing a largely model-independent view near the potential’s minimum (red bands); and an approach that incorporates higher-order effects, which extend the reach over a broader range of the Higgs field (blue bands).

Since the previous update of the European Strategy for Particle Physics, the projected sensitivity has vastly improved. The combined ATLAS and CMS results are now expected to yield a discovery significance exceeding 7σ, should HH production occur at the Standard Model rate. By the end of the HL-LHC programme, the two experiments are expected to determine λ₃ with a 1σ uncertainty of about 30% – enough to exclude the considered BSM potentials at the 95% confidence level if the self-coupling matches the Standard Model prediction.

What lurks beyond the Standard Model?

Puzzles such as the origin of dark matter and the nature of neutrino masses suggest that new physics must lie beyond the Standard Model. With greatly expanded data sets at the HL-LHC, new phenomena may become detectable as resonant peaks from undiscovered particles or deviations in precision observables.

As an example, consider a BSM scenario that includes an additional scalar boson “S” that mixes with the Higgs boson but remains blind to other Standard Model fields (see “Spotting a new scalar” figure). S could induce observable differences in λ₃ (horizontal axis) and the coupling of the Higgs boson to the Z boson, g_HZZ (vertical axis). Both couplings are plotted as a factor of their expected Standard Model values. The figure explores scenarios where the coupling deviates from its Standard Model value by as little as a tenth of a permille, and where the trilinear self-coupling may be between 0.5 and 2.5 times the value. Such models could prove to be the underlying cause of deviations from the Standard Model such as contributing to the matter–antimatter asymmetry in the universe. Combinations of model parameters that could allow for a strong FOPT in the early universe are plotted as black dots.

This example analysis serves to illustrate the complementarity of precision measurements and direct searches at the HL-LHC. The parameter space can be narrowed by measuring the axis variables λ₃ and g_HZZ (blue and orange bands). Direct searches for S → HH and S → ZZ will be able to probe or exclude many of the remaining models (red and purple regions), leaving room for scenarios in which new physics is almost entirely decoupled from the Standard Model.

What’s next?

What once might have seemed like science fiction has become a milestone in our understanding of nature. When Ursula von der Leyen, president of the European Commission, last visited CERN, she reflected on recent progress in the field.

“When you designed a 27 km underground tunnel where particles would clash at almost the speed of light, many thought you were daydreaming. And when you started looking for the Higgs boson, the chances of success seemed incredibly low, but you always proved the sceptics wrong. Your story is one of progress against all odds.”

Today, at a pivotal moment for particle physics, we are redefining what we believe is possible. Plucked from the ATLAS and CMS collaborations’ inputs to the 2026 update to the European Strategy for Particle Physics (CERN Courier November/December 2025 p23), the analy­ses described in this article are just a snapshot of what will be possible at the HL-LHC. In close collaboration with the theory community, experimentalists will use the unmatched datasets and detector capabilities of the HL-LHC and allow the field to explore a rich landscape of anticipated phenomena, including many signatures yet to be imagined.

The future starts now, and it is for us to build.

A leap in technology

The HL-LHC will deliver proton–proton collisions at least five times more intensely than the LHC’s original design. By the end of its lifetime, the HL-LHC is expected to accumulate an integrated dataset of around 3 ab^–1 of proton–proton collisions – about six times the data collected during the LHC era.

ATLAS and CMS are undergoing extensive upgrades to cope with the intense environment created by a “pileup” of up to 200 simultaneous proton–proton interactions per bunch crossing. For this, researchers are building ever more precise particle detectors and developing faster, more intelligent software.

The ATLAS and CMS collaborations will implement a full upgrade of their tracking systems, providing extended detector coverage and improved spatial resolution (see “Tracking upgrades” figure). New capabilities are added to either or both experiments, such as precision timing layers outside the tracker, a more performant high-granularity forward calorimeter, new muon detectors designed to handle the increased particle flux, and modernised front- and back-end electronics across the calorimeter and muon systems, among other improvements.

Major advances are also being made in data readout, particle reconstruction and event selection. These include track reconstruction capabilities in the trigger and a significantly increased latency, allowing for more advanced decisions about which collisions to keep for offline analysis. Novel selection techniques are also emerging to handle very high event rates with minimal event content, along with AI-assisted methods for identifying anomalous events already in the first stages of the trigger chain.

Finally, detector advancements go hand-in-hand with innovation in algorithms. The reconstruction of physics objects is being revolutionised by higher detector granularity, precise timing, and the integration of machine learning and hardware accelerators such as modern GPUs. These developments will significantly enhance the identification of charged-particle tracks, interaction vertices, b-quark-initiated jets, tau leptons and other signatures – far surpassing the capabilities foreseen when the HL-LHC was first conceived.

The post What can you do with 380 million Higgs bosons? appeared first on CERN Courier.

There is an overwhelming amount of evidence for the existence of dark matter in our universe. This type of matter is approximately five times more abundant than the matter that makes up everything we observe: ourselves, the Earth, the Milky Way, all galaxies, neutron stars, black holes and any other imaginable structure.

We call it dark because it has not yet been probed through electroweak or strong interactions. We know it exists because it experiences and exerts gravity. That gravity may be the only bridge between dark matter and our own “baryonic” matter, is a scenario that is as plausible as it is intimidating, since gravitational interactions are too weak to produce detectable signals in laboratory-scale experiments, all of which are made of baryonic matter.

However, dark matter may interact with ordinary matter through non-gravitational forces as well, possibly mediated by new particles. Our optimism is rooted in the need for new physics. We also require new mechanisms to generate neutrino masses and the matter–antimatter asymmetry of the universe, and these new mechanisms may be intimately connected to the physics of dark matter. This view is reinforced by a surprising coincidence: the abundances of baryonic and dark matter are of the same order of magnitude, a fact that is difficult to explain without invoking a non-gravitational connection between the two sectors.

It may be that we have not yet detected dark matter simply because we are not looking in the right place. Like good sailors, the first question we ask is how far the boundaries of the territory to be explored extend. Cosmological and astrophysical observations allow dark-matter masses ranging from ultralight values of order 10^–22 eV up to masses of the order of thousands of solar masses. The lower bound arises from the requirement that the dark-matter de Broglie wavelength not exceed the size of the smallest gravitationally bound structures, dwarf galaxies, such that quantum pressure does not suppress their formation (see “Leo P” image). The upper limit can be understood from the requirement that dark matter behave as a smooth, effectively collision-less medium on these small astrophysical structures. This leaves us with a range of possibilities spanning about 90 orders of magnitude, a truly overwhelming landscape. Given that our resources, and our own lifetimes, are finite, we guide our expedition both by theoretical motivation and the capabilities of our experiments to explore this vast territory.

Dark matter could be connected to the Standard Model in alternative ways

The canonical dark-matter candidate where theoretical motivation and experimental capability coincides is the weakly interacting massive particle. “WIMPs” are among the most theoretically economical dark-matter candidates, as they naturally arise in theories with new physics at the electroweak scale and can achieve the observed relic abundance through weak-scale interactions. The latter requirement implies that the mass of thermal WIMPs must lie above the GeV scale – approximately a nucleon mass. This “Lee–Weinberg” bound arises because lighter particles would not have annihilated fast enough in the early universe, leaving behind far more dark matter than we observe today.

WIMPs can be probed using a wide range of experimental strategies. At high-energy colliders, searches rely on missing transverse energy, providing sensitivity to the production of dark-matter particles or to the mediators that connect the dark and visible sectors. Beam dump and fixed-target experiments offer complementary sensitivity to light mediators and portal states. Direct-detection experiments measure nuclear recoils of heavy and stable targets, such as noble liquids like xenon or argon, which are sensitive to energy depositions at the keV scale, allowing us to probe dark-matter masses in the light end of the typical WIMP range with extraordinary sensitivity.

Light dark matter

So far, no conclusive signal has been observed, and the simplest realisations of the WIMP paradigm are becoming increasingly constrained. However, dark matter could be connected to the Standard Model in alternative ways, for example through new force carriers, allowing its mass to fall below the Lee–Weinberg bound. This sub-GeV dark matter, also referred to as light dark matter, appears in highly motivated theoretical frameworks such as asymmetric dark matter, in which an asymmetry between dark-matter particles and antiparticles sets the relic abundance, analogously to the baryon asymmetry that determines the visible matter abundance. In some of the best motivated realisations of this scenario, the dark-matter candidate resides in a confining “hidden sector” (see, for example, “Soft clouds probe dark QCD”). A dark-baryon symmetry may guarantee the stability of such composite dark-matter states, with the baryonic and dark asymmetries being generated by related mechanisms.

Dark matter could be even lighter and behave as a wave. This occurs when its mass is below the eV-to-10 eV scale, comparable to the ionisation energy of hydrogen. In this case, its de Broglie wavelength exceeds the typical separation between particles, allowing it to be described as a coherent, classical field. In the ultralight dark-matter regime, the leading candidate is the axion. This particle is a prediction of theories beyond the Standard Model that provide a solution to the strong charge–parity (CP) problem.

In the Standard Model, there is no fundamental reason for CP to be conserved by strong interactions. In fact, two terms in the Lagrangian, of very different origin, contribute to an effective CP-violating angle, which would generically induce an electric dipole moment of hadrons, corresponding phenomenologically to a misalignment of their electromagnetic charge distributions. But remarkably – and this is at the heart of the puzzle – high-precision experiments measuring the neutron electric dipole moment show that this angle cannot be larger than 10^–10 radians.

Why is this? To quote Murray Gell-Mann, what is not forbidden tends to occur. This unnaturally precise alignment in the strong sector strongly suggests the presence of a symmetry that forces this angle to vanish.

One of the most elegant and widely studied solutions, proposed by Roberto Peccei and Helen Quinn, consists of extending the Standard Model with a new global symmetry that appears at very high energies and is later broken as the universe cools. Whenever such a symmetry breaks, the theory predicts the appearance of one or more new, extremely light particles. If the symmetry is not perfect, but is slightly disturbed by other effects, this particle is no longer exactly massless and instead acquires a small mass controlled by the symmetry-breaking effects. A familiar example comes from ordinary nuclear physics: pions are light particles because the symmetry that would make them massless is slightly broken by the tiny masses of its constituent quarks.

In this framework, the new light particle is called the axion, independently proposed by Steven Weinberg and Frank Wilczek. The axion has remarkable properties: it naturally drives the unwanted CP-violating angle to zero, and its interactions with ordinary matter are not arbitrary but tightly controlled by the same underlying physics that gives it its tiny mass. Strong-interaction effects predict a narrow, well-defined “target band” relating how heavy the axion is to how strongly it interacts with matter, providing a clear roadmap for current experimental searches (the yellow band in the “In pursuit of the QCD axion” figure).

An excellent candidate

Axions also emerge as excellent dark-matter candidates. They can account for the observed cosmic dark matter through a purely dynamical mechanism in which the axion field begins to oscillate around the minimum of its potential in the early universe, and the resulting oscillations redshift as non-relativistic dark matter. Inflation is a little understood rapid expansion of the early universe by more than 26 orders of magnitude in scale factor that cosmologists invoke to explain large-scale correlations in the cosmic microwave background and cosmic structure. If the Peccei–Quinn symmetry was broken after inflation, the axion field would take random initial values in different regions of space, leading to domains with uncorrelated phases and the formation of cosmic strings. Averaging over these regions removes the freedom to tune the initial angle and makes the axion relic density highly predictive. When the additional axions from cosmic strings and domain walls are included, this scenario points to a well defined axion mass in the tens to few-hundreds of μeV range.

There is now a wide array of ingenious experiments, the result of the work of large international collaborations and decades of technological development, that aim to probe the QCD-axion band in parameter space. Despite the many experimental proposals, so far only ADMX, CAPP and HAYSTAC have reached sensitivities close to this target (see “Cavity haloscope” image). These experiments, known as haloscopes, operate under the assumption that axions constitute the dark matter in our universe. In these setups, a high–quality-factor electromagnetic cavity is placed inside a strong magnetic field in which axions from the dark-matter halo of the Milky Way are expected to convert into photons. The resonant frequency of the cavity is tuned like a radio scanning axion masses. This technique allows experiments to probe couplings many orders of magnitude weaker than typical Standard Model interactions. However, scaling these resonant experiments to significantly different axion masses is challenging as a cavity’s resonant frequency is tied to its size. Moving away from its optimal axion-mass range either forces the cavity volume to become very small, reducing the signal power, or requires geometries that are difficult to realise in a laboratory environment.

Other experimental approaches, such as helioscopes, focus on searching for axions produced in the Sun. These experiments mainly probe the higher-mass region of the QCD-axion band and also place strong constraints on axion-like particles (ALPs). ALPs are also light fields that arise from the breaking of an almost exact global symmetry, but unlike the QCD axion, the symmetry is not explicitly broken by strong-interaction effects, so their masses and couplings are not fixedly related. While such particles do not solve the strong CP problem, they can be viable dark-matter candidates that naturally arise in many extensions of the Standard Model, especially in theories with additional global symmetries and in quantum-gravity frameworks.

Among the proposed experimental efforts to observe post-inflation QCD axions, two stand out as especially promising: MADMAX and ALPHA. Both are haloscopes, designed to detect QCD axions in the galactic dark-matter halo. Neither is traditional. Each uses a novel detector concept to target higher axion masses – a regime that is especially well motivated if the Peccei–Quinn symmetry is broken after inflation (see “In pursuit of the post-inflation axion”).

We are living in an exciting era for dark-matter research. Experimental efforts continue and remain highly promising. A large and well-motivated region of parameter space is likely to become accessible in the near future, and upcoming experiments are projected to probe a significant fraction of the QCD axion parameter space over the coming decades. Clear communication, creativity, open-mindedness in exploring new ideas, and strong coordination and sharing of expertise across different physics communities, will be more important than ever.

The post Introducing the axion appeared first on CERN Courier.

One hundred µeV. 25 GHz. 10 m. This is the mass, frequency and de Broglie wavelength of a typical post-inflation axion. Though well motivated as a potential explanation for both the nature of dark matter and the absence of CP violation in the strong interaction, such axions subvert the “particle gas” picture of dark matter familiar to many high-energy physicists, and pose distinct challenges for experimentalists.

Axions could occupy countless orders of magnitude in mass, but those that result from symmetry breaking after cosmic inflation are a particularly interesting target, as their mass is predicted to lie within a narrow window of just one or two orders of magnitude, up to and around 100 µeV (see “Introducing the axion”). Assuming a mass of 100 µeV and a local dark-matter density of 0.4 GeV/cm³ in the Milky Way’s dark-matter halo, a back-of-the-envelope calculation indicates that every cubic de Broglie wavelength should contain more than 10²¹ axions. Such a high occupation number means that axion dark matter would act like a classical field. Moving through the Earth at several hundreds of kilometres per second, the Milky Way’s axion halo would be nonrelativistic and phase coherent over domains metres in width and tens of microseconds in duration.

Axion haloscopes seek to detect this halo via faint electric-field oscillations. The same couplings that should allow axions to decay to pairs of photons on timescales many orders of magnitude longer than the age of the universe should allow them to “mix” with photons in a strong magnetic field. The magnetic field provides a virtual photon, and the axion oscillates into a real photon. For several decades, the primary detection strategy has been to seek to detect their resonant conversion into an RF signal in a microwave cavity permeated by a magnetic field. The experiment is like a car radio. The cavity is tuned very slowly. At the frequency corresponding to the cosmic axion’s mass, a faint signal would be amplified.

The ADMX, CAPP and HAYSTAC experiments have led the search below 25 μeV. These searches are dauntingly difficult, requiring the whole experiment to be cooled down to around 100 mK. Quantum amplifiers must be able to read out signals as weak as 10^–24 W. The current generation of experiments can tune over about 10% of the resonant frequency, remaining stable at each small frequency step for 15 minutes before moving onto the next frequency. The steps are determined by the expected lineshape of the axion signal. Axion velocities in the Milky Way’s dark-matter halo should follow a thermal distribution set by the galaxy’s gravitational potential. This produces a spread of kinetic energies that broadens the corresponding photon frequency spectrum into a boosted-Maxwellian shape with a width about 10^–6 of the frequency. For a mass around 100 μeV, the expected width is about 25 kHz.

The trouble is that the resonance frequency of a cavity is set by its diameter: the larger the cavity, the smaller the accessible frequency. Because the signal power scales with the cavity volume, it is increasingly difficult to achieve a good sensitivity at higher masses. For a 100 µeV axion with frequency 25 GHz that oscillates into a 25 GHz photon, the cavity would have to be of order only a centimetre wide.

Probing this parameter space calls for novel detector concepts that decouple the mass of the axion from the volume where axions convert into radio photons. This realisation has motivated a new generation of haloscopes built around electromagnetic structures that no longer rely on the resonant frequency of a closed cavity, but instead engineer large effective volumes matched to high axion masses.

Two complementary approaches – dielectric haloscopes and plasma haloscopes – exploit this idea in different ways. Each offers the possibility of discovering a post-inflation axion in the coming decade.

The MADMAX dielectric haloscope

Thanks to their electromagnetic coupling, a galactic halo of axions would drive a spatially uniform electric field oscillation parallel to an external magnetic field. For 100 µeV axions, it would oscillate at about 25 GHz. In such a field, a dielectric disc will emit photons perpendicular to its surfaces due to an electromagnetic boundary effect: the discontinuity in permittivity forces the axion-induced field to readjust, producing outgoing microwaves.

The Magnetized Disc and Mirror Axion (MADMAX) collaboration seeks to boost this signal through constructive interference. The trick is multiple discs, with tuneable spacing and a mirror to reflect the photons. As the axion halo would be a classical field, each disc should continuously emit radiation in both directions. For multiple dielectric discs, coherent radiation from all disc surfaces leads to constructive interference when the distance between the discs is about half the electromagnetic wavelength, potentially boosting axion-to-photon conversion in a broad frequency range. The experiment can be tuned for a given axion mass by controlling the spacing between the discs with micron-level precision. Arbitrarily many discs can be incorporated, thereby decoupling the volume where axions can convert into photons from the axion’s mass.

The MADMAX collaboration has developed two indirect techniques to measure the “boost factor” of its dielectric haloscopes. In the first method, scanning a bead along the volume maps the three-dimensional induced electric field, from which the boost factor is then computed as the integral of the electric field over the sensitive volume. This method yielded 15% uncertainty for a prototype booster with a mirror and three 30 cm-diameter sapphire discs (see “A work in progress” figure). By studying the response of the prototype in the absence of an external magnetic field, the collaboration set the world’s best limits on dark-photon dark matter in the mass range from 78.62 to 83.95 μeV.

The MADMAX collaboration has developed two indirect techniques to measure the “boost factor” of its dielectric haloscopes

The boost factor can alternatively be obtained by modelling the booster’s response using physical properties extracted from reflectivity measurements and the behaviour of the power spectrum in the given frequency range. This method was applied to MADMAX prototypes inside the world’s largest warm-bore superconducting dipole magnet. Named after the Italian physicist who designed it in the 1970s, the Morpurgo magnet is normally used to test subdetectors of the ATLAS experiment using beams from CERN’s North Area. Since MADMAX requires no beam, a first axion search using the diameter aperture took place during the 2024 winter shutdown of the LHC. The prototype booster included a 20 cm-diameter mirror and three sapphire discs separated by aluminium rings. Frequencies around 19 GHz were explored by adjusting the mirror position. No significant excess consistent with an axion signal was observed. Despite coming from a small prototype, these results surpass astrophysical bounds and constraints from the CERN Axion Solar Telescope (CAST), demonstrating the detection power of dielectric haloscopes.

As a next step, a prototype booster with a mirror and up to twenty 30 cm-diameter discs is expected to deliver a factor 10 to 100 improvement over the 2024 tests. The positions of its discs will be adjusted inside its stainless-steel cryo­stat using cryogenic piezo motors. The setup is currently being commissioned and is set for installation in the Morpurgo magnet during the third long shutdown of the LHC from mid-2026 to 2029. An important goal is to prove the broad-band scanning capacity of dielectric haloscopes at cryogenic temperatures and conditions close to those of the final MADMAX design. Operating at 4 K will enhance MADMAX’s sensitivity by reducing noise from thermal radiation. A prototype has already been successfully tested inside a custom-made glass fibre cryostat in the Morpurgo magnet in cooperation with CERN’s cryogenic laboratory.

The final baseline detector foresees a 9 T superconducting dipole magnet with a warm bore of about 1.3 m. A first design has been developed and important aspects of its technological feasibility have already been tested, such as quench protection and conductor performance. As a first step, an intermediate 4 T warm-bore magnet is being purchased. It should be available around 2030. Once constructed, the magnet will be installed at DESY’s axion platform inside the former HERA H1 iron yoke, where preparations for the required cryogenic infrastructure are underway.

With MADMAX’s prototype booster scaling towards its final size, and quantum detection techniques such as travelling-wave parametric amplifiers and single-photon detectors being developed, significant improvements in sensitivity are on the horizon for dielectric haloscopes. MADMAX is on a promising path to probing axion dark matter in the 40 to 400 µeV mass range at sensitivities sufficient to discover axion dark matter at the classic Dine–Fischler–Srednicki–Zhitnitsky (DFSZ) and Kim–Shifman–Vainshtein–Zakharov (KSVZ) theory benchmarks.

The ALPHA plasma haloscope

In a plasma, photons acquire an effective mass determined by the plasma frequency, which depends on the density of charge carriers. If the plasma frequency is close to the axion’s Compton frequency, axion–photon mixing is resonantly enhanced. As the plasma could in principle be of any volume, the volume in which the axion field converts into photons has been decoupled from the axion mass – but tuning the plasma frequency is not feasible, preventing a detector based on this effect from scanning a wide range of masses.

In 2019, Matthew Lawson, Alexander Millar, Matteo Pancaldi, Edoardo Vitagliano and Frank Wilczek proposed performing this experiment using a metamaterial plasma with a tunable electromagnetic dispersion which mimics that of a real plasma. In a plasma haloscope, this metamaterial is a lattice of thin metallic wires embedded in vacuum. By adjusting the wire spacing, the diameter of the wires and their arrangement, the resonant plasma frequency can be tuned over a wide range.

The ALPHA collaboration was formed in 2021 to build a full-scale plasma haloscope capable of probing axion masses from 40 to 400 μeV, corresponding to axion frequencies from 10 to 100 GHz. While challenges related to detecting an extremely feeble signal remain, the simplicity of the cavity design, particularly in the magnet geometry and the tuning mechanism, offers flexibility.

ALPHA’s design can be pictured as a large-bore superconducting solenoid magnet, and a resonator housing an array of thin copper or superconducting wires stretched along the field direction. Photons are extracted through waveguides and fed into an ultra-low-noise microwave receiver chain, cooled by a dilution refrigerator to below 100 mK, developing quantum-sensing techniques developed in close collaboration with the HAYSTAC collaboration. Photons are amplified with Josephson parametric amplifiers – the same technique used for qubits used in quantum computers, and the topic of the 2025 Nobel Prize in Physics awarded to John Clarke, Michel Devoret and John Martinis. Tests at room temperature in 2022 and 2023 demonstrated that the response of the meta-plasma can be tuned across the 10 to 20 GHz range with a modest number of configuration changes, and that the quality factors exceed 10⁴ even before cooling down to cryogenic temperatures.

Two designs are being pursued to design a tuning mechanism that allows precise adjustment of the plasma frequency with minimal mechanical intervention: a spiral design where a single rotating rod tunes a set of three spiral arms relative to another set of fixed spiral arms (see “Plasma tuning” figure); and a design with multiple spinners rotating groups of wires relative to a fixed grid of wires.

It is an exciting time for axion searches

ALPHA’s development plan proceeds in two main stages. Phase I is currently being constructed at Yale University’s Wright Laboratory, and focuses on employing established technology to demonstrate the technique and search for axions with masses from 40 to 80 μeV. Phase I’s cavity, consisting of copper plasma resonators, will be immersed in a 9 T magnet, 17.5 cm in diameter and 50 cm tall. The expected conversion power in ALPHA’s frequency range is of order 10^–24 W – comparable to the thermal noise in a 50 Ω resistor cooled to 50 mK. The read-out chain therefore employs Josephson parametric amplifiers whose noise temperatures approach the standard quantum limit. The system is designed to scan continuously while maintaining sensitivity close to the KSVZ axion-photon coupling, a benchmark for well-motivated axion models. The data-acquisition strategy builds on techniques developed in ADMX and HAYSTAC: fast Fourier transforms of the time-stream, coherent stacking across overlapping frequency bins and real-time evaluation of excess-power statistics.

Several improvements are being developed in parallel for Phase II. Quantum sensing techniques have the potential to boost the signal while reducing noise. Such techniques include HAYSTAC-style noise squeezing, using cavity entanglement and state swapping to enhance the signal, and single-photon detection. Dramatically increasing the quality factor of superconducting plasma resonators will also significantly boost the signal. Last but not least, magnets with a larger bore and higher field, such as the ones being deployed at the neutron scattering facilities at Oak Ridge National Laboratory, are expected to expand the experimental reach up to 200 μeV and push the sensitivity to below the axion–photon coupling of the DFSZ model, another classic theoretical benchmark.

Beginning in 2026, ALPHA Phase I will start taking its first physics data, initially searching for dark photons – a dark-matter candidate that interacts with plasma without requiring the presence of a magnetic field. After commissioning ALPHA’s magnet, a full axion search will commence during 2027 and 2028.

It is an exciting time for axion searches. New experiments are coming online, implementing new ideas to expand the accessible mass ranges. Groups in Italy, Japan and Korea are exploring alternative metamaterial geometries, including superconducting wire meshes and photonic crystals that replicate plasma behaviour at higher frequencies. European teams linked to the IAXO collaboration are considering hybrid systems that couple plasma-like resonators to strong dipole magnets. ALPHA will search for axions in the well-motivated region, first focusing between 40 and 80 μeV, and then between 80 and 200 μeV.

Intense efforts are underway. Discoveries may be just around the corner.

The post In pursuit of the post-inflation axion appeared first on CERN Courier.

Chen-Ning Yang, a towering figure in science whose numerous insights shaped contemporary theoretical physics, passed away in Beijing on 18 October 2025 at the age of 103. Yang was one of the greatest physicists of the 20th century, whose profound contributions, often based on principles of symmetry, are central to our contemporary understanding of nature.

Yang was born in 1922 in China’s Anhui province, moving as a child to Tsinghua University in Beijing, when his father was appointed professor of mathematics. Displaced by war, in 1938 he enrolled at the National Southwest Associated University in Kunming, where he earned his Master of Science in 1944, not fully removed from ongoing hostilities in the Second Sino–Japanese War. Yang wrote that his taste in physics was already formed from his education in Kunming.

He was awarded a fellowship for further graduate study in the US and enrolled in 1945 at the University of Chicago. He studied with Enrico Fermi and wrote his thesis on applications of group theory to nuclear physics in 1948 with Edward Teller as his advisor. In 1949, Yang joined the Institute for Advanced Study in Princeton, New Jersey, where he emerged as one of the world’s leading scientists. He wrote that he would probably have taken Fermi’s advice and returned to Chicago, but remained in Princeton to be nearer to Chih Li Tu, whom he married in 1951.

Landmark papers

His years in Princeton were extraordinarily productive, with many landmark papers in particle physics, including a famous analysis of particle decays into two photons, and statistical mechanics, including the celebrated Ising model Lee–Yang circle theorem. Most significantly of all, Yang developed non-abelian gauge theories with Robert Mills in 1954. These have the property that once the gauge groups are identified, new gauge particles and their interactions are determined. Over the subsequent 30 years, a combination of theoretical advances and experimental discoveries identified the gauge particles of our world, establishing Yang–Mills theories as a cornerstone of modern physics, alongside Maxwell’s equations and Einstein’s theory of general relativity. A spontaneously broken Yang–Mills theory, incorporating the Higgs boson, and combined with a Maxwell field, describes the electromagnetic and weak interactions, while a fully unbroken theory, quantum chromodynamics, describes the strong interactions. None of this could have been foreseen in 1954, but as Yang later wrote, “we thought it was beautiful and should be published”.

Yang’s collaboration with Tsung-Dao Lee in 1956 on the groundbreaking possibility of parity non-conservation in weak interactions earned them the 1957 Nobel Prize in Physics, making them the first Nobel laureates of Chinese origin. The confirmation of non-conservation in the experiments of Chien-Shiung Wu and other groups led to further work, with Lee and Rudolf Oehme, on the possibility of charge conjugation and time reversal non-invariance, which were subsequently observed and are now recognised as relevant to the predominance of matter over antimatter in the universe. Around the time of the Nobel Prize, Yang, now famous, reunited with his father from China at CERN. This was their first time together since he left for his doctoral studies in Chicago.

In 1966, Yang accepted the position of Albert Einstein Professor at the new State University of New York at Stony Brook, to which he relocated with his family. In the same year, the Institute for Theoretical Physics, now the C.N. Yang Institute for Theoretical Physics, was founded, and he led it until his retirement from Stony Brook in 1999. At Stony Brook, he continued work in particle physics, and broke new ground in the quantum structure of integrable models and the geometry of gauge field theories. He also profoundly shaped statistical physics, in 1967, discovering the pivotal relation for one-dimensional quantum many-body problems, the Yang–Baxter equation, which opened new directions for research in statistical physics, integrable models, quantum groups and related fields of physics and mathematics.

Building bridges

In 1971, his visit to China sparked a wave of visits there by other well-known scholars, earning him recognition as a pioneer in building bridges of academic exchange between China and the US. As a prominent public figure, he went on to support the restoration and strengthening of basic scientific research in China. He also helped inspire a renaissance of fruitful interplay between physics and mathematics, through his work on the geometry of gauge fields, relating gauge theories to the mathematical concept of fibre bundles, a realisation that grew out of conversations in the 1970s with the mathematician James Simons.

Starting in 1997, he served as honorary director of the newly established Center for Advanced Study at Tsinghua University, now the Institute for Advanced Study, and became a professor at Tsinghua University in 1999. In 2003, he returned as a widower to his childhood home, the campus of Tsinghua University, also spending time at the Chinese University in Hong Kong. In his words, his “life can be said to form a circle”, including a second marriage, with Fan Weng. He took on developing the Institute for Advanced Study as his new mission. Yang poured immense effort into advancing fundamental disciplines and cultivating talents at Tsinghua, making contributions that greatly impacted the reform and development of Chinese higher education.

Yang was elected member or foreign member of more than 10 national and regional academies of sciences, received honorary doctorates from more than 20 prestigious universities worldwide, and was honoured with numerous awards.

In his collected papers, Yang wrote that “taste and style are so important in scientific research, as they are in literature, art and music.” With his own taste having served as his guide, Chen-Ning Yang leaves an opus of exceptional creativity and breadth, providing tools that have enabled generations of physicists to make new discoveries of their own.

The post Chen-Ning Yang 1922–2025 appeared first on CERN Courier.

“The electron–positron Future Circular Collider (FCC-ee) is recommended as the preferred option for the next flagship collider at CERN,” explains strategy secretary Karl Jakobs of the University of Freiburg. “A descoped FCC-ee is the preferred alternative option. Descoping scenarios include removing the top-quark run, constructing two rather than four interaction regions and experiments, and decreasing the RF-system power.”

The ESG drafted its recommendations in a dedicated meeting at Monte Verità in Ascona, Switzerland. From 1 to 5 December, 62 delegates from across the field built on community inputs and the work of the Physics Preparatory Group to elaborate a proposal for the update to the European Strategy for Particle Physics. The recommendations address a broad range of topics and goals related to research in high-energy physics in Europe and beyond (CERN Courier November/December 2025 p23).

Seven large-scale collider projects have been the subject of a comparative assessment: CLIC, FCC-ee, FCC-hh, LCF, LEP3, LHeC and a muon collider (see “Seven colliders for CERN”). Following community submissions to the strategy process in March 2025 and at the open symposium in Venice in June 2025, a consensus emerged that an electron–positron Higgs and electroweak factory is the optimal collider to follow the High-Luminosity LHC (HL-LHC), with FCC-ee the favoured machine of a strong majority of the community (CERN Courier September/October 2025 p24). The identification of a descoped FCC-ee as the preferred alternative option was a new development in Ascona.

“Descoping would reduce the construction cost of FCC-ee by approximately 15%,” says Jakobs. “Although this would have a significant impact on the breadth of the physics programme and the precision achieved, the descoped FCC-ee would still provide a very strong physics programme and a viable path towards high energies, compared to the alternative collider options. Should additional resources become available, these descoping scenarios would be reversible.”

“The other electron-positron collider options offer substantially reduced precision physics programmes and would not be competitive with a collider like the FCC-ee,” continues Jakobs. “Moreover, in themselves, they currently lack a viable path towards energies of 10 TeV.”

The FCC-ee would maintain European leadership in high-energy particle physics

In preparation for the Ascona meeting, working groups were set up to study national inputs, the physics and technology of the large-scale flagship collider projects, the implementation of the strategy, relations with other fields of physics, sustainability and environmental impact, public engagement, education and communication, as well as social and career aspects, and knowledge and technology transfer.

According to the ESG, the FCC-ee would deliver the world’s broadest high-precision particle-physics programme, with an outstanding discovery potential through the Higgs, electroweak, flavour and top-quark sectors, as well as advances in QCD. Its technical feasibility, scope and cost are defined by the FCC Feasibility Study (CERN Courier May/June 2025 p9). The FCC-ee would maintain European leadership in high-energy particle physics, says the ESG, as well as advancing technology and providing significant societal benefits.

“The FCC-ee or the descoped version would also pave the way towards a hadron collider reusing the tunnel and much of the infrastructure, providing direct discovery reach well beyond the 10 TeV parton energy scale, in line with the community’s ambition for exploration at the highest achievable energy,” concludes Jakobs. “The overwhelming endorsement of the FCC-ee by the particle-physics communities of CERN’s Member and Associate Member States further reinforces it as the preferred path.”

The recommendations of the ESG advise but do not constrain the CERN Council, which is expected to formally deliberate on the official update to the European Strategy for Particle Physics at a dedicated Council Session in Budapest in May 2026.

The post European Strategy Group recommends FCC-ee appeared first on CERN Courier.

Thus was born the gallium anomaly, which was carefully checked and confirmed by SAGE’s successor, the BEST experiment, as recently as 2022. The most tempting explanation is the existence of a new particle: a “sterile” neutrino flavour that doesn’t interact via any Standard Model interaction. Neutrino oscillations would transform the missing 20% of electron neutrinos into undetectable sterile neutrinos. It would nevertheless have remained invisible to LEP’s famous measurement of the number of neutrino flavours as it would not couple to the Z boson.

Out the window

This interpretation has been in tension with neutrino-oscillation fits for some time, but a new measurement at the KATRIN experiment likely excludes a sterile-neutrino explanation of the gallium anomaly, says Patrick Huber (Virginia Tech). “There was a strong hint of that from solar neutrinos, but the KATRIN result really nails this window shut. That is not to say the gallium anomaly went away; the experimental evidence here is firm and stands at more than five sigma significance, even under the most conservative assumptions about nuclear cross sections and systematics. So this still requires an explanation, but due to KATRIN we now know for sure it can’t be a vanilla sterile neutrino.”

KATRIN’s main objective is to measure the mass of the electron neutrino (CERN Courier January/February 2020 p28). Though neutrino oscillations imply that the particle is massive, its mass has thus far proved to be below the sensitivity of experiments. The KATRIN experiment, based at the Karlsruhe Institute of Technology in Germany, seeks to remedy this with precise observations of the beta decay of tritium. The heavier the electron neutrino, the lower the maximum energy of the beta-decay electrons. Though KATRIN has not yet been able to uncover evidence for the tiny mass of the electron neutrino, the much larger mass of any sterile neutrino able to explain the gallium anomaly would have made itself felt in precise observations of the endpoint of the energy spectrum of beta-decay electrons thanks to mixing between the neutrino flavours.

After the new KATRIN analysis, the best fit of the sterile neutrino from the gallium anomaly is excluded at 96.6% confidence

“A sterile neutrino would manifest itself as a model-independent kink-like distortion in the beta-decay spectrum, rather than as a deficit in the event rate,” explains lead analyst Thierry Lasserre of the Max-Planck-Institut für Kernphysik, in Heidelberg, Germany. “After the new KATRIN analysis, including 36 million electrons in the last 40 electron volts below the endpoint, the best fit of the sterile neutrino from the gallium anomaly is excluded at 96.6% confidence.”

Though heavy sterile neutrinos remain a well motivated completion of the Standard Model of particle physics with the potential to solve problems in cosmology, light sterile neutrinos struck out a second time in the same volume of Nature last month, thanks to a new measurement at the MicroBooNE experiment at Fermilab, near Chicago.

The MicroBooNE collaboration was following up on a persistent anomaly uncovered by their sister experiment, MiniBooNE, which was itself following up on the infamous LSND anomaly of 2001 (CERN Courier July/August 2020 p32). Both experiments had reported an excess of electron neutrinos in a beam of muon neutrinos generated using a particle accelerator. Here, the sterile-neutrino explanation would be more subtle: muon neutrinos would have to oscillate twice, once into sterile neutrinos and then into electron neutrinos. Using a bespoke liquid-argon time projection chamber, the MicroBooNE collaboration excludes the single-light-sterile-neutrino interpretation of the LSND and MiniBooNE anomalies at 95% confidence.

“The MicroBooNE result is just confirming what we knew from global fits for a long time,” clarifies Huber. “We cannot treat the appearance of electron neutrinos in a muon neutrino beam as a two-flavour problem if a sterile neutrino is involved – if we accept this simple fact of quantum mechanics then LSND and MiniBooNE’s excess of electron neutrinos cannot be due to mixing with a sterile neutrino since the corresponding disappearance of electron and muon neutrinos has not been observed.”

One sterile-neutrino anomaly remains unmentioned, the reactor anomaly, but it has already evaporated into statistical insignificance thanks to new experiments and careful modelling of the flux of electron antineutrinos from nuclear reactors. The promise of experiments with reactor neutrinos is now exemplified by the rapid progress of the Jiangmen Underground Neutrino Observatory (JUNO) in China, which started data taking on 26 August last year (CERN Courier November/December 2025 p9).

Back to the standard paradigm

While the recent KATRIN and MicroBooNE analyses sought evidence for a hypothetical sterile neutrino beyond the standard scenario, JUNO operates within the standard three-flavour framework. Using just 59 days of data, the experiment independently exceeded the precision of previous global fits on two out of six of the parameters governing neutrino oscillations. These are the same mixing angle and mass splitting that govern the oscillations of solar electron neutrinos into other flavours – the very effect that GALLEX and SAGE were initially designed to study in the 1990s. As JUNO gathers data, it will resolve a fine-toothed comb that modulates this oscillation spectrum – the effect of a smaller mass splitting between the three neutrinos. JUNO is designed to resolve these tiny oscillations, revealing a fundamental aspect of nature’s design: the hierarchy of the small and large mass splittings.

“The JUNO result is very exciting,” says Huber, “not so much because of its immediate impact, but because it marks the very successful start of an experiment that will deeply change neutrino physics.”

The JUNO result is exciting because it marks the successful start of an experiment that will deeply change neutrino physics

JUNO is the first of a trio of a new generation of large-scale neutrino-oscillation experiments using controlled sources. Concluding a busy two-month period for neutrinos since the previous edition of CERN Courier was published, the launch of the nuSCOPE collaboration now dangles the promise of a valuable boost to the other two. One hundred physicists attended its kick-off workshop at CERN from 13 to 15 October 2025. The collaboration seeks to implement a concept first proposed 50 years ago by Bruno Pontecorvo: nuSCOPE will eliminate systematic uncertainties related to neutrino flux by measuring the energy and flavour of neutrinos as they are created as well as when they interact with a target.

If approved, nuSCOPE will study neutrino–nucleus interactions with a level of accuracy comparable to that in electron–nucleus scattering, and control the sources of uncertainty projected to be dominant in the DUNE experiment under construction in the US and at the Hyper-Kamiokande experiment under construction in Japan. DUNE and Hyper-Kamiokande both plan to study the oscillations of accelerator-produced beams of muon neutrinos. Their most specialised design goal is to observe another fundamental aspect of physics: whether the weak interaction treats neutrinos and antineutrinos symmetrically.

With three ambitious and sharply divergent experimental concepts, DUNE, Hyper-Kamiokande and JUNO promise substantial progress in neutrino physics in the coming decade. But KATRIN and MicroBooNE now leave precious little merit for the once compelling phenomenology of the single light sterile neutrino.

Two strikes, and you’re out.

The post Two strikes for the light sterile neutrino appeared first on CERN Courier.

“It’s the first time in history that private donors wish to partner with CERN to build an extraordinary research instrument that will allow humanity to take major steps forward in our understanding of fundamental physics and the universe. I am profoundly grateful to them for their generosity, vision and unwavering commitment to knowledge and exploration. Their support is essential to the prospective realisation of the FCC and to enabling future generations of scientists to push the frontiers of scientific discovery and technology,” said CERN Director-General Fabiola Gianotti.

Understanding the fundamental nature of our universe is the mission that unites humanity

“Understanding the fundamental nature of our universe is the mission that unites humanity,” said Pete Worden, chairman of the Breakthrough Prize Foundation. “We’re proud to support the creation of the most powerful scientific instrument in history, that can shed new light on the deepest questions humanity can ask.”

“The Future Circular Collider is an instrument that could push the boundaries of human knowledge and deepen our understanding of the fundamental laws of the universe,” said Eric Schmidt. “Beyond the science, the technologies emerging from this project could benefit society in profound ways, from medicine to computing to sustainable energy, while training a new generation of innovators and problem-solvers. Wendy and I are inspired by the ambition of this project and by what it could mean for the future of humanity.”

“CERN’s Member States are extremely grateful for the interest expressed by our donors in contributing to the funding of the Laboratory’s next flagship project. This once again demonstrates CERN’s relevance and positive impact on society, and the strong interest in CERN’s future that exists well beyond our own particle-physics community,” said the president of the CERN Council Costas Fountas.

The FCC has also been included among 11 proposed “Moonshot” projects in the draft Multiannual Financial Framework for the years 2028–2034, released by the European Commission in July.

Based on strong input from the international particle-physics community, the FCC has been recommended as the preferred option for the next flagship collider at CERN in the ongoing process to update the European Strategy for Particle Physics, which will be concluded by the CERN Council in May 2026 (see “European Strategy Group recommends FCC-ee“). A decision by the CERN Council on the construction of the FCC is expected around 2028.

The post Private donors pledge support for FCC appeared first on CERN Courier.

Cosmology has long predicted that the first generation of stars should differ strongly from those forming today. Born out of pristine gas of only hydrogen and helium, they could have reached masses between a thousand and ten thousand times that of the Sun, before collapsing after only a few million years. Such “primordial monsters” have been proposed as the seeds of the first quasars (see “Collapsing monster” image), but clear observations had until now been lacking.

An analysis of the galaxy GS 3073 using the James Webb Space Telescope (JWST) now carries an unexpectedly loud message from the first generation of stars: there is far too much nitrogen to be explained by known stellar populations. This mismatch suggests a different kind of stellar ancestor, one no longer present in our universe. It is the first indirect evidence for the long-sought primordial monsters, first proposed in the early 1960s by Fred Hoyle and William Fowler in the US, and independently by Yakov Zel’dovich and Igor Novikov in the Soviet Union, in attempts to explain the newly discovered quasars.

Black-hole powered

JWST’s near-infrared spectroscopy of GS 3073 reveals the highest nitrogen-to-oxygen ratio yet measured while surveying the universe’s first billion years. Its dense central gas contains almost as many nitrogen atoms as oxygen, while carbon and neon are comparatively modest. In addition, the galaxy has an active nucleus powered by a black hole that is already millions to hundreds of millions of times the mass of the Sun, despite the galaxy’s low metallicity.

Could a primordial monster explain GS 3073? The answer lies in how these huge stars mix and burn their fuel.

GS 3073 could offer the first chemical evidence for the largest stars the universe ever formed and to the early production of massive black holes

Simulations reveal that after an initial phase of hydrogen burning in the core, these stars ignite helium, producing large amounts of carbon and oxygen. Because the stars are so luminous and extended, their interiors are strongly convective. Hot material rises, cool material sinks and chemical elements are constantly stirred. Freshly made carbon from the helium-burning core leaks outward into a surrounding shell where hydrogen is still burning. There, a sequence of reactions known as the CNO cycle converts hydrogen into helium while steadily turning carbon into nitrogen. Over time, this process loads the outer parts of the star with nitrogen, while also moderately enhancing oxygen and neon. The heaviest elements produced in the final burning stages remain trapped in the core and never reach the surface before the star collapses.

Mass loss from such primordial stars is uncertain. Without metals, they cannot generate the strong line-driven winds familiar from massive stars today. Instead, mass may be lost through pulsations, eruptions or interactions in dense environments. But simulations allow a robust conclusion: supermassive primordial stars between roughly one thousand and ten thousand solar masses naturally produce gas with nitrogen-to-oxygen, carbon-to-oxygen and neon-to-oxygen ratios that match those measured in the dense regions of GS 3073. Stars significantly lighter or heavier than this range cannot reproduce the extreme nitrogen-to-oxygen ratio, even before carbon and neon are taken into account.

Under pressure

Radiation pressure could have supported these primordial monsters for no more than a few million years. As their cores contract and heat, photons become energetic enough to convert into electron–positron pairs, reducing the radiation pressure. For classical massive stars with masses in the range of nine to 120 times the mass of the sun, this instability leads to a thermonuclear explosion that we refer to as a supernova. By contrast, supermassive stars are so dominated by gravity due to their much larger mass that they collapse directly into black holes, without undergoing a supernova explosion.

This provides a natural path from supermassive primordial stars to the over-massive black hole now seen in GS 3073’s nucleus. In this scenario, one or a few such giants enrich the surrounding gas with nitrogen-rich material through mass loss during their lives, and leave behind black-hole seeds that later grow by accretion. If this picture is correct, GS 3073 offers the first chemical evidence for the largest stars the universe ever formed and ties them directly to the early production of massive black holes. Future JWST observations, together with next-generation ground-based telescopes, will search for more nitrogen-loud galaxies and map their chemical structures in greater detail.

The post First indirect evidence for primordial monsters appeared first on CERN Courier.

The high-energy transient sky is filled with a cacophony of exotic explosions produced by stellar death. Short GRBs of less than two seconds are produced by the merging of compact objects such as black holes and neutron stars. Longer GRBs are produced by the death of massive stars, with “ultralong” GRBs most often hypothesised to originate in the collapse of massive blue supergiants, as they would allow for accretion onto their central black-hole engines over a period from tens of minutes to hours.

Peculiar observations

GRB 250702B lasted for at least 25,000 seconds (7 hours), superseding the previous longest GRB 111209A by over 10,000 seconds. However, the duration alone was not enough to identify this event as a different class of GRB or as an extreme outlier. Two other observations immediately marked GRB 250702B as peculiar: the multiple gamma-ray episodes seen by Fermi and other high-energy satellites; and the soft X-rays from 0.5 to 4 keV seen by China’s Einstein Probe over a period extending a full day before gamma rays were detected.

No previous GRB is known to have been preceded by X-ray emission over such a period. Nor is it an expectation of standard GRB models, even those invoking a blue supergiant. Instead, these X-rays suggest a relativistic tidal disruption event (TDE) – the shredding of a star by a massive black hole, launching a jet that moves near the speed of light. All known relativistic TDE systems are produced by supermassive black holes weighing a million times the mass of our Sun, or more. Such black holes are found at the centre of their host galaxies, but the HST and JWST observations revealed that the transient had occurred near the edge of its host galaxy’s disc (see “Not from the nucleus” image).

This peripheral origin opens the door to a more exotic scenario involving an intermediate-mass black hole (IMBH) weighing hundreds to thousands of solar masses. IMBHs are a missing link in black-hole evolution between the stellar-mass black holes that gravitational-wave detectors frequently see merging and the supermassive black holes found at the centre of most galaxies. Alternative scenarios reduce the black-hole mass even further, and include a micro-TDE, where a star is shredded by a stellar-mass black hole, or a helium star being eaten by a stellar-mass black hole.

There is little consensus on the origin of GRB 250702B, beyond that it involved an accreting black hole

The rapid gamma-ray variability observed by Fermi and other high-energy satellites is an important clue. The time variability of relativistic jets is thought to be orders of magnitude slower than the characteristic scale set by a black hole’s Schwarzschild radius. While an intermediate-mass black hole of a few hundred solar masses is not incompatible, the observed variability is nearly 100 times faster than that seen in relativistic TDEs. By contrast, with characteristic physical scales smaller in proportion to the smaller masses of their black holes, micro-TDEs and helium-star black-hole mergers have no difficulty accommodating such short-timescale variability.

The environment of the transient also provides crucial clues into its origin. JWST spectroscopy revealed that the light from the transient and its host galaxy was emitted 8 billion years ago, when the universe was just a teenager. The galaxy is among the largest and most massive at that age in the universe, and – unusually for galaxies hosting GRBs – a massive dust lane splits its disc in half. Ongoing star formation at the transient’s location suggests a stellar-mass progenitor, as opposed to an IMBH.

Despite numerous studies, there is little consensus on the origin of GRB 250702B, beyond that it involved an accreting black hole. Its exceptional duration and early X-ray emission initially suggested a supermassive black hole, but its rapid variability and location in its host galaxy instead point to a stellar-mass black hole, with a far rarer IMBH potentially splitting the difference. Given that it is a notably rare once-every-50-years event, the wait for the next ultralong GRB may be long, but astrophysicists are optimistic that theoretical advances will disentangle the different progenitor scenarios and reveal the origin of this extraordinary transient.

The post Longest gamma-ray burst confounds astrophysicists appeared first on CERN Courier.

George Smoot, who led the team that first measured tiny fluctuations in the cosmic microwave background (CMB) and began a revolution in cosmology, passed away in Paris on 18 September 2025.

George earned his undergraduate and doctoral degrees at the Massachusetts Institute of Technology (MIT), and then moved to Berkeley, where he held positions at Lawrence Berkeley National Laboratory (Berkeley Lab) and the Space Sciences Laboratory at the University of California, Berkeley (UC Berkeley). Though trained as a particle physicist, he switched to cosmology and developed research projects, including using differential microwave radiometers (DMRs) on U-2 spy planes to detect the dipole anisotropy of the CMB, a consequence of the motion of the Earth relative to the universe as a whole. He then devoted himself to the measurement of the CMB in detail, and this undertaking occupied him from his proposal of a satellite experiment using DMRs in 1974 to the results of the Cosmic Background Explorer (COBE) satellite in 1992. George subsequently continued research and teaching as a member of the faculty of the UC Berkeley physics department.

In 2006, the Nobel Prize committee recognised John Mather for leading a team that determined the CMB spectrum was a blackbody (arising from thermal equilibrium) to exquisite precision, and George for leading a team that detected temperature variations across the sky in the CMB at the level of one part in a hundred thousand. Those variations were signatures of the primordial density fluctuations that gave rise to galaxies, and so eventually to us. They have been called the DNA of cosmic structure and provide a remarkable window on the early universe and high-energy physics beyond our particle accelerators. The excitement caused by the COBE CMB results was dramatically expressed by Stephen Hawking, who declared them to be “the discovery of the century, if not all time.”

After the Nobel Prize, George intensified his efforts in science education and training young scientists. Indeed, on the day of the prize, George continued to teach his undergraduate introductory physics class.

George created new research institutes internationally to support young scientists. He used his prize money to found the Berkeley Center for Cosmological Physics, a joint effort between UC Berkeley and Berkeley Lab. He also started an annual Berkeley Lab summer workshop for high-school students and teachers, now in its 19th year. Later, he founded the Instituto Avanzado de Cosmología and the international Essential Cosmology for the Next Generation winter schools in Mexico, the Paris Centre for Cosmological Physics, the Institute for the Early Universe in South Korea at the world’s largest women’s university, and more. Many of the scientists trained at those institutes went on to become faculty in their home countries and internationally, and formed their own research groups.

His open online course “Gravity! From the Big Bang to Black Holes” taught nearly 100,000 students

George took special pride in the Oersted Medal awarded to him by the American Association of Physics Teachers in 2009 for “outstanding, widespread, and lasting impact” on the teaching of physics. His massive open online course “Gravity! From the Big Bang to Black Holes” with Pierre Binétruy taught nearly 100,000 students.

In his later years, George’s scientific interests spanned not only the CMB (in particular the Planck satellite), but new sensor technologies such as kinetic inductance detectors and ultrafast detectors that could open up new windows on astrophysical phenomena, gravitational waves and gravitational lensing, features in the inflationary primordial fluctuation spectrum, and dark-matter properties.

The primordial density fluctuations for which George was awarded the Nobel Prize lie at the heart of almost every aspect of cosmology. The revolution started by the COBE results led to the convergence of cosmology and particle physics, exemplified by the centrality of dark matter as a primary issue for both disciplines. George will be remembered for this, for the many students whose lives he touched and whose research he inspired, and for his advocacy of international science.

The post George Smoot 1945–2025 appeared first on CERN Courier.

The workshop highlighted how much progress ATLAS and CMS have made in searches for long-lived particles, hidden-valley scenarios (see “Soft cloud” figure) and a host of other unconventional possibilities that now occupy centre stage. Although these ideas were once considered exotic, they have become natural extensions of models connected to cosmology, dark matter and electroweak symmetry breaking. Their experimental signatures are equally rich: displaced vertices, delayed showers, emerging jets or unusual track topologies that demand a rethinking of reconstruction strategies from the ground up.

Deep learning

The most transformative change since previous editions of SEARCH is the integration of AI-based algorithms into every layer of analysis. Deep-learning-driven b-tagging has dramatically increased sensitivity to final states involving heavy flavour, while machine learning is being embedded directly into hardware trigger systems to identify complex event features in real time. This is not technological novelty for its own sake: these tools directly expand the discovery reach of the experiments.

Novel ideas in reconstruction also stood out. Talks showcased how muon detectors can be repurposed as calorimeters to detect late-developing showers, and how tracking frameworks can be adapted to capture extremely displaced tracks that were once discarded as outliers. Such techniques illustrate a broader cultural shift: expanding the search frontier now often comes from reinterpreting detector capabilities in creative ways.

The most transformative change since previous editions of SEARCH is the integration of AI-based algorithms into every layer of analysis

Anomaly detection – the use of unsupervised or semi-supervised deep-learning models to identify data that deviate from learned patterns – was another major focus. These methods, used both offline and in level-one triggers, enable model-agnostic searches that do not rely on an explicit beyond-the-Standard-Model target. Participants noted that this is especially valuable for scenarios like quirks in dark-sector models, where realistic event-generation tools still do not exist. In these cases, anomaly detection may be the only feasible path to discovery.

The rising importance of precision was another theme threading through the discussions. The detailed understanding of detector performance achieved in recent years is unprecedented for a hadron collider. CMS’s muon calibration, which is crucial for its W-mass analy­sis, and ATLAS’s record-breaking jet-calibration accuracy exemplify the progress. This maturity opens the possibility that new physics could first appear as subtle deviations rather than as striking anomalies. As the era of the High-Luminosity LHC approaches, the upcoming additions of precision timing layers and advanced early-tracking capabilities will further strengthen this dimension of the search programme.

The workshop also provided a platform to explore connections between collider searches and other experimental efforts across particle physics. Strong first-order phase transitions, relevant to electroweak baryogenesis, motivated renewed interest in an additional scalar that would modify the Higgs potential. Such a particle could lie anywhere from the MeV scale up to hundreds of GeV – often below the mass ranges targeted by standard resonance searches. Alternative data-taking strategies such as data scouting and data parking offer new opportunities to probe this wide mass window systematically.

Complementarity with flavour physics at LHCb, long-lived particle searches at FASER, and precision experiments seeking electric dipole moments, axion-like particles and other ultralight states, was also highlighted. In a moment without an obvious theoretical favourite, this diversification of experimental approaches is a key strategic strength.

New directions in science are launched by new tools much more often than by new concepts

A recurring sentiment was that the LHC remains a formidable discovery machine, but the community must continue pushing its tools beyond their traditional boundaries. Many discussions at SEARCH 2025 echoed a famous remark by Freeman Dyson: “New directions in science are launched by new tools much more often than by new concepts.” The upcoming upgrades to ATLAS and CMS – precision timing, enhanced tracking earlier in the trigger chain and high-granularity readout – exemplify the kinds of new tools that can reshape the search landscape.

If SEARCH 2025 underscored the need to explore new signatures, technologies and experimental ideas, it also highlighted an equally important message: we must not lose sight of the physics questions that originally motivated the LHC programme. The hierarchy problem, the apparent fine tuning of quantum corrections to the Higgs mass that prevent it rising to the Planck scale, remains unresolved, and supersymmetry continues to offer its most compelling and robust solution by stabilising it through partner particles. With the dramatic advances in reconstruction, triggering and analysis techniques, and with the enormous increase in recorded data from Run 1 through Run 3, the time is ripe to revitalise the inclusive SUSY search programme. A comprehensive, modernised SUSY effort should be a defining element of the combined ATLAS and CMS legacy physics programme, ensuring that the field fully exploits the discovery potential of the LHC dataset accumulated so far.

The post From theories to signals appeared first on CERN Courier.

The LHC’s increased collision energies have opened new territory for TeV-scale searches, but its vast datasets also provide unparalleled opportunities to thoroughly explore the electroweak scale. A new ATLAS result uses an unconventional trigger-level analysis (TLA) of the full Run 2 dataset to achieve record sensitivity to low-mass particles decaying into quarks or gluons. ATLAS employs a two-stage trigger system, with a fast hardware-based first-level trigger selecting about 100 kHz of events from the 40 MHz bunch-crossing rate, followed by a software high-level trigger (HLT) that performs detailed event reconstruction and further reduces the accepted event rate by about two orders of magnitude. By recording a much reduced event format at the trigger level, TLA preserves a substantially larger fraction of events than would normally be output by the HLT.

New particles that decay with a two-jet final state feature in many Standard Model (SM) extensions. For example, the properties of “dark mediators” that couple to both quarks and dark matter could explain the present abundance of dark matter by controlling how much of it remains after falling out of equilibrium with normal matter in the early universe. At the LHC, the coupling of dark mediators to quarks would enable both production and decay into quark–antiquark pairs. This should appear as resonances in the dijet mass distribution.

Searching for dijet resonances at low mass is challenging. Dijet production from strong interactions is one of the LHC’s most abundant signatures. Beyond requiring a precise understanding of these enormous backgrounds and the detector response, the low-mass dijet rate far exceeds what ATLAS can record. Only the most energetic dijet events can be kept, limiting conventional dijet searches to masses above approximately 1 TeV.

To access the low-mass region, ATLAS used TLA to record multi-jet events throughout Run 2. By dropping the raw detector data from the readout, these TLA events were ~200 times smaller than standard events while retaining all high-level jet and calorimeter-based variables reconstructed in real-time by the HLT.

The size reduction allowed ATLAS to record TLA events at rates of up to 27 kHz – compared to an average 1.2 kHz for the full detector readout. This rate was achieved in conjunction with the additional trigger bandwidth allocated to TLA at the end of LHC fills and a more efficient use of this bandwidth for dijet events. In Run 2, this was aided by ATLAS’s L1Topo trigger processor, which applies simple topological selections – such as angular correlations between jets – already at first level. The new result uses 1 billion dijet events, or up to 75 times the data sample available to the equivalent conventional search, achieving unprecedented statistical precision.

The new result achieves record sensitivity to low-mass particles decaying into quarks or gluons

This enormous dataset demands excellent control of systematic uncertainties. ATLAS developed a dedicated multi-step calibration for trigger-level jets, achieving a jet energy scale precision of 1 to 4%, comparable to calibrations using full detector readout. The overwhelming SM background was modelled using a data-driven fitting technique, reaching a relative precision better than 1 part in 10⁴.

The search has found the dijet invariant-mass distribution to be consistent with the background expectation. The analysis provides numerical results that can be used to constrain any of the numerous models of dijet resonances, as well as explicit constraints on a specific dark mediator model used as a common benchmark for many ATLAS and CMS searches. The result sets ATLAS’s most stringent exclusion limits to date on the potential coupling of such a mediator to quarks, across a broad range of mediator masses reaching as low as 375 GeV (see figure 1).

The dijet TLA during Run 2 has established a foundation for an expanded trigger-level physics programme. In Run 3, trigger-level jets incorporate tracking information, allowing flavour tagging and improving jet energy resolution and robustness against pile-up. ATLAS also records trigger-level photons and uses them in combination with partial detector readout at full granularity. These and other advances in TLA should enable future ATLAS searches to probe a wider variety of signatures at the electroweak scale.

The post Trigger-level search for dijet resonances appeared first on CERN Courier.

Planetary defence

“Planetary defence represents a scientific challenge,” says Karl-Georg Schlesinger, co-founder of OuSoCo, a start-up developing advanced material-response models used to benchmark large-scale nuclear deflection simulations. “The world must be able to execute a nuclear deflection mission with high confidence, yet cannot conduct a real-world test in advance. This places extraordinary demands on material and physics data.”

Accelerator facilities play a key role in understanding how asteroid mat­erial behaves under extreme conditions, providing controlled environments where impact-relevant pressures and shock conditions can be reproduced. To probe the material response directly, the team conducted experiments at CERN’s HiRadMat facility in 2024 and 2025, as a part of the Fireball collaboration with the University of Oxford. A sample of the Campo del Cielo meteorite, a metal-rich iron-nickel body, was exposed to 27 successive short, intense pulses of the 440 GeV SPS proton beam, reproducing impact-relevant shock conditions that cannot be achieved with conventional laboratory techniques.

“The material became stronger, exhibiting an increase in yield strength, and displayed a self-stabilising damping behaviour,” explains Melanie Bochmann, co-founder and co-team lead alongside Schlesinger. “Our experiments indicate that – at least for metal-rich asteroid material – a larger device than previously thought can be used without catastrophically breaking the asteroid. This keeps open an emergency option for situations involving very large objects or very short warning times, where non-nuclear methods are insufficient and where current models might assume fragmentation would limit the usable device size.”

Throughout the experiments at the SPS, the team monitored each pulse using laser Doppler vibrometry alongside temperature sensors, capturing in real time how the meteorite softened, flexed and then unexpectedly re-strengthened without breaking. This represents the first experimental evidence that metal-rich asteroid material may behave far more robustly under extreme, sudden energy loading than predicted.

The experiments could also provide valuable insights into planetary formation processes

After the SPS campaign, initial post-irradiation measurements were performed at CERN. These revealed that magnesium inclusions had been activated to produce sodium-22, a radioactive isotope that decays to produce a positron, allowing diagnostics similar to those used in medical imaging. Following these initial measurements, the irradiated meteorite has been transferred to the ISIS Neutron and Muon Source at the Rutherford Appleton Laboratory in the UK, where neutron diffraction and positron annihilation lifetime spectroscopy measurements are planned.

“These analyses are intended to examine changes in the meteorite’s internal structure caused by the irradiation and to confirm, at a microscopic level, the increase in material strength by a factor of 2.5 indicated by the experimental results,” explains Bochmann.

Complementary information can be gathered by space missions. Since NASA’s NEAR Shoemaker spacecraft successfully landed on asteroid Eros in 2001, two Japanese missions and a further US mission have visited asteroids, collecting samples and providing evidence that some asteroids are loosely bound rocky aggregates. In the next mission, NASA and ESA plan to study Apophis, an asteroid several hundreds of metres in size in each dimension that will safely pass closer to Earth than many satellites in geosynchronous orbit on 13 April 2029 – a close encounter expected only once every few thousand years.

The missions will observe how Apophis is twisted, stretched and squeezed by Earth’s gravity, providing a rare opportunity to observe asteroid-scale material response under natural tidal stresses. Bochmann and Schlesinger’s team now plan to study asteroids with a similar rocky composition.

Real-time data

“In our first experimental campaign, we focused on a metal-rich asteroid material because its more homogeneous structure is easier to control and model, and it met all the safety requirements of the experimental facility,” they explain. “This allowed us to collect, for the first time, non-destructive, real-time data on how such material responds to high-energy deposition.”

“As a next step, we plan to study more complex and rocky asteroid materials. One example is a class of meteorites called pallasites, which consist of a metal matrix similar to the meteorite material we have already studied, with up to centimetre-sized magnesium-rich crystals embedded inside. Because these objects are thought to originate from the core–mantle boundary of early planetesimals, such experiments could also provide valuable insights into planetary formation processes.”

The post Asteroid tests challenge nuclear-deflection models appeared first on CERN Courier.

Rohini Madhusudan Godbole, one of India’s most influential particle physicists, passed away in her hometown of Pune on 25 October 2024.

Rohini was born on 12 November 1952 to Madhusudan and Malati Godbole. Theirs was a cultured and highly educated family, and she grew up in an atmosphere of intellectual freedom and progressive ideas. Educated at the best schools and colleges in Pune, she joined the Indian Institute of Technology at Bombay, from which she graduated in 1972. She then moved to Stony Brook, where she completed her PhD in particle physics with Jack Smith in 1979. Returning to India, she worked temporarily at the Tata Institute of Fundamental Research before joining the faculty at the University of Bombay (now Mumbai). There she remained until 1997, when she moved to the Centre for High Energy Physics at the Indian Institute of Science at Bangalore (now Bengaluru). She worked there for the rest of her life, continuing after her formal retirement as an emeritus professor. It was only a few months before the end that she moved back to her hometown, to be with her family in her last days.

Rohini was a prolific researcher. She will probably be best remembered pioneering the development, with Manuel Drees, of photon structure functions for use with photon beams at future colliders, but her contributions spanned vacuum polarisation, Higgs physics, top-quark physics with polarised beams, and beyond the Standard Model physics, especially low-energy supersymmetry. She authored a well-known textbook on the latter subject with Probir Roy and Drees.

Rohini was indefatigable in promoting the cause of women in science

Rohini’s broad understanding and warm character combined to make her the best-known face of elementary particle physics from India. She worked tirelessly to promote high-energy physics inside India, organising schools and workshops, and often represented the country in international forums, such as to monitor India’s participation in the LHC and other large international collaborative experiments. Rohini was a dedicated teacher and mentor to a long series of graduate students and postdocs, and a universal elder sister or aunt for the entire community of younger particle physicists in India.

No description of Rohini can be complete without mentioning her indefatigable efforts to promote the cause of women in science. Having herself faced gender discrimination in her younger days, she was determined to ensure that young women scientists received proper opportunities and recognition. She authored two books highlighting the work of Indian women scientists, thereby setting up role models to inspire the younger generation. Even more than these books, however, her own presence and encouragement left a mark on two generations of particle physicists, in India and abroad.

Rohini’s signal contributions were recognised by many awards and distinctions. The government of India awarded her the coveted Padma Shri in 2019, and the government of France awarded her the Ordre National du Mérite in 2021, mentioning her important role in furthering scientific collaboration between India and France. But her true memorial lies in the unique place she holds in the hearts of thousands of students, collaborators, friends and acquaintances. She was an extraordinary person who carved out a niche all by herself, with her scientific talents, her indefatigable energy, her universal amiability and her indomitable will. Her loss is sorely felt.

The post Rohini Godbole 1952–2024 appeared first on CERN Courier.

Particles react to magnetic fields like tiny bar magnets, depending on their mass, electric charge and spin – a sort of intrinsic angular momentum lacking a true classical analogue. These properties combine into the magnetic moment, along with a quantum-mechanical g-factor which sets the strength of the response. Dirac computed g to be precisely two for electrons, with a formula that applies equally to the other, then-unknown, leptons. We call any deviation from this value anomalous. The name stuck because the first measurements differed from Dirac’s prediction, which initially was not understood. The anomalous piece is a natural probe of new physics, as it arises entirely from quantum fluctuations that may involve as-yet unseen new particles.

What ingredients from the Standard Model go into computing g–2?

Everything. All sectors, all particles, all Standard Model (SM) forces contribute. The dominant and best quantified contributions are due to QED, having been computed through fifth order in the fine structure constant α. We are talking about two independent calculations of more than 12,000 Feynman diagrams, accounting for more than 99.9% of the total SM prediction. Interestingly, two measurements of α disagree at more than 5σ, resulting in an uncertainty of about two parts per billion. While this discrepancy needs to be resolved, it is negligible for the muon g–2 observable. The electroweak contribution was computed at the two-loop level long ago, and updated with better measured input parameters and calculations of nonperturbative effects in quark loops. The resulting uncertainty is close to 40 times smaller than that of the g–2 experiment. Then, the overall uncertainty is determined by our knowledge of the hadronic corrections, which are by far the most difficult to constrain.

What sort of hadronic effects do you have in mind here? How are they calculated?

There are two distinct effects: hadronic vacuum polarisation (HVP) and hadronic light-by-light (HLbL). The former arises at second order in α, is the larger of the two, and the largest source of uncertainty. While interacting with an external magnetic field, the muon emits a virtual photon that can further split into a quark loop before recombining. The HLbL contribution arises at third order and is now known with sufficient precision. The challenge is that loop diagrams must be computed at all virtual energies, down to where the strong force (QCD) becomes non-perturbative and quarks hadronise. There are two ways to tackle this.

Instead of computing the hadronic bubble directly, the data-driven “dispersive” approach relates it to measurable quantities, for example the cross section for electron–positron annihilation into hadrons. About 75% of the total HVP comes from e⁺e^– → π⁺π^–, so the measurement errors in this channel determine the overall uncertainty. The decays of tau leptons into hadrons can also be used as inputs. Since the process is mediated by a charged W boson, instead of a photon, it requires an isospin rotation from the charged to the neutral current. At low energies, this is another challenging non-perturbative problem. While there are phenomenological estimates of this effect, no complete theoretical calculation exists – which means that the uncertainties are not fully quantified. Differing opinions on how to assess them led to controversy over the inclusion of tau decays in the SM prediction of g–2. An alternative to data-driven methods is lattice QCD, which allows for ab initio calculations of the hadronic corrections.

What does “ab initio” mean, in this context?

It means that there are no simplifying assumptions in the QCD calculation. The approximations used in the lattice formulation of QCD come with adjustable parameters and can be described by effective field theories of QCD. For example, we discretise space and time: the distance separating nearest-neighbour points is given by the lattice spacing and the effective field theory guides the approach of the lattice theory to the continuum limit, enabling controlled extrapolations. To evaluate path integrals using Monte Carlo methods, which themselves introduce statistical errors, we also rotate to imaginary time. While not affecting the HVP, this limits the quantities we can compute.

How do you ensure that the lattice predictions are unbiased?

Good question! Lattice calculations are complicated, and it is therefore important to have several results from independent groups for consolidating averages. An important cultural shift in the community is that numerical analyses are now routinely blinded to avoid confirmation bias, making agreements more meaningful. This shifts the focus from central values to systematic errors. For our 2025 White Paper (WP25), the main lattice inputs for HVP were obtained from blinded analyses.

How did you construct the SM prediction for your 2025 White Paper?

To summarise how the SM prediction in WP25 was obtained, sufficiently precise lattice results for HVP arrived just in time. Since measurements of the e⁺e^– → π⁺π^– channel are presently in disagreement with each other, the 2025 prediction solely relied on the lattice average for the HVP. In contrast, the 2020 White Paper (WP20) prediction employed the data-driven method, as the lattice-QCD results were not precise enough to weigh in.

With the experiment’s expected precision jump, it seemed vital for theory to follow suit

While the theory error in WP25 is larger than in WP20, it is a realistic assessment of present uncertainties, which we know how to improve. I stress that the combination of the SM theory error being four times larger than the experimental one and the remaining puzzles, particularly on the data-driven side, means that the question “Does the SM account for the experimental value of the muon’s anomalous magnetic moment?” has not yet been satisfactorily answered. Given the high level of activity, this will, however, happen soon.

Where are the tensions between lattice QCD, data-driven predictions and experimental measurements?

All g–2 experiments are beautifully consistent, and the lattice-based WP25 prediction differs from them by less than one standard deviation. At present, we don’t know if the data-driven method agrees with lattice QCD due to the differences in the e⁺e^– → π⁺π^– measurements. In particular, the 2023 CMD-3 results from the Budker Institute of Nuclear Physics are compatible with lattice results, but disagree with CMD-2, KLOE, BaBar, BESIII and SND, which formed the basis for WP20. All the experimental collaborations are now working on new analyses. BaBar is expected to release a new e⁺e^– → π⁺π^– result soon, and others, including Belle II, will follow. There is also ongoing work on radiative corrections and Monte Carlo generators, both of which are important in solving this puzzle. Once the dust settles, we will see whether the new data-driven evaluation agrees with the lattice average and the g–2 experiment. Either way, this may yield profound insights.

How did the Muon g–2 Theory Initiative come into being?

The first spark came when I received a visiting appointment from Fermilab, offering resources to organise meetings and workshops. At the time, my collaborators and I were gearing up to calculate the HVP in lattice QCD, and the Fermilab g–2 experiment was about to start. With the experiment’s expected precision jump, it seemed vital for theory to follow suit by bringing together communities working on different approaches to the SM contributions, with the goal of pooling our knowledge, reducing theoretical uncertainties and providing reliable predictions.

As Fermilab received my idea positively, I contacted the RBC collaboration and Christoph Lehner joined me with great enthusiasm to shape the effort. We recruited leaders in the experimental and theoretical communities to our Steering Committee. Its role is to coordinate efforts, organise workshops to bring the community together and provide the structure to map out scientific directions and decide on the next steps.

What were the main challenges you faced in coordinating such a complex collaboration?

With so many authors and such high stakes, disagreements naturally arise. In WP20, a consensus was emerging around the data-driven method. The challenge was to come up with a realistic and conservative error estimate, given the up to 3σ tensions between different data sets, including the two most precise measurements of e⁺e^– → π⁺π^– at the time.

As we were finalising our WP20, the picture was unsettled by a new lattice calculation from the Budapest–Marseille–Wuppertal (BMW) collaboration, consistent with earlier lattice results but far more precise. While the value was famously in tension with data-driven methods, the preprint also presented a calculation of the “intermediate window” contribution to the HVP– about 30% of the total – which disagreed with a published RBC/UKQCD result and with data-driven evaluations (CERN Courier March/April 2025 p21). Since BMW was still updating their results and the paper wasn’t yet published, we described the result but excluded it from our SM prediction. Later, in 2023, further complications came from the CMD-3 measurement.

Consolidation between lattice results was first observed for the intermediate window contribution, in 2022 and 2023. This, in turn, revealed a tension with the corresponding data-driven evaluations. Results for the difficult-to-compute long-distance contributions arrived in late fall 2024, yielding consolidated lattice averages for the total HVP, where we had to sort out a few subtleties. This was intense – a lot of work in very little time.

On the data-driven side, we faced the aforementioned tensions between the e⁺e^– → π⁺π^– cross-section measurements. In light of these discrepancies, consensus was reached that we would not attempt a new data-driven average of HVP for WP25, leaving it for the next White Paper. Real conflict arose on the assessment of the quality of the uncertainty estimates for HVP contributions from tau decays and on whether to include them.

And how did you navigate these disagreements?

When the discussions around the assessment of tau-decay uncertainties stopped to converge, we proposed a conflict resolution procedure using the Steering Committee (SC) as the arbitration body, which all authors signed. If a conflict is brought to the SC for resolution, SC members first engage all parties involved to seek resolution. If none is found, the SC makes a recommendation and, if appropriate, the differing scientific viewpoints may be reflected in the document, followed by the recommendation. In the end, just having a conflict-resolution process in place was really helpful. While the SC negotiated a couple of presentation issues, the major disagreements were resolved without triggering the process.

The goal of WP25 was to wrap up a prediction before the announcement of the final Fermilab g–2 measurement. Adopting an internal conflict-resolution process was essential in getting our result out just in time, six days before the deadline.

Lattice QCD has really come of age

What other observables can benefit from advances in lattice QCD?

There are many, and their number is growing – lattice QCD has really come of age. Lattice QCD has been used for years to provide precise predictions of the hadronic parameters needed to describe weak processes, such as decay constants and form factors. A classic example, relevant to the LHC experiments, is the rare decay B_s → μ⁺μ^–, where, thanks to lattice QCD calculations of the B_s-meson decay constant, the SM prediction is more precise than current experimental measurements. While precision continues to improve with refined methods, the lattice community is broadening the scope with new theoretical frameworks and improved computational methods, enabling calculations once out of reach – such as the (smeared) R-ratio, inclusive decay rates and PDFs.

The post How I learnt to stop worrying and love QCD predictions appeared first on CERN Courier.

The extraordinary precision of 127 parts per billion (ppb) achieved at Fermilab deserves to be matched by an equally impressive theoretical prediction. At 530 ppb, theory is currently the limiting factor in any comparison. This is the longer-term goal that the Muon g–2 Theory Initiative is now working towards, with inputs from all possible sources (see “How I learnt to stop worrying and love QCD predictions“). In the near future, it will not be possible to reach this precision with lattice QCD alone. Other approaches are needed to make a competitive Standard Model prediction.

Tensions remain

Essentially, all of the uncertainty in g–2 arises from the hadronic vacuum polarisation (HVP) – a quantum correction whereby a radiated virtual photon briefly transforms into a hadronic state before being reabsorbed. Historically, HVP has been evaluated by applying a dispersion relation to cross sections for hadron production in electron–positron collisions, but this method was displaced by lattice–QCD calculations in the theory initiative’s most recent white paper. The lattice community must be congratulated for the level of agreement that has been reached between groups working independently (CERN Courier July/August 2025 p7). By contrast, data-driven predictions are at present inconsistent across the experiments in the low-energy region; even if results from the CMD-3 experiment are excluded as an outlier, tensions remain, suggesting that some systematic errors may not have been completely addressed (CERN Courier March/April 2025 p21). Could a novel experimental technique help resolve the confusion?

The MUonE collaboration proposes a completely independent approach based on a new experimental method. In MUonE, we will determine the running of the electromagnetic coupling, a fundamental quantity that is driven by the same kinds of quantum fluctuations as muon g–2. We will extract it from a precise measurement of the differential cross section for elastic scattering of muons from electrons as a function of the momentum transferred.

MUonE is a relatively inexpensive experiment that we can set up in the existing M2 beamline in CERN’s North Area, already home to the AMBER and NA64-µ experiments. Three years of running, within the conditions of M2 parameters and the performance of the MUonE detector, would reach a statistical precision of approximately 180 ppb with a comparable level of systematic uncertainty.

MUonE will take advantage of silicon sensors that are already being developed for the CMS tracker upgrade. From the results, we will be able to use a dispersion relation to extract HVP’s contribution to g–2. Perhaps more importantly, however, as our method directly measures a function that is part of the lattice calculation, we can directly verify that method. The big challenge will be to keep the systematic uncertainties in the measurement small enough. However, MUonE does not suffer from the intrinsic problem that existing data-driven techniques have, which is that they must numerically integrate over the sharp peaks of hadron production by low-energy resonances. In contrast, the function derived from the space-like process that it will measure is smooth and well-behaved.

Piecing the puzzle 

CERN was the origin of the first brilliant muon g–2 measurements starting back in the 1950s (CERN Courier September/October 2024 p53), and now the laboratory has an opportunity to put another important piece into the g–2 puzzle through the MUonE project. Another component of great importance in this domain will be the new g-2/EDM experiment planned for J-PARC, which will also be performed in completely different conditions, and therefore with very different systematics to the Fermilab experiment.

The post There’s more g–2 physics over the horizon appeared first on CERN Courier.

Despite decades of searches, experiments have yet to find evidence for a new particle that could account for dark matter on its own. This has strengthened interest in richer “dark-sector” scenarios featuring multiple new states and interactions, potentially analogous to those of the Standard Model (SM). The CMS collaboration targeted one of the most distinctive possible signatures of a dark strong force in proton–proton collisions: a dense, nearly isotropic cloud of low-momentum particles known as a soft unclustered energy pattern (SUEP).

Searches in the LHC proton–proton collision data for events with many low-momentum particles are plagued by overwhelming backgrounds from pileup and soft QCD interactions. The CMS collaboration has recently overcome this challenge by using large-radius clusters of charged particle tracks and relying on quantities that characterise the expected isotropy of SUEP decays.

The 125 GeV Higgs boson serves in many theoretical models as a natural mediator between the SM and a hidden sector, and current experimental constraints still leave room for exotic decays. Motivated by this possibility, CMS focused on Higgs-boson production in association with a vector (W or Z) boson that decays into leptons. While these modes account for < 1% of Higgs bosons produced at the LHC, the leptons provide significant handles for triggering and background suppression.

Rather than relying on SM simulations, which face modelling and statistical challenges for such soft interactions, the background was extrapolated from events with low isotropy or relatively few charged-particle tracks per cluster, using a method that accounts for small correlations between the quantities used in the extrapolation. To validate the approach, an orthogonal sample of events with a high-momentum photon was studied, taking advantage of the Higgs boson’s minuscule coupling to photons and the similarity of background processes in W/Z + jet and photon + jet events that could mimic a SUEP signal.

The data in the search region, consisting of events with a W or Z boson candidate and many isotropically distributed charged particles, was found to be consistent with the SM expectation. Stringent limits were placed on the branching ratio of the 125 GeV Higgs boson decaying to a SUEP shower for a wide range of parameters (see figure 1).

This analysis complements a previous CMS search that primarily targeted much heavier mediators produced via gluon fusion, improving limits on the H → SUEP branching ratio by two orders of magnitude. It additionally provides model-agnostic limits and detailed reinterpretation recipes, maximising the usability of this data for testing alternative theoretical frameworks.

SUEP signatures are not unique to the benchmark scenarios under scrutiny. They naturally emerge in hidden-valley models, where mediators connect the SM to a new, otherwise isolated sector. If the hidden states interact through a “dark QCD”, proton–proton collisions would trigger a crowded cascade of dark partons rather than the familiar collimated showers.

Crucially, unlike in ordinary QCD – where the coupling quickly weakens at energies above confinement – the dark coupling could remain large well beyond its typically low confinement scale. This sustained strong coupling would drive frequent interactions and efficiently redistribute momentum, producing an almost isotropic radiation pattern. As the system cooled, it would then hadronise into numerous soft dark hadrons whose decays back to SM particles would retain this softness and isotropy – yielding the characteristic SUEP probed by CMS.

The post Soft clouds probe dark QCD appeared first on CERN Courier.

On 13 September 2025, 40 representatives of all currently operating neutron-lifetime experiments came together at the Paul Scherrer Institute (PSI) to discuss the current status of the tension and the path forward. Geoffrey Greene (University of Tennessee) opened the workshop by reflecting on five decades of neutron-lifetime measurements from the 1960s to the present.

The beam method employs cold-neutron beams, with protons from neutron beta-decays collected in a magnetic trap and counted. The lifetime is then inferred from the ratio of proton counts to neutron flux. Fred Wietfeldt (Tulane University) highlighted the huge efforts undertaken at the National Institute of Standards and Technology (NIST) in Gaithersburg, most importantly on the absolute calibration of the neutron detector.

Susan Seestrom (Los Alamos National Laboratory) described today’s most precise experiment, the UCNτ experiment at Los Alamos National Laboratory, which uses the magnetic-bottle trap method. It confines ultracold neutrons (UCNs) via their magnetic and gravitational interaction and counts the surviving ones at different times. She also provided an outlook on its next phase, UCNτ⁺, with increased statistics goals. The τSPECT experiment at PSI’s UCN facility is also based on magnetic confinement of neutrons and has recently started data taking, but has distinct differences. As explained by Martin Fertl from Johannes Gutenberg-University Mainz, τSPECT uses a double-spin-flip method to increase the UCN filling of the purely magnetic trap, and a detector moving in and out of the storage volume to first remove slightly higher-energetic neutrons before storage, and then measures the surviving neutrons in situ after storage.

Kenji Mishima (University of Osaka) presented the neutron-lifetime experiment at J-PARC, based on a new principle: the detection of the charged decay products in an active time-projection-chamber, where the neutrons are captured on a small ³He admixture. This experiment’s systematics are entirely different from those of previous efforts and may offer a unique contribution to the field. Other studies largely excluded the possibility that the beam–bottle discrepancy could be explained by hypothetical exotic decay channels or other non-standard processes.

New results from LANL, NIST, J-PARC and PSI should clarify the currently puzzling situation in the coming years.

The post The beam–bottle debate at PSI appeared first on CERN Courier.

Marietta Blau was one of the first women to study physics at the University of Vienna. As recalled by Brigitte Strohmaier (University of Vienna), who summarised Blau’s biography, she became best known for her work at the Institute for Radium Research between 1923 and 1938, where she developed the nuclear-emulsion technique for detecting charged particles with micrometre-scale precision.

Together with Hertha Wambacher, Blau exposed nuclear emulsions to cosmic rays at Victor Hess’s observatory near Innsbruck, producing photographic evidence of the interactions between high-energy particles and matter.

Staying in Scandinavia when Nazi Germany annexed Austria in 1938, Blau could not return to Vienna. She secured a position at the Polytechnic Institute of Mexico City on the recommendation of Albert Einstein, but found herself isolated from colleagues. From 1944 on, she worked in the US before returning to Vienna in 1960, where she supervised the evaluation of photographic plates from CERN.

Her method of nuclear emulsions was further advanced by Cecil Powell in Bristol, who was awarded the Nobel Prize in Physics in 1950 for discoveries regarding mesons made with this method. On this and other occasions, Marietta Blau was also nominated, but never recognised for her groundbreaking research.

Joachim Kopp, chair of the Scientific Advisory Board of HEPHY, introduced the institute’s scientific outlook. He highlighted the breadth of MBI’s programme, which includes major contributions to CERN experiments such as CMS and ALICE at the LHC, and ASACUSA at the AD/ELENA facility, where antimatter is studied using low-energy antiprotons.

Groups at MBI are also involved in the Belle II experiment at KEK, as well as the dark-matter experiments CRESST and COSINUS at the LNGS underground lab. Neutrino physics, gravitational-wave studies at the Einstein Telescope, as well as tests of fundamental symmetries using ultra-cold hydrogen and deuterium beams, are also part of the research programme. The MBI also builds on the long tradition of detector development and construction for future experiments, complemented by a dedicated theory group.

The post Vienna’s new hub for particle physics appeared first on CERN Courier.

József Zimányi was a pioneer of Hungarian and international heavy-ion physics, playing a central role in establishing relativistic heavy-ion research in Hungary and contributing key developments to hydrodynamic descriptions of nuclear collisions. Much of the week’s programme revisited the problems that occupied his career, including how the hot, dense system created in a collision evolves and how it converts its energy into the observed hadrons.

Giuseppe Verde (INFN Catania) and Máté Csanád (ELTE) emphasised the role of femtoscopic methods, rooted in the Hanbury Brown–Twiss interferometry originally developed for stellar measurements, in understanding the system that emerges from heavy-ion collisions. Quantum entanglement in high-energy nuclear collisions – a subject closely connected to the 2025 Nobel Prize in Physics – was also explored in a dedicated, invited lecture by Dmitri Kharzeev (Stony Brook University), who described the approach and the results of his team that suggest the origin of the observed thermodynamic properties is quantum entanglement itself.

The NA61/SHINE collaboration reported ongoing studies of isospin-symmetry breaking, including a recent result where the charged-to-neutral kaon ratio in argon–scandium collisions deviates at 4.7σ from expectations based on approximate isospin symmetry (CERN Courier March/April 2025 p9). Further detailed studies are planned, with potential implications for improving the understanding of antimatter production.

Hydrodynamic modelling remains one of the most successful tools in heavy-ion physics. Tetsufumi Hirano (Sophia University, Japan), the first recipient of the Zimányi Medal, discussed how the collision system behaves like an expanding relativistic fluid, whose collective motion encodes its initial conditions and transport properties. Hydrodynamic approaches incorporating spin effects – and the resulting polarisation effects in heavy-ion collisions – were discussed by Wojciech Florkowski (Jagiellonian University) and Victor E Ambrus (West University of Timisoara).

The post Budapest brims with heavy ions appeared first on CERN Courier.

Peilian Li (UCAS) described how, thanks to an upgraded trigger that is fully software-based, the dataset gathered in 2025 alone already exceeded the total one from Run 1 and Run 2 combined. The future of LHCb was discussed, with prospects for an upgrade targeting the high-luminosity phase of the LHC, where timing information will be introduced. Theorist Monika Blanke (KIT) concluded the workshop with a keynote on the status of B-decay anomalies, highlighting the importance of LHCb measurements on constraining new physics models.

Much attention went to the long-standing discrepancies between data and theory on lepton–flavour–universality tests – such as the measurement of the R(D) and R(D^*) ratios in semileptonic B-meson decays. Marzia Bordone (UZH) gave a theoretical overview of the determination of the form factors describing B → D^* transitions, highlighting discrepancies in the determination of some form-factor shapes, both among different lattice–QCD determinations and within extractions from different experimental datasets.

A new combination of all LHCb measurements of the CKM angle γ, which quantifies a key CP-violating phase in b-hadron decays, yielded an overall value of (62.8 ± 2.6)°. The collaboration reported flagship electroweak precision measurements of the effective weak mixing angle and the W-boson mass, as well as the first dedicated measurement of the Z-boson mass at the LHC.

An exciting focus for 2026 will be the search for the double open-beauty tetraquark T_bb(bbud)

An exciting focus for 2026 will be the search for the double open-beauty tetraquark T_bb(bbud) – the first accessible exotic hadron expected to be stable against strong decay (CERN Courier November/December 2024 p34). Saša Prelovšek (UL) presented the first lattice-QCD calculation of the state’s electromagnetic form factors, allowing her to rule out an interpretation of the tetraquark as a loosely–bound B–B^* molecule.

The legacy Run 1+2 B → K^*μ⁺μ^– angular analysis, based on a dataset roughly twice as large as that used in previous ones, was presented. Previously seen tensions were confirmed with much increased precision and new observables are reported for the first time. Theorists Arianna Tinari (UZH), Giuseppe Gagliardi (INFN Rome3) and Nazila Mahmoudi (IP2I, CERN) reviewed the status of the non-local hadronic contributions that could affect this channel, discussing how the use of different theoretical approaches can be employed to determine these contributions and how compatible the current results are with the theoretical expectations.

Zhengchen Lian (THU, INFN Firenze) showed the characteristic “bowling–pin” deformation of neon nuclei as it was recently observed using the SMOG2 apparatus, which allows collisions of LHC protons with a variety of fixed-target light nuclei injected into the beampipe (CERN Courier November/December 2025 p8).

The post The many flavours of LHCb appeared first on CERN Courier.

Two cosmological tensions will play a key role in the coming decades. One concerns how strongly matter clumps together to form structures such as galaxy clusters and filaments. The other involves the universe’s expansion rate, H₀. In both cases, measurements based on early-universe data differ from those conducted in the local universe. The discrepancy on H₀ has now reached about 6σ (CERN Courier March/April 2025 p28). Independent methods, such as strong lensing, lensed supernovae and gravitational-wave standard sirens, are essential to confirm or resolve this discrepancy. Several of these techniques are expected to reach 1% precision in the near future. More broadly, upcoming large-scale cosmological missions, including Euclid, DESI, LiteBIRD and the Legacy Survey of Space and Time (LSST) – which released its world-leading camera’s first images in June – are set to deliver important insights into inflation, dark energy and the cosmological effects of neutrino masses.

The dark universe featured prominently. Participants discussed an excess of gamma rays from the galactic centre detected by the Fermi telescope, which is consistent with the self-annihilation of weakly interacting massive particles (WIMPs) and may represent one of the strongest experimental hints for dark matter. Recent analyses on more than 40 million galaxies and quasars in DESI’s Data Release 2 show that fits to baryon acoustic oscillation distances deviate from the standard ΛCDM model at the 2.8 to 4.2σ level, with a dynamical dark energy providing a better match. Euclid, having identified approximately 26 million galaxies out to over 10.5 billion light-years, is poised to constrain the nature of dark matter by combining measurements of large-scale structure, gravitational-lensing statistics, small-scale substructure, dwarf-galaxy populations and stellar streams. Experiments such as XENONnT and PandaX-4T are instead pursuing a mature direct-detection programme.

Future colliders were a central topic at P2I. While new physics has long been expected to emerge near the TeV scale to stabilise the Higgs mass, the Standard Model remains in excellent agreement with current data, and precision flavour measurements constrain many possible new particles to lie at much higher energies. The LHC collaborations presented a flurry of new results and superb prospects for its high–luminosity phase, alongside new results from Belle II and NA64. Looking ahead, a major future collider will be essential for exploring and probing the laws connecting particle physics with the earliest moments of the universe.

The conference hosted the first-ever public presentation of JUNO’s experimental results, only a few hours after their appearance on arXiv. Despite relying on only 59.1 days of data, the experiment has already demonstrated excellent detector performance and produced competitive measurements on solar-neutrino oscillation that are fully consistent with previous results. This level of precision is remarkable, after barely two months of data collection. Three major questions in neutrino physics remain unresolved: the ordering of neutrino masses, the value of the CP-violating parameter and the octant of the mixing angle θ₃₂. The next generation of experiments, including JUNO, DUNE, Hyper-K and upgraded neutrino telescopes, are specifically designed to answer these questions. Meanwhile, DESI has reported a new, stringent upper limit of 0.064 eV on the sum of neutrino masses, within a flat ΛCDM framework. It is the tightest cosmological constraint to date.

The LHC collaborations presented a flurry of new results and superb prospects for its high–luminosity phase

New data from the JWST, Subaru and ALMA telescopes revealed an unexpectedly rich population of galaxies only 200–300 million years after the Big Bang. Many of these early systems appear to grow far more rapidly than predicted by the ΛCDM model, raising questions such as whether star formation efficiency was significantly higher in the early universe or whether we currently underestimate the growth of dark-matter halos (CERN Courier November/December 2025 p11). These data also highlighted a surprisingly abundant population of high-redshift active galactic nuclei, with important implications for black-hole seeding and early supermassive black-hole formation. A comprehensive review of the rapidly evolving field of supernova and transient astronomy was also presented. The mechanisms behind core-collapse supernovae remain only partially understood, and the thermonuclear explosions of white dwarfs continue to pose open questions. At the same time, observations keep identifying new transient classes, whose physical origins are still under investigation. Important insights into protostars, discs and planet formation were also discussed. Observations show that interstellar bubbles and molecular filaments shape the formation of stars and planets across a vast range of physical scales. More than 6000 exoplanets have today been detected, from hot Jupiters to super Earths and ocean planets, many without counterparts in our Solar System.

With more than 150 new gravitational-wave (GW) candidates now identified, including extreme ones with rapid spins and highly asymmetric component masses, GW astronomy offers outstanding opportunities to investigate gravity in the strong-field regime. Notably, the GW250114 event was shown to obey Hawking’s area law, which states that the total horizon area cannot decrease during a black-hole merger, providing strong confirmation of general relativity in the most nonlinear regime. Next-generation observatories such as the Einstein Telescope, Cosmic Explorer and LISA will allow detailed black-hole spectroscopy and impose tighter constraints on alternative theories of gravity.

Even if the transition to multi-messenger astronomy began in the late 20th century, the first binary neutron-star merger, GW170817, remains its landmark event. An extraordinary global effort – more than 70 teams and 100 instruments pointed at the event for years – highlighted several historic firsts: the first gravitational-wave “standard siren” measurement of the Hubble constant, the first association between a neutron-star merger and a short gamma-ray burst, the first observed kilonovae confirming the astrophysical site of heavy-element production, and the first direct test comparing the speed of gravity and light. Very-high-energy gamma-ray astronomy (HESS, MAGIC and VERITAS) also reported impressive results, with more than 300 sources above 100 GeV observed, and bright prospects, as the Cherenkov Telescope Array Observatory (CTAO) is about to start operations.

The post Tokyo targets the two infinities appeared first on CERN Courier.

As the Standard Model (SM) withstands increasingly stringent experimental tests, rare decays remain a prime hunting ground for new physics. In a recent paper, the LHCb collaboration reports its first dedicated searches for the decays B⁰ → K⁺π^–τ⁺τ^– and B_s⁰ → K⁺K^–τ⁺τ^–, pushing hadron–collider flavour physics further into tau-rich territory.

At the quark level, the B⁰ → K⁺π^–τ⁺τ^– and B_s⁰ → K⁺K^–τ⁺τ^– decays happen via the flavour-changing process b → sτ⁺τ^–, which is highly suppressed in the SM. The expected branching fractions of around 10^–7 would place these decays well below the current experimental sensitivity. However, many new-physics scenarios, such as those involving leptoquarks or additional Z′ bosons, predict mediators that couple preferentially to third-generation leptons.

The tensions with the SM observed in the ratios of semileptonic branching fractions R(D^(*)) and in b → sμ⁺μ^– processes could, for example, result in an enhancement of b → sτ⁺τ^– decays. Yet despite its potential to yield signs of new physics, the tau sector remains largely unexplored.

The LHCb analysis only considered tau decays to muons, in order to exploit the detector’s excellent muon identification systems. Reconstructing decays to final states with tau leptons at a hadron collider is notoriously challenging, particularly when relying on leptonic decays such as τ⁺ → μ⁺ν_τν_μ, which result in multiple unreconstructed neutrinos. Using the Run 2 data set of about 5.4 fb^–1 of proton–proton collisions, the collaboration applied machine-learning techniques to extract the topological and isolation features of suppressed tau-pair signals from the background.

Due to the large amount of missing energy in the final state, the B-meson mass cannot be fully reconstructed and the output of the machine-learning algorithm was instead fitted to search for a b → sτ⁺τ^– component. The search was primarily limited by the size of the control samples used to constrain the background shapes – a limitation that will be alleviated by the larger datasets expected in future LHC runs.

No significant signal excess was observed in either the K⁺π^–τ⁺τ^– or the K⁺K^–τ⁺τ^– final states. Upper limits on the branching fractions were then established in bins of the dihadron invariant masses, allowing separate exploration of regions dominated by dihadron resonances and those expected to be primarily non-resonant.

These results represent the world’s most stringent limits on b → sτ⁺τ^– transitions

When interpreted in terms of resonant modes, the limits are B(B⁰ → K^*(892)⁰τ⁺τ^–) < 2.8 × 10^–4 and B(B_s⁰ → φ(1020)τ⁺τ^–) < 4.7 × 10^–4 at the 95% confidence level. The B⁰ → K^*(892)⁰τ⁺τ^– limit improves on previous bounds by approximately an order of magnitude, while the limit on B_s⁰ → φ(1020)τ⁺τ^– is the first ever established.

These results represent the world’s most stringent limits on b → sτ⁺τ^– transitions. The analysis lays essential groundwork for future searches, as the larger LHCb datasets from LHC Run 3 and beyond are expected to open a new frontier in measurements of rare b-hadron transitions involving heavy leptons.

With the upgraded detector and the novel fully software-based trigger, the efficiency in selecting low-p_T muons – and consequently the tau leptons from which they originate – will be much improved. Sensitivity to b → sτ⁺τ^– transitions is therefore expected to increase substantially in the coming years.

The post Tau leptons join the hunt appeared first on CERN Courier.

Strangeness production in high-energy hadron collisions is a powerful tool for exploring quantum chromodynamics (QCD). Unlike up and down, strange quarks are not present as valence quarks in colliding protons and neutrons, and must therefore appear through interactions. They are, however, still light enough to be abundantly produced at the LHC.

Over the past 15 years, the ALICE collaboration has shown that the abundance of strange over non-strange hadrons grows with event multiplicity in all collision systems. In particular, high-multiplicity proton–proton (pp) collisions display a significant strangeness enhancement, reaching saturation levels similar to those in heavy-ion collisions. In one of the most precise studies of strange-to-non-strange hadron production to date, the ALICE collaboration has reported its recent results from pp and lead–lead collisions at the LHC.

Strange hadrons (K_s⁰, Λ, Ξ, Ω) were reconstructed from their weak-decay topologies. Candidates were then selected by applying geometrical and kinematic cuts, estimating and subtracting backgrounds, and correcting the resulting distributions using detector-response simulations. The analyses were carried out at a centre-of-mass energy per nucleon pair of 5.02 TeV and span a wide multiplicity range, from 2 to 2000 charged particles at mid-rapidity.

To better understand how strangeness is produced, the collaboration has taken a significant step by measuring the probability distribution of forming a specific number of strange particles of the same species per event. This study, based on event-by-event strange-particle counting, moves beyond average yields and probes higher orders in the strange-particle production probability distribution. To account for the response of the detector, each candidate is assigned a probability of being genuine rather than background, and a Bayesian unfolding method iteratively corrects for particles that were missed or misidentified to reconstruct the true counts. This provides a novel technique for testing theoretical strangeness-production mechanisms, particularly in events characterised by a significant imbalance between strange and non-strange particles.

Exploiting a large dataset of pp collisions, the probability of producing n particles of a given species S (S = K_s⁰, Λ, Ξ^– or Ω^–) per event, P(n_S), could be determined up to a maximum of n_S = 7 for K_s⁰, n_S = 5 for Λ, n_S = 4 for Ξ and n_S = 2 for Ω (see figure 1). An increase of P(n_S) with charged-particle multiplicity is observed, becoming more pronounced for larger n, as reflected by the growing separation between the curves corresponding to low- and high-multiplicity classes in the high-n tail of the distributions.

The average production yield of n particles per event can be calculated from the P(n_S) distributions, taking into account all possible combinations that result in a given multiplet. This makes it possible to compare events with the same or a different overall strange quark content that hadronise into various combinations of hadrons in the final state. While the ratio between Ω triplets to single K_s⁰ shows an extreme strangeness-enhancement pattern up to two orders of magnitude across multiplicity, comparing hadron combinations that differ in up- and down-quark content but share the same total s-quark content (for instance, Ω singlets compared to Λ triplets) helps isolate the part of the enhancement unrelated to strangeness.

Comparisons with state-of-the-art phenomenological models show that this new approach greatly enhances sensitivity to the underlying physics mechanisms implemented in different event generators. Together with the traditional strange-to-pion observables, the multiplicity-differential probability distributions of strange hadrons provide a more detailed picture of how strange quarks are produced and hadronise in high-energy collisions, offering a stringent benchmark for the phenomenological description of non-perturbative QCD.

The post Strangeness at its extremes appeared first on CERN Courier.

Neutrino physics is a vibrant field of study, with spectacular recent advances. To this day, neutrino oscillations are the only experimental evidence of physics beyond the Standard Model, and, 25 years after this discovery, breathtaking progress has been achieved in both theory and experiment. Giulia Ricciardi’s new textbook provides a timely new resource in a fast developing field.

Entering this exciting field of research can be intimidating, thanks to the breadth of topics that need to be mastered. As well as particle physics, neutrinos touch astroparticle physics, cosmology, astrophysics, nuclear physics and geophysics, and many neutrino textbooks assume advanced knowledge of quantum field theory and particle theory. Ricciardi achieves a brilliant balance by providing a solid foundation in these areas, alongside a comprehensive overview of neutrino theory and experiment. This sets her book apart from most other literature on the subject and makes it a precious resource for newcomers and experts alike. She provides a self-contained introduction to group theory, symmetries, gauge theories and the Standard Model, with an approach that is both accessible and scientifically rigorous, putting the emphasis on understanding key concepts rather than abstract formalisms.

With the theoretical foundations in place, Ricciardi then turns to neutrino masses, neutrino mixing, astrophysical neutrinos and neutrino oscillations. Dirac, Majorana and Dirac-plus-Majorana mass terms are explored, alongside the “see-saw” mechanism and its possible implementations. A full chapter is devoted to neutrino oscillations in the vacuum and in matter, preparing the reader to explore neutrino oscillations in experiments, first from natural sources, such as the Sun, supernovae, the atmosphere and cosmic neutrinos; a subsequent chapter then covers reactor and accelerator neutrinos, giving a detailed overview of the key theoretical and experimental issues. Ricciardi avoids a common omission in neutrino textbooks by addressing neutrin–nucleus interactions – a fast developing topic in theory and a crucial aspect of interpreting current and future experiments. The book concludes with a look at the current research and future prospects, including a discussion of neutrino-mass measurements and neutrinoless double-beta decay.

The clarity with which Ricciardi links theoretical concepts to experimental observations is remarkable. Her book is engaging and eminently enjoyable. I highly recommend it.

The post Introduction to neutrino and particle physics appeared first on CERN Courier.

The core part of Aspect’s book covers his own contributions to the experimental test of Bell’s inequality spanning 1975 to 1985. He gives a very personal account of his involvement as an experimental physicist in this matter, starting soon after he visited Bell at CERN in spring 1975 for advice concerning his French Thèse d’État. With anecdotes that give the reader the impression of sitting next to the author and listening to his stories, Aspect recounts how, in 1975, captivated by Bell’s work, he set up experiments in underground rooms at the Institut d’Optique in Orsay to test hidden-variable theories. He explains his experiments in detail with diagrams and figures from his original publications as well as images of the apparatus used. By 1981 and for several years to come, it was Aspect’s experiments that came closest to Bell’s idea on how to test the inequality formulated in 1964. Aspect defended his thesis in 1983 in a packed auditorium with illustrious examiners such as J S Bell, C Cohen-Tannoudji and B d’Espagnat. Not long afterwards, Cohen-Tannoudji invited him to the Collège de France and the Paris ENS to work on the laser cooling and manipulation of atoms – a quite different subject. At that time, Aspect didn’t see any point in closing some of the remaining loopholes in his experiments.

To prepare the terrain for his story, Aspect first tells the history of quantum mechanics from 1900 to 1935. He begins with a discussion of Planck’s blackbody radiation (1900), Einstein’s description of the photoelectric effect (1905) and the heat capacity of solids (1907), the wave–particle duality of light, first Solvay Congress (1911), Bohr’s atomic model (1913) and matter–radiation interaction according to Einstein (1916). He then covers the Einstein–Bohr debates at the Solvay congresses of 1927 and 1930 on the interpretation of the probability aspects of quantum mechanics.

Aspect then turns to the Einstein, Pod­olsky, Rosen (EPR) paper of 1935, which discusses a gedankenexperiment involving two entangled quantum mechanical particles. Whereas the previous Einstein–Bohr debates ended with convincing arguments by Bohr refuting Einstein’s point of view, Bohr didn’t come up with a clear answer to Einstein’s objection of 1935, namely that he considered quantum mechanics to be incomplete. In 1935 and the following years, for most physicists the Einstein–Bohr debate had been considered uninteresting and purely philosophical. It had practically no influence on the success of the application of quantum mechanics. Between 1935 and 1964, the EPR subject was nearly dormant, apart from David Bohm’s interventions during the 1950s. In 1964 Bell took up the EPR paradox, which had been advanced as an argument that quantum mechanics should be supplemented by additional variables (CERN Courier July/August 2025 p21).

Aspect describes clearly and convincingly how Bell entered the scene and how the inequality with his name triggered experimentalists to get involved: experiments with polarisation-entangled photons and their correlations could decide whether Einstein or Bohr’s view of quantum mechanics was correct. Bell’s discovery transferred the Einstein–Bohr debate from epistemology to the realm of experimental physics. At the end of the 1960s the first experiments based on Bell’s inequality started to take form. Aspect describes how these analysed the polarisation correlation of the entangled photons at a separation of a few metres. He discusses their difficulties and limitations, starting with the experiments launched by Clauser et al.

In the final chapter, covering 1985 to the present, Aspect explains why he decided not to continue his research with entangled photons and to switch subject. His opinion was that the technology at the time wasn’t ripe enough to close some of the remaining loopholes in his experiments – loopholes of a type that Bell considered less important. Aspect was convinced that if quantum mechanics was faulty, one would have seen indications of that in his experiments. It took until 2015 for two of the loopholes left open by Aspect’s experiments (the locality and detection loophole) to be simultaneously closed. Yet no experiment, as ideal as it is, can be said to be totally loophole-free, as Aspect says. The final chapter also covers more philosophical aspects of quantum non-locality and speculations about how Einstein would have reacted to the violation of Bell’s inequalities. In complementary sections, Aspect speaks about the no-cloning theorem, technological applications of quantum optics like quantum cryptography according to Ekert, quantum teleportation and quantum random number generators.

Who will profit from reading this book? First one should say that it is not a quantum-mechanics or quantum-optics textbook. Most of the material is written in such a way that it will be accessible and enjoyable to the educated layperson. For the more curious reader, supplementary sections cover physical aspects in deeper detail, and the book cites more than 80 original references. Aspect’s long experience and honed pedagogical skills are evident throughout. It is an engaging and authoritative introduction to one of the most profound debates in modern physics.

The post If Einstein had known appeared first on CERN Courier.

Hendrik Verweij, who was for many years a driving force in the development of electronics for high-energy physics, passed away on 11 August 2025 in Meyrin, Switzerland, at the age of 93.

Born in Linschoten near Gouda in the Netherlands, Henk earned a degree in electrical engineering at the Technical High School in Hilversum and started his career as an instrumentation specialist at Philips, working on oscilloscopes. He joined CERN in July 1956, bringing his expertise in electronics to the newly founded laboratory. With Ian Pizer, group leader of the electronics group of the nuclear-physics-division, he published CERN Yellow Report 61-15 on a nanosecond-sampling oscilloscope, followed by a paper on a fast amplifier one year later.

During the next four decades, developments in electronics profoundly transformed the world. Henk played a crucial role in bringing this transformation to CERN’s electronics instrumentation, and he eventually succeeded Pizer as group leader. Over the years he worked with numerous colleagues on fast signal-processing circuits. The creation of a collection of standardised modules facilitated the setup of a variety of CERN experiments. With Bjorn Hallgren and others, he realised the simultaneous, fast time and amplitude digitisation of the inner drift detector of the innovative UA1 experiment at CERN’s Super Proton Synchrotron, which discovered the W and Z bosons together with the UA2 experiment.

In the 1960s, recognising the importance of standardisation for engaging industry, Henk built close ties with colleagues in the US, including at Lawrence Berkeley Laboratory, SLAC and the National Bureau of Standards (NBS). He took part in the discussions that led to the Nuclear Instrumentation Module (NIM) standard, defined in 1964 by the US Atomic Energy Commission, and served on the NIM committee chaired by Lou Costrell of the NBS.

Henk was also a member of the ESONE committee for the CAMAC and later FASTBUS standards, working alongside colleagues such as Bob Dobinson, Fred Iselin, Phil Ponting, Peggie Rimmer, Tim Berners-Lee and many others from across Europe and the US in this international effort. He contributed hardware for standard modules both before and after the publication of the FASTBUS specification in 1984, and reported regularly at conferences on the status of European developments. A strong advocate of collaboration with industry, he also helped persuade LeCroy to establish a facility near CERN.

A driving force in the development of electronics for high-energy physics

Towards the end of his career, Henk became group leader of the microelectronics group at CERN, closing the loop in this transformational electronics evolution with integrated circuit developments for silicon microstrip, hybrid pixel and other detectors. When he retired in the 1990s, the group had built up the necessary expertise to design optimised application-specific integrated circuits (ASICs) for the LHC detectors. Ultimately, these allow the recording of millions of frames per second and event selection from the on-chip stored data.

Retirement did not diminish Henk’s interest in CERN and its electronics activities. He often passed by in the microelectronics group at CERN, regularly participating in Medipix meetings on the development of hybrid pixel-detector read-out chips for medical imaging and other applications.

Henk played an important role in making advances in microelectronics available to the high-energy physics community. His friends and colleagues will miss his experience, vision and irrepressible enthusiasm.

The post Hendrik Verweij 1931–2025 appeared first on CERN Courier.

The briefing book compiled by the Physics Preparatory Group is an impressive distillation of our current knowledge of particle physics, and a preview of the exciting prospects offered by future programmes. It provides the scientific basis for defining Europe’s long-term particle-physics priorities and determining the flagship collider that will best advance the field. To this end, it presents comparisons of the physics reach of the different candidate machines, which often have different strengths in probing new physics beyond the Standard Model (SM).

Condensing all this in a few sentences is difficult, though two messages are clear: if the next collider at CERN is an electron–positron collider, the exploration of new physics will proceed mainly through high-precision measurements; and the highest physics reach into the structure of physics beyond the SM via indirect searches will be provided by the combined exploration of the Higgs, electroweak and flavour domains.

Following a visionary outlook for the field from theory, the briefing book divides its exploration of the future of particle physics into seven sectors of fundamental physics and three technology pillars that underpin them.

1. Higgs and electroweak physics

In the new era that has dawned with the discovery of the Higgs boson, numerous fundamental questions remain, including whether the Higgs boson is an elementary scalar, part of an extended scalar sector, or even a portal to entirely new phenomena. The briefing book highlights how precision studies of the Higgs boson, the W and Z bosons, and the top quark will probe the SM to unprecedented accuracy, looking for indirect signs of new physics.

Addressing these requires highly precise measurements of its couplings, self-interaction and quantum corrections. While the High-Luminosity LHC (HL-LHC) will continue to improve several Higgs and electroweak measurements, the next qualitative leap in precision will be provided by future electron–positron colliders, such as the FCC-ee, the Linear Collider Facility (LCF), CLIC or LEP3. And while these would provide very important information, it would fall upon the shoulders of an energy-frontier machine like the FCC-hh or a muon collider to access potential heavy states. Using the absolute HZZ coupling from the FCC-ee, such machines would measure the single-Higgs-boson couplings with a precision better than 1%, and the Higgs self-coupling at the level of a few per cent (see “Higgs self-coupling” figure).

This anticipated leap in experimental precision will necessitate major advances in theory, simulation and detector technology. In the coming decades, electroweak physics and the Higgs boson in particular will remain a cornerstone of particle physics, linking the precision and energy frontiers in the search for deeper laws of nature.

2. Strong interaction physics

Precise knowledge of the strong interaction will be essential for understanding visible matter, exploring the SM with precision, and interpreting future discoveries at the energy frontier. Building upon advanced studies of QCD at the HL-LHC, future high-luminosity electron–positron colliders such as FCC-ee and LEP3 would, like LHeC, enable per-mille precision on the strong coupling constant, and a greatly improved understanding of the transition between the perturbative and non-perturbative regimes of QCD. The LHeC would bring increased precision on parton-distribution functions that would be very useful for many physics measurements at the FCC-hh. FCC-hh would itself open up a major new frontier for strong-interaction studies.

A deep understanding of the strong interaction also necessitates the study of strongly interacting matter under extreme conditions with heavy-ion collisions. ALICE and the other experiments at the LHC will continue to illuminate this physics, revealing insights into the early universe and the interiors of neutron stars.

3. Flavour physics

With high-precision measurements of quark and lepton processes, flavour studies test the SM at energy scales far above those directly accessible to colliders, thanks to their sensitivity to the effects of virtual particles in quantum loops. Small deviations from theoretical predictions could signal new interactions or particles influencing rare processes or CP-violating effects, making flavour physics one of the most sensitive paths toward discovering physics beyond the SM.

The book highlights how precision studies of the Higgs boson, the W and Z bosons, and the top quark will probe the SM to unprecedented accuracy

Global efforts are today led by the LHCb, ATLAS and CMS experiments at the LHC and by the Belle II experiment at SuperKEKB. These experiments have complementary strengths: huge data samples from proton–proton collisions at CERN and a clean environment in electron–positron collisions at KEK. Combining the two will provide powerful tests of lepton-flavour universality, searches for exotic decays and refinements in the understanding of hadronic effects.

The next major step in precision flavour physics would require “tera-Z” samples of a trillion Z bosons from a high-luminosity electron–positron collider such as the FCC-ee, alongside a spectrum of focused experimental initiatives at a more modest scale.

4. Neutrino physics

Neutrino physics addresses open fundamental questions related to neutrino masses and their deep connections to the matter–antimatter asymmetry in the universe and its cosmic evolution. Upcoming experiments including long-baseline accelerator-neutrino experiments (DUNE and Hyper-Kamiokande), reactor experiments such as JUNO (see “JUNO takes aim at neutrino-mass hierarchy” and astroparticle observatories (KM3NeT and IceCube, see also CERN Courier May/June 2025 p23) will likely unravel the neutrino mass hierarchy and discover leptonic CP violation.

In parallel, the hunt for neutrinoless-double-beta decay continues. A signal would indicate that neutrinos are Majorana fermions, which would be indisputable evidence for new physics! Such efforts extend the reach of particle physics beyond accelerators and deepen connections between disciplines. Efforts to determine the absolute mass of neutrinos are also very important.

The chapter highlights the growing synergy between neutrino experiments and collider, astrophysical and cosmological studies, as well as the pivotal role of theory developments. Precision measurements of neutrino interactions provide crucial support for oscillation measurements, and for nuclear and astroparticle physics. New facilities at accelerators explore neutrino scattering at higher energies, while advances in detector technologies have enabled the measurement of coherent neutrino scattering, opening new opportunities for new physics searches. Neutrino physics is a truly global enterprise, with strong European partici­pation and a pivotal role for the CERN neutrino platform.

5. Cosmic messengers

Astroparticle physics and cosmology increasingly provide new and complementary information to laboratory particle-physics experiments in addressing fundamental questions about the universe. A rich set of recent achievements in these fields includes high-precision measurements of cosmological perturbations in the cosmic microwave background (CMB) and in galaxy surveys, a first measurement of an extragalactic neutrino flux, accurate antimatter fluxes and the discovery of gravitational waves (GWs).

Leveraging information from these experiments has given rise to the field of multi-messenger astronomy. The next generation of instruments, from neutrino telescopes to ground- and space-based CMB and GW observatories, promises exciting results with important clues for
particle physics.

6. Beyond the Standard Model

The landscape for physics beyond the SM is vast, calling for an extended exploration effort with exciting prospects for discovery. It encompasses new scalar or gauge sectors, supersymmetry, compositeness, extra dimensions and dark-sector extensions that connect visible and invisible matter.

Many of these models predict new particles or deviations from SM couplings that would be accessible to next-generation accelerators. The briefing book shows that future electron–positron colliders such as FCC-ee, CLIC, LCF and LEP3 have sensitivity to the indirect effects of new physics through precision Higgs, electroweak and flavour measurements. With their per-mille precision measurements, electron–positron colliders will be essential tools for revealing the virtual effects of heavy new physics beyond the direct reach of colliders. In direct searches, CLIC would extend the energy frontier to 1.5 TeV, whereas FCC-hh would extend it to tens of TeV, potentially enabling the direct observation of new physics such as new gauge bosons, supersymmetric particles and heavy scalar partners. A muon collider would combine precision and energy reach, offering a compact high-energy platform for direct and indirect discovery.

This chapter of the briefing book underscores the complementarity between collider and non-collider experiments. Low-energy precision experiments, searches for electric dipole moments, rare decays and axion or dark-photon experiments probe new interactions at extremely small couplings, while astrophysical and cosmological observations constrain new physics over sprawling mass scales.

7. Dark matter and the dark sector

The nature of dark matter, and the dark sector more generally, remains one of the deepest mysteries in modern physics. A broad range of masses and interaction strengths must be explored, encompassing numerous potential dark-matter phenomenologies, from ultralight axions and hidden photons to weakly interacting massive particles, sterile neutrinos and heavy composite states. The theory space of the dark sector is just as crowded, with models involving new forces or “portals” that link visible and invisible matter.

As no single experimental technique can cover all possibilities, progress will rely on exploiting the complementarity between collider experiments, direct and indirect searches for dark matter, and cosmological observations. Diversity is the key aspect of this developing experimental programme!

8. Accelerator science and technology

The briefing book considers the potential paths to higher energies and luminosities offered by each proposal for CERN’s next flagship project: the two circular colliders FCC-ee and FCC-hh, the two linear colliders LCF and CLIC, and a muon collider; LEP3 and LHeC are also considered as colliders that could potentially offer a physics programme to bridge the time between the HL-LHC and the next high-energy flagship collider. The technical readiness, cost and timeline of each collider are summarised, alongside their environmental impact and energy efficiency (see “Energy efficiency” figure).

The two main development fronts in this technology pillar are high-field magnets and efficient radio-frequency (RF) cavities. High-field superconducting magnets are essential for the FCC-hh, while high-temperature superconducting magnet technology, which presents unique opportunities and challenges, might be relevant to the FCC-hh as a second-stage machine after the FCC-ee. Efficient RF systems are required by all accelerators (CERN Courier May/June 2025 p30). Research and development (R&D) on advanced acceleration concepts, such as plasma-wakefield acceleration and muon colliders, also present much promise but necessitate significant work before they can present a viable solution for a future collider.

Preserving Europe’s leadership in accelerator science and technology requires a broad and extensive programme of work with continuous support for accelerator laboratories and test facilities. Such investments will continue to be very important for applications in medicine, materials science and industry.

9. Detector instrumentation

A wealth of lessons learned from the LHC and HL-LHC experiments are guiding the development of the next generation of detectors, which must have higher granularity, and – for a hadron collider – a higher radiation tolerance, alongside improved timing resolution and data throughput.

As the eyes through which we observe collisions at accelerators, detectors require a coherent and long-term R&D programme. Central to these developments will be the detector R&D collaborations, which have provided a structured framework for organising and steering the work since the previous update to the European Strategy for Particle Physics. These span the full spectrum of detector systems, with high-rate gaseous detectors, liquid detectors and high-performance silicon sensors for precision timing, precision particle identification, low-mass tracking and advanced calorimetry.

If detectors are the eyes that explore nature, computing is the brain that deciphers the signals they receive

All these detectors will also require advances in readout electronics, trigger systems and real-time data processing. A major new element is the growing role of AI and quantum sensing, both of which already offer innovative methods for analysis, optimisation and detector design (CERN Courier July/August 2025 p31). As in computing, there are high hopes and well-founded expectations that these technologies will transform detector design and operation.

To maintain Europe’s leadership in instrumentation, it is important to maintain sustained investments in test-beam infrastructures and engineering. This supports a mutually beneficial symbiosis with industry. Detector R&D is a portal to sectors as diverse as medical diagnostics and space exploration, providing essential tools such as imaging technologies, fast electronics and radiation-hard sensors for a wide range of applications.

10. Computing

If detectors are the eyes that explore nature, computing is the brain that deciphers the signals they receive. The briefing book pays much attention to the major leaps in computation and storage that are required by future experiments, with simulation, data management and processing at the top of the list (see “Data challenge” figure). Less demanding in resources, but equally demanding of further development, is data analysis. Planning for these new systems is guided by sustainable computing practices, including energy-efficient software and data centres. The next frontier is the HL-LHC, which will be the testing ground and the basis for future development, and serves as an example for the preservation of the current wealth of experimental data and software (CERN Courier September/October 2025 p41).

Several paradigm shifts hold great promise for the future of computing in high-energy physics. Heterogeneous computing integrates CPUs, GPUs and accelerators, providing hugely increased capabilities and better scaling than traditional CPU usage. Machine learning is already being deployed in event simulation, reconstruction and even triggering, and the first signs from quantum computing are very positive. The combination of AI with quantum technology promises a revolution in all aspects of software and of the development, deployment and usage of computing systems.

Some closing remarks

Beyond detailed physics summaries, two overarching issues appear throughout the briefing book.

First, progress will depend on a sustained interplay between experiment, theory and advances in accelerators, instrumentation and computing. The need for continued theoretical development is as pertinent as ever, as improved calculations will be critical for extracting the full physics potential of future experiments.

Second, all this work relies on people – the true driving force behind scientific programmes. There is an urgent need for academia and research institutions to attract and support experts in accelerator technologies, instrumentation and computing by offering long-term career paths. A lasting commitment to training the new generation of physicists who will carry out these exciting research programmes is equally important.

Revisiting the briefing book to craft the current summary brought home very clearly just how far the field of particle physics has come – and, more importantly, how much more there is to explore in nature. The best is yet to come!

The post Ten windows on the future of particle physics appeared first on CERN Courier.

Charged hadrons offer distinct advantages. Though they are more challenging to manipulate in a clinical environment, protons and heavy ions deposit most of their energy just before they stop, at the so-called Bragg peak, allowing medical physicists to spare healthy tissue and target cancer cells precisely. Particle therapy has been an effective component of the most advanced cancer therapies for nearly 80 years, since it was proposed by Robert R Wilson in 1946.

With the incidence of cancer rising across the world, research into particle therapy is more valuable than ever to human wellbeing – and the science isn’t slowing down. Today, progress requires adapting accelerator physics to the demands of the burgeoning field of radiobiology. This is the scientific basis for developing and validating a whole new generation of treatment modalities, from FLASH therapy to combining particle therapy with immunotherapy.

Here are the top five facts accelerator physicists need to know about biology at the Bragg peak.

1. 100 keV/μm optimises damage to DNA

Almost every cell’s control centre is contained within its nucleus, which houses DNA – your body’s genetic instruction manual. If the cell’s DNA becomes compromised, it can mutate and lose control of its basic functions, leading the cell to die or multiply uncontrollably. The latter results in cancer.

For more than a century, radiation doses have been effective in halting the uncontrollable growth of cancerous cells. Today, the key insight from radiobiology is that for the same radiation dose, biological effects such as cell death, genetic instability and tissue toxicity differ significantly based on both beam parameters and the tissue being targeted.

Biologists have discovered that a “linear energy transfer” of roughly 100 keV/μm produces the most significant biological effect. At this density of ionisation, the distance between energy deposition events is roughly equal to the diameter of the DNA double helix, creating complex, repair-resistant DNA lesions that strongly reduce cell survival. Beyond 100 keV/μm, energy is wasted.

DNA is the main target of radiotherapy because it holds the genetic information essential for the cell’s survival and proliferation. Made up of a double helix that looks like a twisted ladder, DNA consists of two strands of nucleotides held together by hydrogen bonds. The sequence of these nucleotides forms the cell’s unique genetic code. A poorly repaired lesion on this ladder leaves a permanent mark on the genome.

When radiation induces a double-strand break, repair is primarily attempted through two pathways: either by rejoining the broken ends of the DNA, or by replacing the break with an identical copy of healthy DNA (see “Repair shop” image). The efficiency of these repairs decreases dramatically when the breaks occur in close spatial proximity or if they are chemically complex. Such scenarios frequently result in lethal mis-repair events or severe alterations in the genetic code, ultimately compromising cell survival.

This fundamental aspect of radiobiology strongly motivates the use of particle therapy over conventional radiotherapy. Whereas X-rays deliver less than 10 keV/μm, creating sparse ionisation events, protons deposit tens of keV/μm near the Bragg peak, and heavy ions 100 keV/μm or more.

2. Mitochondria and membranes matter too

For decades, radiobiology revolved around studying damage to DNA in cell nuclei. However, mounting evidence reveals that an important aspect of cellular dysfunction can be inflicted by damage to other components of cells, such as the cell membrane and the collection of “organelles” inside it. And the nucleus is not the only organelle containing DNA.

Mitochondria generate energy and serve as the body’s cellular executioners. If a mitochondrion recognises that its cell’s DNA has been damaged, it may order the cell membrane to become permeable. Without the structure of the cell membrane, the cell breaks apart, its fragments carried away by immune cells. This is one mechanism behind “programmed cell death” – a controlled form of death, where the cell essentially presses its own self-destruct button (see “Self-destruct” image).

Irradiated mitochondrial DNA can suffer from strand breaks, base–pair mismatches and deletions in the code. In space-radiation studies, damage to mitochondrial DNA is a serious health concern as it can lead to mutations, premature ageing and even the creation of tumours. But programmed cell death can prevent a cancer cell from multiplying into a tumour. By disrupting the mitochondria of tumour cells, particle irradiation can compromise their energy metabolism and amplify cell death, increasing the permeability of the cell membrane and encouraging the tumour cell to self-destruct. Though a less common occurrence, membrane damage by irradiation can also directly lead to cell death.

3. Bystander cells exhibit their own radiation response

For many years, radiobiology was driven by a simple assumption: only cells directly hit by radiation would be damaged. This view started to change in the 1990s, when researchers noticed something unexpected: even cells that had not been irradiated showed signs of stress or injury when they were near the irradiated cells. This phenomenon, known as the bystander effect, revealed that irradiated cells can send bio-­chemical signals to their neighbours, which may in turn respond as if they themselves had been exposed, potentially triggering an immune response (see “Communication” image).

“Non-targeted” effects propagate not only in space, but also in time, through the phenomenon of radiation-induced genomic instability. This temporal dimension is characterised by the delayed appearance of genomic alterations across multiple cell generations. Radiation damage propagates across cells and tissues, and over time, adding complexity beyond the simple dose–response paradigm.

Although the underlying mechanisms remain unclear, the clustered ionisation events produced by carbon ions generate complex DNA damage and cell death, while largely preserving nearby, unirradiated cells.

4. Radiation damage activates the immune system

Cancer cells multiply because the immune system fails to recognise them as a threat (see “Immune response” image). The modern pharmaceutical-based technique of immunotherapy seeks to alert the immune system to the threat posed by cancer cells it has ignored by chemically tagging them. Radiotherapy seeks to activate the immune system by inflicting recognisable cellular damage, but long courses of photon radiation can also weaken overall immunity.

This negative effect is often caused by the exposure of circulating blood and active blood-producing organs to radiation doses. Fortunately, particle therapy’s ability to tightly conform the dose to the target and subject surrounding tissues to a minimal dose can significantly mitigate the reduction of immune blood cells, better preserving systemic immunity. By inflicting complex, clustered DNA lesions, heavy ions have the strongest potential to directly trigger programmed cell death, even in the most difficult-to-treat cancer cells, bypassing some of the molecular tricks that tumours use to survive, and amplifying the immune response beyond conventional radiotherapy with X-rays. This is linked to the complex, clustered DNA lesions induced by high-energy-transfer radiation, which triggers the DNA damage–repair signals strongly associated with immune activation.

These biological differences provide a strong rationale for the rapidly emerging research frontier of combining particle therapy with immunotherapy. Particle therapy’s key advantage is its ability to amplify immunogenic cell death, where the cell’s surface changes, creating “danger tags” to recruit immune cells to come and kill it, recognise others like it, and kill those too. This ability for particle therapy to mitigate systemic immuno­suppression makes it a theoretically superior partner for immunotherapy compared to conventional X-rays.

5. Ultra-high dose rates protect healthy tissues

In recent years, the attention of clinicians and researchers has focused on the “FLASH” effect– a groundbreaking concept in cancer treatment where radiation is delivered at an ultra-high dose rate in excess of 40 J/kg/s. FLASH radiotherapy appears to minimise damage to healthy tissues while maintaining at least the same level of tumour control as conventional methods. Inflammation in healthy tissues is reduced, and the number of immune cells entering the tumour increased, helping the body fight cancer more effectively. This can significantly widen the therapeutic window – the optimal range of radiation doses that can successfully treat a tumour while minimising toxicity to healthy tissues.

Though the radiobiological mechanisms behind this protective effect remain unclear, several hypotheses have been proposed. A leading theory focuses on oxygen depletion or “hypoxia”.

As tumours grow, they outpace the surrounding blood vessels’ ability to provide oxygen (see “Oxygen depletion” image). By condensing the dose in a very short time, it is thought that FLASH therapy may induce transient hypoxia within normal tissues too, reducing oxygen-dependent DNA damage there, while killing tumour cells at the same rate. Using a similar mechanism, FLASH therapy may also preserve mitochondrial integrity and energy production in normal tissues.

It is still under investigation whether a FLASH effect occurs with carbon ions, but combining the biological benefits of high-energy-transfer radiation with those of FLASH could be very promising.

The post Biology at the Bragg peak appeared first on CERN Courier.

2025 has been a very exciting year. We just published a paper in Nature Physics about radioactive ion beams.

I also received an ERC Advanced Grant to study the FLASH effect with neon ions. We plan to go back to the 1970s, when Cornelius Tobias in Berkeley thought of using very heavy ions against radio-resistant tumours, but now using FLASH’s ultrahigh dose rates to reduce its toxicity to healthy tissues. Our group is also working on the simultaneous acceleration of different ions: carbon ions will stop in the tumour, but helium ions will cross the patient, providing an online monitor of the beam’s position during irradiation. The other big news in radiotherapy is vertical irradiation, where we don’t rotate the beam around the patient, but rotate the patient around the beam. This is particularly interesting for heavy-ion therapy, where building a rotating gantry that can irradiate the patient from multiple angles is almost as expensive as the whole accelerator. We are leading the Marie Curie UPLIFT training network on this topic.

Why are heavy ions so compelling?

Close to the Bragg peak, where very heavy ions are very densely ionising, the damage they cause is difficult to repair. You can kill the tumours much better than with protons. But carbon, oxygen and neon run the risk of inducing toxicity in healthy tissues. In Berkeley, more than 400 patients were treated with heavy ions. The results were not very good, and it was realised that these ions can be very toxic for normal tissue. The programme was stopped in 1992, and since then there has been no more heavy-ion therapy in the US, though carbon-ion therapy was established in Japan not long after. Today, most of the 130 particle-therapy centres worldwide use protons, but 17 centres across Asia and Europe offer carbon-ion therapy, with one now under construction at the Mayo Clinic in the US. Carbon is very convenient, because the plateau of the Bragg curve is similar to X-rays, while the peak is much more effective than protons. But still, there is evidence that it’s not heavy enough, that the charge is not high enough to get rid of very radio-resistant hypoxic tumours – tumours where you don’t have enough oxygenation. So that’s why we want to go heavier: neon. If we show that you can manage the toxicity using FLASH, then this is something that can be translated into the clinics.

There seems to be a lot of research into condensing the dose either in space, in microbeams or, in time, in the FLASH effect…

Absolutely.

Why does that spare healthy tissue at the expense of cancer cells?

That is a question I cannot answer. To be honest, nobody knows. We know that it works, but I want to make it very clear that we need more research to translate it completely to the clinic. It is true that if you either fractionate in space or compress in time, normal tissue is much more resistant, while the effect on the tumour is approximately the same, allowing you to increase the dose without harming the patient. The problem is that the data are still controversial.

So you would say that it is not yet scientifically established that the FLASH effect is real?

There is an overwhelming amount of evidence for the strong sparing of normal tissue at specific sites, especially for the skin and for the brain. But, for example, for gastrointestinal tumours the data is very controversial. Some data show no effect, some data show a protective effect, and some data show an increased effectiveness of FLASH. We cannot generalise.

Is it surprising that the effect depends on the tissue?

In medicine this is not so strange. The brain and the gut are completely different. In the gut, you have a lot of cells that are quickly duplicating, while in the brain, you almost have the same number of neurons that you had when you were a teenager – unfortunately, there is not much exchange in the brain.

So, your frontier at GSI is FLASH with neon ions. Would you argue that microbeams are equally promising?

Absolutely, yes, though millibeams more so than microbeams, because microbeams are extremely difficult to go into clinical translation. In the micron region, any kind of movement will jeopardise your spatial fractionation. But if you have millimetre spacing, then this becomes credible and feasible. You can create millibeams using a grid. Instead of having one solid beam, you have several stripes. If you use heavier ions, they don’t scatter very much and remain spatially fractionated. There is mounting evidence that fractionated irradiation of the tumour can elicit an immune response and that these immune cells eventually destroy the tumour. Research is still ongoing to understand whether it’s better to irradiate with a spatial fractionation of 1 millimetre or to only radiate the centre of the tumour, allowing the immune cells to migrate and destroy the tumour.

What’s the biology of the body’s immune response to a tumour?

To become a tumour, a cell has to fool the immune system, otherwise our immune system will destroy it. So, we are desperately trying to find a way to teach the immune system to say: “look, this is not a friend – you have to kill it, you have to destroy it.” This is immunotherapy, the subject of the Nobel Prize in medicine in 2018 and also related to the 2025 Nobel Prize in medicine on regulation of the immune system. But these drugs don’t work for every tumour. Radiotherapy is very useful in this sense, because you kill a lot of cells, and when the immune system sees a lot of dead cells, it activates. A combination of immunotherapy and radiotherapy is now being used more and more in clinical trials.

You also mentioned radioactive ion beams and the simultaneous acceleration of carbon and helium ions. Why are these approaches advantageous?

The two big problems with particle therapy are cost and range uncertainty. Having energy deposition concentrated at the Bragg peak is very nice, but if it’s not in the right position, it can do a lot of damage. Precision is therefore much more important in particle therapy than in conventional radiotherapy, as X-rays don’t have a Bragg peak – even if the patient moves a little bit, or if there is an anatomical change, it doesn’t matter. That’s why many centres prefer X-rays. To change that, we are trying to create ways to see the beam while we irradiate. Radioactive ions decay while they deposit energy in the tumour, allowing you to see the beam using PET. With carbon and helium, you don’t see the carbon beam, but you see the helium beam. These are both ways to visualise the beam during irradiation.

How significantly does radiation therapy improve human well-being in the world today?

When I started to work in radiation therapy at Berkeley, many people were telling me: “Why do you waste your time in radiation therapy? In 10 years everything will be solved.” At that time, the trend was gene therapy. Other trends have come and gone, and after 35 years in this field, radiation therapy is still a very important tool in a multidisciplinary strategy for killing tumours. More than 50% of cancer patients need radiotherapy, but, even in Europe, it is not available to all patients who need it.

Accelerator and detector physicists have to learn to speak the language of the non-specialist

What are the most promising initiatives to increase access to radiotherapy in low- and middle-income countries?

Simply making the accelerators cheaper. The GDP of most countries in Africa, South America and Asia is also steadily increasing, so you can expect that – let’s say – in 20 or 30 years from now, there will be a big demand for advanced medical technologies in these countries, because they will have the money to afford it.

Is there a global shortage of radiation physicists?

Yes, absolutely. This is true not only for particle therapy, which requires a high number of specialists to maintain the machine, but also for conventional X-ray radiotherapy with electron linacs. It’s also true for diagnostics because you need a lot of medical physicists for CT, PET and MRI.

What is your advice to high-energy physicists who have just completed a PhD or a postdoc, and want to enter medical physics?

The next step is a specialisation course. In about four years, you will become a specialised medical physicist and can start to work in the clinics. Many who take that path continue to do research alongside their clinical work, so you don’t have to give up your research career, just reorient it toward medical applications.

How does PTCOG exert leadership over global research and development?

The Particle Therapy Co-Operative Group (PTCOG) is a very interesting association. Every particle-therapy centre is represented in its steering committee. We have two big roles. One is research, so we really promote international research in particle therapy, even with grants. The second is education. For example, Spain currently has 11 proton therapy centres under construction. Each will need maybe 10 physicists. PTCOG is promoting education in particle therapy to train the next generation of radiation-therapy technicians and medical oncologists. It’s a global organisation, representing science worldwide, across national and continental branches.

Do you have a message for our community of accelerator physicists and detector physicists? How can they make their research more interdisciplinary and improve the applications?

Accelerator physicists especially, but also detector physicists, have to learn to speak the language of the non-specialist. Sometimes they are lost in translation. Also, they have to be careful not to oversell what they are doing, because you can create expectations that are not matched by reality. Tabletop laser-driven accelerators are a very interesting research topic, but don’t oversell them as something that can go into the clinics tomorrow, because then you create frustration and disappointment. There is a similar situation with linear accelerators for particle therapy. Since I started to work in this field, people have been saying “Why do we use circular accelerators? We should use linear accelerators.” After 35 years, not a single linear accelerator has been used in the clinics. There must also be a good connection with industry, because eventually clinics buy from industry, not academia.

Are there missed opportunities in the way that fundamental physicists attempt to apply their research and make it practically useful with industry and medicine?

In my opinion, it should work the other way around. Don’t say “this is what I am good at”; ask the clinical environment, “what do you need?” In particle therapy, we want accelerators that are cheaper and with a smaller footprint. So in whatever research you do, you have to prove to me that the footprint is smaller, and the cost lower.

Do forums exist where medical doctors can tell researchers what they need?

PTCOG is definitely the right place for that. We keep medicine, physics and biology together, and it’s one of the meetings with the highest industry participation. All the industries in particle therapy come to PTCOG. So that’s exactly the right forum where people should talk. We expect 1500 people at the next meeting, which will take place in Deauville, France, from 8 to 13 June 2026, shortly after IPAC.

Are accelerator physicists welcome to engage in PTCOG even if they’ve not previously worked on medical applications?

Absolutely. This is something that we are missing. Accelerator physicists mostly go to IPAC but not to PTCOG. They should also come to PTCOG to speak more with medical physicists. I would say that PTCOG is 50% medical physics, 30% medicine and 20% biology. So, there are a lot of medical physicists, but we don’t have enough accelerator physicists and detector physicists. We need more particle and nuclear physicists to come to PTCOG to see what the clinical and biology community want, and whether they can provide something.

Do you have a message for policymakers and funding agencies about how they can help push forward research in radiotherapy?

Unfortunately, radiation therapy and even surgery are wrongly perceived as old technologies. There is not much investment in them, and that is a big problem for us. What we miss is good investment at the level of cooperative programmes that develop particle therapy in a collaborative fashion. At the moment, it’s becoming increasingly difficult. All the money goes into prevention and pharmaceuticals for immunotherapy and targeted therapy, and this is something that we are trying to revert.

Are large accelerator laboratories well placed to host cooperative research projects?

Both GSI and CERN face the same challenge: their primary mission is nuclear and particle physics. Technological transfer is fine, but they may jeopardise their funding if they stray too far from their primary goal. I believe they should invest more in technological transfer, lobbying their funding agencies to demonstrate that there is a translation of their basic science into something that is useful for public health.

How does your research in particle therapy transfer to astronaut safety?

Particle therapy and space-radiation research have a lot in common. They use the same tools and there are also a lot of overlapping topics, for example radiosensitivity. One patient is more sensitive, one patient is more resistant, and we want to understand what the difference is. The same is true of astronauts – and radiation is probably the main health risk for long-term missions. Space is also a hostile environment in terms of microgravity and isolation, but here we understand the risks, and we have countermeasures. For space radiation, the problem is that we don’t understand the risk very well, because the type of radiation is so exotic. We don’t have that type of radiation on Earth, so we don’t know exactly how big the risk is. Plus, we don’t have effective countermeasures, because the radiation is so energetic that shielding will not be enough to protect the crews effectively. We need more research to reduce the uncertainty on the risk, and most of this research is done in ground-based accelerators, not in space.

Radiation therapy is probably the best interdisciplinary field that you can work in

I understand that you’re even looking into cryogenics…

Hibernation is considered science fiction, but it’s not science fiction at all – it’s something we can recreate in the lab. We call it synthetic torpor. This can be induced in animals that are non-hibernating. Bears and squirrels hibernate; humans and rats don’t, but we can induce it. And when you go into hibernation, you become more radioresistant, providing a possible countermeasure to radiation exposure, especially for long missions. You don’t need much food, you don’t age very much, metabolic processes are slowed down, and you are protected from radiation. That’s for space. This could also be applied to therapy. Imagine you have a patient with multiple metastasis and no hope for treatment. If you can induce synthetic torpor, all the tumours will stop, because when you go into a low temperature and hibernation, the tumours don’t grow. This is not the solution, because when you wake the patient up, the tumours will grow again, but what you can do is treat the tumours while you are in hibernation, while healthy tissue is more radiation resistant. The number of research groups working on this is low, so we’re quite far from considering synthetic torpor for spaceflight or clinical trials for cancer treatment. First of all, we have to see how long we can keep an animal in synthetic torpor. Second, we should translate into bigger animals like pigs or even non-human primates.

In the best-case scenario, what can particle therapy look like in 10 years’ time?

Ideally, we should probably at least double the amount of particle-therapy centres that are now available, and expand into new regions. We finally have a particle-therapy centre in Argentina, which is the first one in South America. I would like to see many more in South America and in Africa. I would also like to see more centres that try to tackle tumours where there is no treatment option, like glioblastoma or pancreatic cancer, where the mortality is the same as the incidence. If we can find ways to treat such cancers with heavy ions and give hope to these patients, this would be really useful.

Is there a final thought that you’d like to leave with readers?

Radiation therapy is probably the best interdisciplinary field that you can work in. It’s useful for society and it’s intellectually stimulating. I really hope that big centres like CERN and GSI commit more and more to the societal benefits of basic research. We need it now more than ever. We are living in a difficult global situation, and we have to prove that when we invest money in basic research, this is very well invested money. I’m very happy to be a scientist, because in science, there are no barriers, there is no border. Science is really, truly international. I’m an advocate of saying scientific collaboration should never stop. It didn’t even stop during the Cold War. At that time, the cooperation between East and West at the scientist level helped to reduce the risk of nuclear weapons. We should continue this. We don’t have to think that what is happening in the world should stop international cooperation in science: it eventually brings peace.

The post The future of particle therapy appeared first on CERN Courier.

Herwig Schopper was born on 28 February 1924 in the German-speaking town of Landskron (today, Lanškroun) in the then young country of Czechoslovakia. He enjoyed an idyllic childhood, holidaying at his grandparents’ hotel in Abbazia (today, Opatija) on what is now the Croatian Adriatic coast. It was there that his interest in science was awakened through listening in on conversations between physicists from Budapest and Belgrade. In Landskron, he developed an interest in music and sport, learning to play both piano and double bass, and skiing in the nearby mountains. He also learned to speak English, not merely to read Shakespeare as was the norm at the time, but to be able to converse, thanks to a Jewish teacher who had previously spent time in England. This skill was to prove transformational later in life.

The idyll began to crack in 1938 when the Sudetenland was annexed by Germany. War broke out the following year, but the immediate impact on Herwig was limited. He remained in Landskron until the end of his high-school educ ation, graduating as a German citizen – and with no choice but to enlist. Joining the Luftwaffe signals corps, because he thought that would help him develop his knowledge of physics, he served for most of the war on the Eastern Front ensuring that communication lines remained open between military headquarters and the troops on the front lines. As the war drew to a close in March 1945, he was transferred west, just in time to see the Western Allies cross the Rhine at Remagen. Recalled to Berlin and given orders to head further west, Herwig instructed his driver to first make a short detour via Potsdam. This was a sign of the kind of person Herwig was that, amidst the chaos of the fall of Berlin, he wanted to see Schloss Sanssouci, Frederick the Great’s temple to the enlightenment, while he had the chance.

By the time Herwig arrived in Schleswig–Holstein, the war was over, and he found himself a prisoner of the British. He later recalled, with palpable relief, that he had managed to negotiate the war without having to shoot at anyone. On discovering that Herwig spoke English, the British military administration engaged him as a translator. This came as a great consolation to Herwig since many of his compatriots were dispatched to the mines to extract the coal that would be used to reconstruct a shattered Germany. Herwig rapidly struck up a friendship with the English captain he was assigned to. This in turn eased his passage to the University of Hamburg, where he began his research career studying optics, and later enabled him to take the first of his scientific sabbaticals when travel restrictions on German academics were still in place (see “Academic overture” image).

In 1951, Herwig left for a year in Stockholm, where he worked with Lise Meitner on beta decay. He described this time as his first step up in energy from the eV-energies of visible light to the keV-energies of beta-decay electrons. A later sabbatical, starting in 1956, would see him in Cambridge, where he worked under Meitner’s nephew, Otto Frisch, in the Cavendish laboratory. As Austrian Jews, both Meitner and Frisch had sought exile before the war. By this time, Frisch had become director of the Cavendish’s nuclear physics department and a fellow of the Royal Society.

Initial interactions

While at Cambridge, Herwig took his first steps in the emerging field of particle physics, and became one of the first to publish an experimental verification of Lee and Yang’s proposal that parity would be violated in weak interactions. His single-author paper was published soon after that by Chien-Shiung Wu and her team, leading to a lifelong friendship between the two (see “Virtuosi” image).

Following Wu’s experimental verification of parity violation, cited by Herwig in his paper, Lee and Yang received the Nobel Prize. Wu was denied the honour, ostensibly on the basis that she was one of a team and the prize can only be shared three ways. It remains in the realm of speculation whether Herwig would have shared the prize had his paper been the first to appear.

A third sabbatical, arranged by Willibald Jentschke, who wanted Herwig to develop a user group for the newly established DESY laboratory, saw the Schopper family move to Ithaca, New York in 1960. At Cornell, Herwig learned the ropes of electron synchrotrons from Bob Wilson. He also learned a valuable lesson in the hands-on approach to leadership. Arriving in Ithaca on a Saturday, Herwig decided to look around the deserted lab. He found one person there, tidying up. It turned out not to be the janitor, but the lab’s founder and director, Wilson himself. For Herwig, Cornell represented another big jump in energy, cementing Schopper as an experimental particle physicist.

Cornell represented another big jump in energy, cementing Schopper as an experimental particle physicist

Herwig’s three sabbaticals gave him the skills he would later rely on in hardware development and physics analysis, but it was back in Germany that he honed his management skills and established himself a skilled science administrator.

At the beginning of his career in Hamburg, Herwig worked under Rudolf Fleischmann, and when Fleischmann was offered a chair at Erlangen, Herwig followed. Among the research he carried out at Erlangen was an experiment to measure the helicity of gamma rays, a technique that he’d later deploy in Cambridge to measure parity violation.

It was not long before Herwig was offered a chair himself, and in 1958, at the tender age of 34, he parted from his mentor to move to Mainz. In his brief tenure there, he set wheels in motion that would lead to the later establishment of the Mainz Microtron laboratory, today known as MAMI. By this time, however, Herwig was much in demand, and he soon moved to Karlsruhe, taking up a joint position between the university and the Kernforschungszentrum, KfK. His plan was to merge the two under a single management structure as the Karlsruhe Institute for Experimental Nuclear Physics. In doing so, he laid the seeds for today’s Karlsruhe Institute of Technology, KIT.

Pioneering research

At Karlsruhe, Herwig established a user group for DESY, as Jentschke had hoped, and another at CERN. He also initiated a pioneering research programme into superconducting RF and had his first personal contacts with CERN, spending a year there in 1964. In typical Herwig fashion, he pursued his own agenda, developing a device he called a sampling total absorption counter, STAC, to measure neutron energies. At the time, few saw the need for such a device, but this form of calorimetry is now an indispensable part of any experimental particle physicists’ armoury.

In 1970, Herwig again took leave of absence from Karlsruhe to go to CERN. He’d been offered the position of head of the laboratory’s Nuclear Physics Division, but his stay was to be short lived (see “Prélude” image). The following year, Jentschke took up the position of Director-General of CERN alongside John Adams. Jentschke was to run the original CERN laboratory, Lab I, while Adams ran the new CERN Lab II, tasked with building the SPS. This left a vacancy at Germany’s national laboratory, and the job was offered to Herwig. It was too good an offer to refuse.

As chair of the DESY directorate, Herwig witnessed from afar the discovery of both charm and bottom quarks in the US. Although missing out on the discoveries, DESY’s machines were perfect laboratories to study the spectroscopy of these new quark families, and DESY went on to provide definitive measurements. Herwig also oversaw DESY’s development in synchrotron light science, repurposing the DORIS accelerator as a light source when its physics career was complete and it was succeeded by PETRA.

The ambition of the PETRA project put DESY firmly on course to becoming an international laboratory, setting the scene for the later HERA model. PETRA experiments went on to discover the gluon in 1979.

The following year, Herwig was named as CERN’s next Director-General, taking up office on 1 January 1981. By this time, the CERN Council had decided to call time on its experiment with two parallel laboratories, leaving Herwig with the task of uniting Lab I and Lab II. The Council was also considering plans to build the world’s most powerful accelerator, the Large Electron–Positron collider, LEP.

It fell to Herwig both to implement a new management structure for CERN and to see the LEP proposal through to approval (see “Architects of LEP” image). Unpopular decisions were inevitable, making the early years of Herwig’s mandate somewhat difficult. In order to get LEP approved, he had to make sacrifices. As a result, the Intersecting Storage Rings (ISR), the world’s only hadron collider, collided its final beams in 1984 and cuts had to be made across the research programme. Herwig was also confronted with a period of austerity in science funding, and found himself obliged to commit CERN to constant funding in real terms throughout the construction of LEP, and as it turns out, in perpetuity.

It fell to Herwig both to implement a new management structure for CERN and to see the LEP proposal through to approval

Herwig’s battles were not only with the lab’s governing body; he also went against the opinions of some of his scientific colleagues concerning the size of the new accelerator. True to form, Herwig stuck with his instinct, insisting that the LEP tunnel should be 27 km around, rather than the more modest 22 km that would have satisfied the immediate research goals while avoiding the difficult geology beneath the Jura mountains. Herwig, however, was looking further ahead – to the hadron collider that would follow LEP. His obstinacy was fully vindicated with the discovery of the Higgs boson in 2012, confirming the Brout–Englert–Higgs mechanism, which had been proposed almost 50 years earlier. This discovery earned the Nobel Prize for Peter Higgs and François Englert in 2013 (see “Towards LEP and the LHC” image).

The CERN blueprint

Difficult though some of his decisions may have been, there is no doubt that Herwig’s 1981 to 1988 mandate established the blueprint for CERN to this day. The end of operations of the ISR may have been unpopular, and we’ll never know what it may have gone on to achieve, but the world’s second hadron collider at the SPS delivered CERN’s first Nobel prize during Herwig’s mandate, awarded to Carlo Rubbia and Simon van der Meer in 1984 for the discovery of W and Z bosons.

Herwig turned 65 two months after stepping down as CERN Director-General, but retirement was never on his mind. In the years that followed, he carried out numerous roles for UNESCO, applying his diplomacy and foresight to new areas of science. UNESCO was in many ways a natural step for Herwig, whose diplomatic skills had been honed by the steady stream of high-profile visitors to CERN during his mandate as Director-General. At one point, he engineered a meeting at UNESCO between Jim Cronin, who was lobbying for the establishment of a cosmic-ray observatory in Argentina, and the country’s president, Carlos Menem. The following day, Menem announced the start of construction of the Pierre Auger Observatory. On another occasion, Herwig was tasked with developing the Soviet gift to Cuba of a small particle accelerator into a working laboratory. That initiative would ultimately come to nothing, but it helped Herwig prepare the groundwork for perhaps his greatest post-retirement achievement: SESAME, a light-source laboratory in Jordan that operates as an intergovernmental organisation following the CERN model (see “Science diplomacy” image). Mastering the political challenge of establishing an organisation that brings together countries from across the Middle East – including long-standing rivals – required a skill set that few possess.

Although the roots of SESAME can be traced to a much earlier date, by the end of the 20th century, when the idea was sufficiently mature for an interim organisation to be established, Herwig was the natural candidate to lead the new organisation through its formative years. His experience of running international science coupled with his post-retirement roles at UNESCO made him the obvious choice to steer SESAME from idea to reality. It was Herwig who modelled SESAME’s governing document on the CERN convention, and it was Herwig who secured the site in Jordan for the laboratory. Today, SESAME is producing world-class research – a shining example of what can be achieved when people set aside their differences and focus on what they have in common.

Establishing an organisation that brings together countries from across the Middle East required a skill set few possess

Herwig never stopped working for what he believed in. When CERN’s current Director-General convened a meeting with past Directors-General in 2024, along with the president of the CERN Council, Herwig was present. When initiatives were launched to establish an international research centre in the Balkans, Herwig stepped up to the task. He never lost his sense of what is right, and he never lost his mischievous sense of humour. Following an interview at his house in 2024 for the film The Peace Particle, the interviewer asked whether he still played the piano. Herwig stood up, walked to the piano and started to play a very simple arrangement of Christian Sinding’s “Rustle of Spring”. Just as curious glances started to be exchanged, he transitioned, with a twinkle in his eye, to a beautifully nuanced rendition of Liszt’s “Liebestraum No. 3”.

Herwig Schopper was a rare combination of genius, polymath, humanitarian and gentleman. Always humble, he could make decisions with nerves of steel when required. His legacy spans decades and disciplines, and has shaped the field of particle physics in many ways. With his passing, the world has lost a truly remarkable individual. He will be sorely missed.

The post Polymath, humanitarian, gentleman appeared first on CERN Courier.

Still, the discovery of the fundamental particles of the Standard Model has been speedy in comparison to another longstanding quest in natural philosophy: chrysopoeia, the medieval alchemists’ dream of transforming the “base metal” lead into the precious metal gold. This may have been motivated by the observation that the dull grey, relatively abundant metal lead is of similar density to gold, which has been coveted for its beautiful colour and rarity for millennia.

The quest goes back at least to the mythical, or mystical, notion of the philosopher’s stone and Zosimos of Panopolis around 300 CE. Its evolution, in various cultures, through medieval times and up to the 19th century, is a fascinating thread in the emergence of modern empirical science from earlier ways of thinking. Some of the leaders of this transition, such as Isaac Newton, also practised alchemy. While the alchemists pioneered many of the techniques of modern chemistry, it was only much later that it became clear that lead and gold are distinct chemical elements and that chemical methods are powerless to transmute one into the other.

With the dawn of nuclear physics in the 20th century, it was discovered that elements could transform into others through nuclear reactions, either naturally by radioactive decay or in the laboratory. In 1940, gold was produced at the Harvard Cyclotron by bombarding a mercury target with fast neutrons. Some 40 years ago, tiny amounts of gold were produced in nuclear reactions between beams of carbon and neon, and a bismuth target at the Bevalac in Berkeley. Very recently, gold isotopes were produced at the ISOLDE facility at CERN by bombarding a uranium target with proton beams (see “Historic gold” images).

Now, tucked away discreetly in the conclusions of a paper recently published by the ALICE collaboration, one can find the observation, originating from Igor Pshenichnov, Uliana Dmitrieva and Chiara Oppedisano, that “the transmutation of lead into gold is the dream of medieval alchemists which comes true at the LHC.”

ALICE has finally measured the transmutation of lead into gold, not via the crucibles and alembics of the alchemists, nor even by the established techniques of nuclear bombardment used in the experiments mentioned above, but in a novel and interesting way that has become possible in “near-miss” interactions of lead nuclei at the LHC.

At the LHC, lead has been transformed into gold by light.   

Since the first announcement, this story has attracted considerable attention in the media. Here I would like to put this assertion in scientific context and indicate its relevance in testing our understanding of processes that can limit the performance of the LHC and future colliders such as the FCC.

Electromagnetic pancakes

Any charged particle at rest is surrounded by lines of electric fields radiating outwards in all directions. These fields are particularly strong close to a lead nucleus because it contains 82 protons, each with one elementary charge. In the LHC, the lead nuclei travel at 99.999994% of the speed of light, squeezing the field lines into a thin pancake transverse to the direction of motion in the laboratory frame of reference. This compression is so strong that, in the vicinity of the nucleus, we find the strongest magnetic and electric fields known in the universe, trillions of times stronger than even the prodigiously powerful superconducting magnets of the LHC, and orders of magnitude greater than the Schwinger limit where the vacuum polarises or the magnetic fields found in rare, rapidly spinning neutron stars called magnetars. Of course, these fields extend only over a very short time as one nucleus passes by the other. Quantum mechanics, via a famous insight of Fermi, Weizsäcker and Williams, tells us that this electromagnetic flash is equivalent to a pulse of quasi-real photons whose intensity and energy are greatly boosted by the large charge and the relativistic compression.

When two beams of nuclei are brought into collision in the LHC, some hadronic interactions occur. In the unimaginable temperatures and densities of this ultimate crucible we create droplets of the quark–gluon plasma, the main subject of study of the heavy-ion programme. However, when nuclei “just miss” each other, the interactions of these electromagnetic fields amount to photon–photon and photon–nucleus collisions. Some of the processes occurring in these so-called ultra-peripheral collisions (UPCs) are so strong that they would limit the performance of the collider, were it not for special measures implemented in the last 10 years.

The ALICE paper is one among many exploring the rich field of fundamental physics studies opened up by UPCs at the LHC (CERN Courier January/February 2025 p31). Among them are electromagnetic dissociation processes where a photon interacting with a nucleus can excite oscillations of its internal structure and result in the ejection of small numbers of neutrons and protons that are detected by ALICE’s zero degree calorimeters (ZDCs). The ALICE experiment is unique in having calorimeters to detect spectator protons as well as neutrons (see “Spotting spectators” figure). The residual nuclei are not detected although they contribute to the signals measured by the beam-loss monitor system of the LHC.

Each ²⁰⁸Pb nucleus in the LHC beams contains 82 protons and 208–82 = 126 neutrons. To create gold, a nucleus with a charge of 79, three protons must be removed, together with a variable number of neutrons.    

Alchemy in ALICE

While less frequent than the creation of the elements thallium (single-proton emission) or mercury (two-proton emission), the results of the ALICE paper show that each of the two colliding lead-ion beams contribute a cross section of 6.8 ± 2.2 barns to gold production, implying that the LHC now produces gold at a maximum rate of about 89 kHz from lead–lead collisions at the ALICE collision point, or 280 kHz from all the LHC experiments combined. During Run 2 of the LHC (2015–2018), about 86 billion gold nuclei were created at all four LHC experiments, but in terms of mass this was only a tiny 2.9 × 10^–11 g of gold. Almost twice as much has already been produced in Run 3 (since 2023).

The transmutation of lead into gold is the dream of medieval alchemists which comes true at the LHC

Strikingly, this gold production is somewhat larger than the rate of hadronic nuclear collisions, which occur at about 50 kHz for a total cross section of 7.67 ± 0.25 barns.

Different isotopes of gold are created according to the number of neutrons that are emitted at the same time as the three protons. To create ¹⁹⁷Au, the only stable isotope and the main component of natural gold, a further eight neutrons must be removed – a very unlikely process. Most of the gold produced is in the form of unstable isotopes with lifetimes of the order of a minute.

Although the ZDC signals confirm the proton and neutron emission, the transformed nuclei are not themselves detected by ALICE and their fate is not discussed in the paper. These interaction products nevertheless propagate hundreds of metres through the beampipe in several secondary beams whose trajectories can be calculated, as seen in the “Ultraperipheral products” figure.

The ordinate shows horizontal displacement from the central path of the outgoing beam. This coordinate system is commonly used in accelerator physics as it suppresses the bending of the central trajectory – downwards in the figure – and its separation into the beam pipes of the LHC arcs.   

The “5σ” envelope of the intense main beam of ²⁰⁸Pb nuclei that did not collide is shown in blue. Neutrons from electromagnetic dissociation and other processes are plotted in magenta. They begin with a certain divergence and then travel down the LHC beam pipe in straight lines, forming a cone, until they are detected by the ALICE ZDC, some 114 m away from the collision, after the place where the beam pipe splits in two. Because of the coordinate system, the neutron cone appears to bend sharply at the first separation dipole magnet.

Protons are shown in green. As they only have 40% of the magnetic rigidity of the main beam, they bend quickly away from the central trajectory in the first separation magnet, before being detected by a different part of the ZDC on the other side of the beam pipe.

Photon–photon interactions in UPCs copiously produce electron–positron pairs. In a small fraction of them, corresponding nevertheless to a large cross-section of about 280 barns, the electron is created in a bound state of one of the ²⁰⁸Pb nuclei, generating a secondary beam of ²⁰⁸Pb⁸¹⁺ single-electron ions. The beam from this so-called bound-free pair production (BFPP), shown in red, carries a power of about 150 W – enough to quench the superconducting coils of the LHC magnets, causing them to transition from the superconducting to the normal resistive state. Such quenches can seriously disrupt accelerator operation, as the stored magnetic energy is rapidly released as heat within the affected magnet.

To prevent this, new “TCLD” collimators were installed on either side of ALICE during the second long shutdown of the LHC. Together with a variable-amplitude bump in the beam orbit, which pulls the BFPP beam away from the first impact point so that it can be safely absorbed on the TCLD, this allowed the luminosity to be increased to more than six times the original LHC design, just in time to exploit the full capacity of the upgraded ALICE detector in Run 3.

Light-ion collider

Besides lead, the LHC has recently collided beams of ¹⁶O and ²⁰Ne (see “First oxygen and neon collisions at the LHC”), and nuclear transmutation has manifested itself in another way. In hadronic or electromagnetic events where equal numbers of protons and neutrons are emitted, the outgoing nucleus has almost the same charge-to-mass ratio, since nuclear binding energies are very small at the top of the periodic table. It may then continue to circulate with the original beam, resulting in a small contamination that increases during the several hours of an LHC fill. Hybrid collisions can then occur, for example including a ¹⁴N nucleus formed by the ejection of a proton and a neutron from ¹⁶O. Fortunately, the momentum spread introduced by the interactions puts many of these nuclei outside the acceptance of the radio-frequency cavities that keep the beams bunched as they circulate around the ring, so the effect is smaller than had first been expected.

The most powerful beam from an electromagnetic-dissociation process is ²⁰⁷Pb from single neutron emission, plotted in green. It has comparable intensity to ²⁰⁸Pb⁸¹⁺ but propagates through the LHC arc to the collimation system at Point 3.

Similar electromagnetic-dissociation processes occur elsewhere, notably in beam interactions with the LHC collimation system. The recent ALICE paper, together with earlier ones on neutron emissions in UPCs, helps to test our understanding of the nuclear interactions that are an essential ingredient of complex beam-physics simulations. These are used to understand and control beam losses that might otherwise provoke frequent magnet quenches or beam dumps. At the LHC, a deep symbiosis has emerged between the fundamental nuclear physics studied by the experiments and the accelerator physics limiting its performance as a heavy-ion collider – or even as a light-ion collider (see “Light-ion collider” panel).

The figure also shows beams of the three heaviest gold isotopes in gold. ²⁰⁴Au has an impact point in a dipole magnet but is far too weak to quench it. ²⁰³Au follows almost the same trajectory as the BFPP beam. ²⁰²Au propagates through the arc to Point 3. The extremely weak flux of ¹⁹⁷Au, the only stable isotope of gold, is also shown.

Worth its weight in gold

Prospecting for gold at the LHC looks even more futile when we consider that the gold nuclei emerge from the collision point with very high energies. They hit the LHC beam pipe or collimators at various points downstream where they immediately fragment in hadronic showers of single protons, neutrons and other particles. The gold exists for tens of milliseconds at most.

And finally, the isotopically pure lead used in CERN’s ion source costs more by weight than gold, so realising the alchemists’ dream at the LHC was a poor business plan from the outset.

The moral of this story, perhaps, is that among modern-day natural philosophers, LHC physicists take issue with the designation of lead as a “base” metal. We find, on the contrary, that ²⁰⁸Pb, the heaviest stable isotope among all the elements, is worth far more than its weight in gold for the riches of the physics discoveries that it has led us to.

The post Alchemy by pure light appeared first on CERN Courier.

Joseph Rotblat’s childhood was blighted by the destruction visited on Warsaw, first by the Tsarist Army, followed by the Central Powers and completed by the Red Army from 1918 to 1920. His father’s successful paper-importing business went bankrupt in 1914, and the family became destitute. After a short course in electrical engineering, Joseph and a teenaged friend became jobbing electricians. A committed autodidact, Rotblat found his way into the Free University, where he studied physics under Ludwik Wertenstein. Wertenstein had worked with Marie Skłodowska-Curie in Paris and was the chief of the Radiological Institute in Warsaw as well as teaching at the Free University. He was the first to recognise Rotblat’s brilliance and retained him as a researcher at the Institute. Rotblat’s main research was neutron-induced artificial radioactivity: he was among the first to induce cobalt-60, which became a standard source in radiotherapy machines before reliable linear accelerators were available.

Chadwick described Rotblat as “very intelligent and very quick”

By the late 1930s, Rotblat had published more than a dozen papers, some in English journals after translation by Wertenstein; the name Rotblat was becoming known in neutron physics. The professor regarded him as the likely next head of the Radiological Institute and thought he should prepare by working outside Poland. Rotblat wanted to gain experience of the cyclotron and, although he could have joined the Joliot–Curie group in Paris, elected to go to Liverpool where James Chadwick was overseeing a machine expected to produce a proton beam within months. He arrived in Liverpool in April 1939 and was shocked by the city’s filth. He also found the scouse dialect of its citizens incomprehensible. Despite the trying circumstances, Rotblat soon impressed Chadwick with his experimental skill and was rewarded with a prestigious fellowship. Chadwick wrote to Wertenstein in June describing Rotblat as “very intelligent and very quick”.

Brimming with enthusiasm

Chadwick had formed a long-distance friendship with Ernest Lawrence, the cyclotron’s inventor, who kept him apprised of developments in Berkeley. At the time of Rotblat’s arrival, Lawrence was brimming with enthusiasm about the potential of neutrons and radioactive isotopes from cyclotrons for medical research, especially in cancer treatment. Chadwick hired Bernard Kinsey, a Cambridge graduate who spent three years with Lawrence, to take charge of the Liverpool cyclotron, and he befriended Rotblat. Liverpool had limited funding: Chadwick complained to Lawrence that the money “this laboratory has been running on in the past few years – is less than some men spend on tobacco.” Chadwick served on a Cancer Commission in Liverpool under the leadership of Lord Derby, which planned to bring cancer research to the Liverpool Radium Institute using products from the cyclotron.

The small stipend from the Oliver Lodge fellowship encouraged Rotblat to return to Warsaw in August 1939 to collect his wife, Tola, and bring her to England. She was recovering from acute appendicitis; her doctors persuaded Joseph that she was not fit to travel. So he returned alone on the last train allowed to pass through Berlin before the Germans attacked Poland once more. Tola wrote her last letter to Joseph in December 1939. While he was in Warsaw, Rotblat confided in Wertenstein about his belief that a uranium fission bomb was feasible using fast neutrons, and he repeated this argument to Chadwick when he returned to Liverpool. Chadwick eventually became the leader of the British contingent on the Manhattan Project and arranged for Rotblat to come to Los Alamos in 1944 while remaining a Polish citizen. Rotblat worked in Robert Wilson’s cyclotron group and survived a significant radiation accident, receiving an estimated dose of 1.5 J/kg to his upper torso and head. The circumstances of his leaving the project in December 1944 were far more complicated than the moralistic account he wrote in The Bulletin of the Atomic Scientists 40 years later, but no less noble.

Tragedy and triumph

As Chadwick wrote to Rotblat in London, he saw “very obvious advantages” for the future of nuclear physics in Britain from Rotblat’s return to Liverpool. For one thing, “Rotblat has a wider experience on the cyclotron than anyone now in England,” and he also possessed “a mass of information on the equipment used in Project Y [Los Alamos] and Chicago.” Chadwick had two major roles in mind for Rotblat. One was to revitalise the depleted Liverpool department and to stimulate cyclotron research in England; and the second to collate the detailed data on nuclear physics brought by British scientists returning from the Manhattan Project. In 1945, Rotblat discovered that six members of his family had miraculously survived the war in Poland, but tragically not Tola. His state of despair deepened after the news of the atomic bombs being used against Japan: he knew about the possibility of a hydrogen bomb, and remembered conversations with Niels Bohr in Los Alamos about the risks of a nuclear arms race. He made two resolutions: to campaign against nuclear weapons and to leave academic nuclear physics and become a medical physicist to use his scientific knowledge for the direct benefit of people.

When Chadwick returned to Liverpool from the US, he found the department in a much better state than he expected. The credit for this belonged largely to Rotblat’s leadership; Chadwick wrote to Lawrence praising his outstanding ability, combined with a truly remarkable concern for the staff and students. Chadwick and Rotblat then agreed to build a synchrocyclotron in Liverpool. Rotblat selected the abandoned crypt of an unbuilt Catholic cathedral as the best site, since the local topography would provide some radiation protection. The post-war shortages, especially of steel, made this an extremely ambitious project. Rotblat presented a successful application for the largest university grant to the Department of Science and Industrial Research, and despite design and construction problems resulting in spiralling costs, the machine was in active research use from 1954 to 1968.

With the encouragement of physicians at Liverpool Royal Infirmary, Rotblat started to dabble in nuclear medicine to image thyroid glands and treat haematological disorders. In 1949 he saw an advert for the chair in physics at the Medical College of St. Bartholomew’s Hospital (Bart’s) in London and applied. While Rotblat was easily the most accomplished candidate, there was a long delay in his appointment on spurious grounds, such as being over-qualified to teach physics to medical students, likely to be a heavy consumer of research funds and xenophobia. Bart’s was a closed, reactionary institution. There was a clear division between the Medical College, with its links to London University, and the hospital, where the post-war teaching was suboptimal as it struggled to recover from the war and adjusted reluctantly to the new National Health Service (NHS). The Medical College, in Charterhouse Square, was severely bombed in the Blitz and the physics department completely destroyed. Rotblat attempted to thwart his main opponent, the dean (described as “secretive and manipulative” in one history), by visiting the hospital and meeting senior clinicians and governors. There was also a determined effort, orchestrated by Chadwick, to retain him in the ranks of nuclear physicists.

When I interviewed Rotblat in 1994, he told me that Chadwick’s final tactic was to tell him that he was close to being elected as a fellow of the Royal Society, but if he took the position at Bart’s, it would never happen. Rotblat poignantly observed: “He was right.” I mentioned this to Lorna Arnold, the nuclear historian, who thought it was a shame. She said she would take it up with her friend Rudolf Peierls. Despite being in poor health, Peierls vowed to correct this omission, and the next year the Royal Society elected Rotblat a fellow at the age of 86.

Full-time medical physicist

Rotblat’s first task at Bart’s, when he finally arrived in 1950, was to prepare a five-year departmental plan: a task he was well-qualified for after his experience with the synchrocyclotron in Liverpool. With wealthy, centuries-old hospitals such as Bart’s allowed to keep their endowments after the advent of the NHS, he also became an active committee member for the new Research Endowment Fund that provided internal grants and hired research assistants. The physics department soon collaborated with the biochemistry, pharmacology and physiology departments that required radioisotopes for research. He persuaded the Medical College to buy a 15 MV linear accelerator from Mullard, an English electronics company, which never worked for long without problems.

Rotblat resolved to campaign against nuclear weapons and use his scientific knowledge for the direct benefit of people

During his first two years, in addition to the radioisotope work, he studied the passage of electrons through biological tissue and the energy dissipation of neutrons in tissue – the 1950s were a golden age for radiobiology in England, and Rotblat forged close relationships with Hal Gray and his group at the Hammersmith Hospital. In the mid-1950s, he was approached by Patricia Lindop, a newly qualified Bart’s physician who had also obtained a first-class degree in physiology. Lindop had a five-year grant from the Nuffield Foundation to study ageing and, after discussions with Rotblat, it was soon arranged that she would study the acute and long-term effects of radiation in mice at different ages. This was a massive, prospective study that would eventually involve six research assistants and a colony of 30,000 mice. Rotblat acted as the supervisor for her PhD, and they published multiple papers together. In terms of acute death (within 30 days of a high, whole-body dose), she found that mice that were one-day old at exposure could tolerate the highest doses, whereas four-week-old mice were the most vulnerable. The interpretation of long-term effects was much less clearcut and provoked major disagreements within the radiobiology community. In a 1994 letter, Rotblat mused on the number of Manhattan Project scientists still alive: “According to my own studies on the effects of radiation on lifespan, I should have been dead a long time, having received a sub-lethal dose in Los Alamos. But here I am, advocating the closure of Los Alamos, Livermore and Sandia, instead of promoting them as health resorts!”

In 1954, the US Bravo test obliterated the Bikini atoll and layered a Japanese fishing boat (Lucky Dragon No. 5) that was outside the exclusion zone in the South Pacific with radioactive dust. American scientists realised that the weapon massively exceeded its designed yield, and there was an unconvincing attempt to allay public fear. Rotblat was invited onto BBC’s flagship current-affairs programme, Panorama, to explain to the public the difference between the original fission bombs and the H-bomb. His lucid delivery impressed Bertrand Russell, a mathematical philosopher and a leading pacifist in World War I, who also spoke on Panorama. The two became close friends. When Rotblat went to a radiobiology conference a few months later, he met a Japanese scientist who had analysed the dust recovered from Lucky Dragon No. 5. The dust was comprised of about 60% rare-earth isotopes, leading Rotblat to believe that most of the explosive energy was due to fission not fusion. He wrote his own report, not based on any inside knowledge and despite official opposition, concluding this was a fission–fusion–fission bomb and that his TV presentation had underestimated its power by orders of magnitude. Rotblat’s report became public just as the British Cabinet decided in secret to develop thermonuclear weapons. The government was concerned that the Americans would view this as another breach of security by an ex-Manhattan Project physicist. Rotblat’s reputation as a man of the political left grew within the conservative institution of Bart’s.

Russell made a radio address at the end of 1954 to address the global existential threat posed by thermonuclear weapons and urged the public to “remember your humanity and forget the rest”. Six months later, Russell announced the Russell–Einstein Manifesto with Rotblat as one of the signatories, and relied upon by Russell to answer questions from the press. The first Pugwash conference followed in 1957 with Rotblat as a prominent contributor. His active involvement, closely supported by Lindop, would last for the rest of his life, as he encouraged communication across the East–West divide and pushed for international arms control agreements. Much of this work took place in his office at Bart’s. Rotblat and the Pugwash conference then shared the 1995 Nobel Peace Prize.

The post The physicist who fought war and cancer appeared first on CERN Courier.

“Building JUNO has been a journey of extraordinary challenges,” says JUNO chief engineer Ma Xiaoyan. “It demanded not only new ideas and technologies, but also years of careful planning, testing and perseverance. Meeting the stringent requirements of purity, stability and safety called for the dedication of hundreds of engineers and technicians. Their teamwork and integrity turned a bold design into a functioning detector, ready now to open a new window on the world of neutrinos.”

Main goals

Neutrinos interact only via the parity-violating weak interaction, providing direct evidence only for left-handed neutrinos. As a result, right-handed neutrinos are not part of the Standard Model (SM) of particle physics. As the SM explains fermion masses by a coupling of the Higgs field to a left-handed fermion and its right-handed counterpart of the same flavour, neutrinos are predicted to be massless – a prediction still consistent with every effort to directly measure a neutrino mass yet attempted. Yet decades of observations of the flavour oscillations of solar, atmospheric, reactor, accelerator and astrophysical neutrinos have provided incontrovertible indirect evidence that neutrinos must have tiny masses below the sensitivity of instruments to detect. Observations of quantum interference between flavour eigenstates – the electron, muon and tau neutrinos – indicate that there must be a small mass splitting between ν₁ and the slightly more massive ν₂, and a larger mass splitting to ν₃. But it is not yet known whether the mass eigenvalues follow a so-called normal hierarchy, m₁ < m₂ < m₃, or an inverted hierarchy, m₃ < m₁ < m₂. Resolving this question is the main physics goal of the JUNO experiment.

JUNO’s determination of the mass ordering is largely free of parameter degeneracies

“Unlike other approaches, JUNO’s determination of the mass ordering does not rely on the scattering of neutrinos with atomic electrons in the Earth’s crust or the value of the leptonic CP phase, and hence is largely free of parameter degeneracies,” explains JUNO spokesperson Wang Yifang. “JUNO will also deliver order‑of‑magnitude improvements in the precision of several neutrino‑oscillation parameters and enable cutting‑edge studies of neutrinos from the Sun, supernovae, the atmosphere and the Earth. It will also open new windows to explore unknown physics, including searches for sterile neutrinos and proton decay.”

Located 700 m underground near Jiangmen city, JUNO detects antineutrinos produced 53 km away by the Taishan and Yangjiang nuclear power plants. At the heart of the detector is a liquid‑scintillator detector inside a 44 m-deep water pool. A stainless-steel truss supports an acrylic sphere housing the liquid scintillator, as well as 20,000 20‑inch photomultiplier tubes (PMTs), 25,600 three‑inch PMTs, front‑end electronics, cabling and anti‑magnetic compensation coils. All the PMTs operate simultaneously to capture scintillation light from neutrino interactions and convert it to electrical signals.

To distinguish the extremely fine flavour oscillations that will allow JUNO to observe the neutrino-mass hierarchy, the experiment must achieve an extremely fine energy resolution of almost 50 keV for a typical 3 MeV reactor antineutrino. To attain this, JUNO had to push performance margins in several areas relative to the KamLAND experiment in Japan, which was previously the world’s largest liquid-scintillator detector.

“JUNO is a factor 20 larger than KamLAND, yet our required energy resolution is a factor two better,” explains Wang. “To achieve this, we have covered the full detector with PMTs with only 3 mm clearance and twice the photo-detection efficiency. By optimising the recipe of the liquid scintillator, we were able to improve its attenuation length by a factor of two to over 20 m, and increase its light yield by 50%.”

Go with the flow

Proposed in 2008 and approved in 2013, JUNO began underground construction in 2015. Detector installation started in December 2021 and was completed in December 2024, followed by a phased filling campaign. Within 45 days, the team filled the detector with 60 ktons of ultra‑pure water, keeping the liquid‑level difference between the inner and outer acrylic spheres within centimetres and maintaining a flow‑rate uncertainty below 0.5% to safeguard structural integrity.

Over the next six months, 20 ktons of liquid scintillator progressively filled the 35.4 m diameter acrylic sphere while displacing the water. Stringent requirements on scintillator purity, optical transparency and extremely low radioactivity had to be maintained throughout. In parallel, the collaboration conducted detector debugging, commissioning and optimisation, enabling a seamless transition to full operations at the completion of filling.

JUNO is designed for a scientific lifetime of up to 30 years, with a possible upgrade path allowing a search for neutrinoless double‑beta decay, says the team. Such an upgrade would probe the absolute neutrino-mass scale and test whether neutrinos are truly Dirac fermions, as assumed by the SM, or Majorana fermions without distinct antiparticles, as favoured by several attempts to address fundamental questions spanning particle physics and cosmology.

The post JUNO takes aim at neutrino-mass hierarchy appeared first on CERN Courier.

“Early analyses have already helped characterise the geometry of oxygen and neon nuclei, including the latter’s predicted prolate ‘bowling-pin’ shape,” says Anthony Timmins of the University of Houston. “More importantly, they appear consistent with the onset of the quark-gluon plasma in light–ion collisions.”

As the quark–gluon plasma appears to behave like a near-perfect fluid with low viscosity, the key to modelling heavy-ion collisions is hydrodynamics – the physics of how fluids evolve under pressure gradients, viscous stresses and other forces. When two lead nuclei collide at the LHC, they create a tiny, extremely hot fireball where quarks and gluons interact so frequently they reach local thermal equilibrium within about 10^–23 s. Measurements of gold–gold collisions at Brookhaven’s RHIC and lead–lead collisions at the LHC suggest that the quark–gluon plasma flows with an extraordinarily low viscosity, close to the quantum limit, allowing momentum to move rapidly across the system. But it’s not clear whether the same rules apply to the smaller nuclear systems involved in light–ion collisions.

“For hydrodynamics to work, along with the appropriate quark-gluon plasma equation of state, you need a separation of scales between the mean free path of quarks and gluons, the pressure gradients and overall system size,” explains Timmins. “As you move to smaller systems, those scales start to overlap. Oxygen and neon are expected to sit near that threshold, close to the limits of plasma formation.”

Across the oxygen–oxygen and neon–neon datasets, the ALICE, ATLAS and CMS collaborations decomposed the transverse distribution of emitted particles into Fourier modes – a way to search for collective, fluid-like behaviour. Measurements of the “elliptic” and “triangular” Fourier components as functions of event multiplicity support the emergence of a collective flow driven by the initial collision geometry. The collaborations observe signs of energetic-probe suppression in oxygen–oxygen collisions – a signature of the droplet “quenching” jets in a way not observed in proton–proton collisions. Similar features appeared in a one-day xenon–xenon run that took place in October 2017.

These initial results are just a smattering of those to come

CMS compared particle yields in light-ion collisions to a proton–proton reference. After scaling for the number of binary nucleon–nucleon interactions, the collaboration observed a maximum suppression of 0.69 ± 0.04 at a transverse momentum of about 6 GeV, more than five standard deviations from unity. While milder than that observed for lead–lead and xenon–xenon collisions, the data point to genuine medium-induced suppression in the smallest ion–ion system studied to date. Meanwhile, ATLAS reported the first dijet transverse-momentum imbalance in a light-ion system. The reduction in balanced jets is consistent with path-length-dependent energy-loss effects, though apparently weaker than in lead–lead collisions.

In “head-on” collisions, ALICE, ATLAS and CMS all observed a neon–oxygen–lead hierarchy in elliptic flow, suggesting that, if a quark–gluon plasma does form, it exhibits the most pronounced “almond shape” in neon collisions. This pattern reflects the expected nuclear geometries of each species. Lead-208 is a doubly magic nucleus, with complete proton and neutron shells that render it tightly bound and nearly spherical in its ground state. Conversely, neon is predicted to be prolate, with its inherent elongation producing a larger elliptic overlap. Oxygen falls in between, consistent with models describing it as roughly spherical or weakly clustered.

ALICE and ATLAS reported a hierarchy of flow coefficients in light-ion collisions, with elliptic, triangular and quadrangular flows progressively decreasing as their Fourier index rises, in line with hydrodynamic expectations. Like CMS’s charged hadron yields, ALICE’s preliminary neutral pion yields exhibit a suppression at large momenta.

In a previous fixed-target study, the LHCb collaboration also measured the elliptic and triangular components of the flow in lead–neon and lead–argon collisions, observing the distinctive shape of the neon nucleus. As for proton–oxygen collisions, LHCb’s forward-rapidity coverage can probe the partonic structure of nuclei at very small values of Bjorken-x – the fraction of the nucleon’s momentum carried by a quark or gluon. Such measurements help constrain nuclear parton distribution functions in the low-x region dominated by gluons and provide rare benchmarks for modelling ultra-high-energy cosmic rays colliding with atmospheric oxygen.

These initial results are just a smattering of those to come. In a whirlwind 11-day campaign, physicists made full use of the brief but precious opportunity to investigate the formation of quark–gluon plasma in the uncharted territory of light ions. Accelerator physicists and experimentalists came together to tackle peculiar problems, such as the appearance of polluting species in the beams due to nuclear transmutation (see “Alchemy by pure light“). Despite the tight schedule, luminosity targets for proton–oxygen, oxygen–oxygen and neon–neon collisions were exceeded by large factors, thanks to high accelerator availability and the high injector intensity delivered by the LHC team.

“These early oxygen and neon studies show that indications of collective flow and parton-energy-loss-like suppression persist even in much smaller systems, while providing new sensitivity to nuclear geometry and valuable prospects for forward-physics studies,” concludes Timmins. “The next step is to pin down oxygen’s nuclear parton distribution function. That will be crucial for understanding the hadron-suppression patterns we see, with proton–oxygen and ultra-peripheral collisions being great ways to get there.”

The post First oxygen and neon collisions at the LHC appeared first on CERN Courier.

When Francesca Luoni logs on each morning at NASA’s Langley Research Center in Virginia, she’s thinking about something few of us ever consider: how to keep astronauts safe from the invisible hazards of space radiation. As a research scientist in the Space Radiation Group, Luoni creates models to understand how high-energy particles from the Sun and distant supernovae interact with spacecraft structures and the human body – work that will help future astronauts safely travel deeper into space.

But Luoni is not a civil servant for NASA. She is contracted through the multinational engineering firm Analytical Mechanics Associates, continuing a professional slingshot from pure research to engineering and back again. Her career is an intriguing example of how to balance research with industrial engagement – a holy grail for early-career researchers in the late 2020s.

Leveraging expertise

Luoni’s primary aim is to optimise NASA’s Space Radiation Cancer Risk Model, which maps out the cancer incidence and mortality risk for astronauts during deep-space missions, such as NASA’s planned mission to Mars. To make this work, Luoni’s team leverages the expertise of all kinds of scientists, from engineers, statisticians and physicists, to biochemists, epidemiologists and anatomists.

“I’m applying my background in radiation physics to estimate the cancer risk for astronauts,” she explains. “We model how cosmic rays pass through the structure of a spacecraft, how they interact with shielding materials, and ultimately, what reaches the astronauts and their tissues.”

Before arriving in Virginia early this year, Luoni had already built a formidable career in space-radiation physics. After a physics PhD in Germany, she joined the GSI Helmholtz Centre for Heavy Ion Research, where she spent long nights at particle accelerators testing new shielding materials for spacecraft. “We would run experiments after the medical facility closed for the day,” she says. “It was precious work because there are so few facilities worldwide where you can acquire experimental data on how matter responds to space-like radiation.”

Her experiments combined experimental measurement data with Monte Carlo simulations to compare model predictions with reality – skills she honed during her time in nuclear physics that she still uses daily at NASA. “Modelling is something you learn gradually, through university, postgrads and research,” says Luoni. “It’s really about understanding physics, maths, and how things come together.”

In 2021 she accepted a fellowship in radiation protection at CERN. The work was different from the research she’d done before. It was more engineering-oriented, ensuring the safety of both scientists and surrounding communities from the intense particle beams of the LHC and SPS. “It may sound surprising, but at CERN the radiation is far more energetic than we see in space. We studied soil and water activation, and shielding geometries, to protect everyone on site. It was much more about applied safety than pure research.”

Luoni’s path through academia and research was not linear, to say the least. From being an experimental physicist collecting data at GSI, to working as an engineer and helping physicists conduct their own experiments at CERN, Luoni is excited to be diving back into pure research, even if it wasn’t her initially intended field.

Despite her industry–contractor title, Luoni’s day-to-day work at NASA is firmly research-driven. Most of her time is spent refining computational models of space-radiation-induced cancer risk. While the coding skills she honed at CERN apply to her role now, Luoni still experienced a steep learning curve when transitioning to NASA.

“I am learning biology and epidemiology, understanding how radiation damages human tissues, and also deepening my statistics knowledge,” she says. Her team codes primarily in Python and MATLAB, with legacy routines in Fortran. “You have to be patient with Fortran,” she remarks. “It’s like building with tiny bricks rather than big built-in functions.”

Luoni is quick to credit not just the technical skills but the personal resilience gained from moving between countries and disciplines. Born in Italy, she has worked in Germany, Switzerland and now the US. “Every move teaches you something unique,” she says. “But it’s emotionally demanding. You face bureaucracy, new languages, distance from family and friends. You need to be at peace with yourself, because there’s loneliness too.”

Bravery and curiosity

But in the end, she says, it’s worth the price. Above all, Luoni counsels bravery and curiosity. “Be willing to step out of your comfort zone,” she says. “It takes strength to move to a new country or field, but it’s worth it. I feel blessed to have experienced so many cultures and to work on something I love.”

While she encourages travel, especially at the PhD and postdoc stages in a researcher’s career, Luoni advises caution when presenting your experience on applications. Internships and shorter placements are welcome, but employers want to see that you have stayed somewhere long enough to really understand and harness that company’s training.

“Moving around builds a unique skill set,” she says. “Like it or not, big names on your CV matter – GSI, CERN, NASA – people notice. But stay in each place long enough to really learn from your mentors, a year is the minimum. Take it one step at a time and say yes to every opportunity that comes your way.”

Luoni had been looking for a way to enter space-research throughout her career, building up a diverse portfolio of skills throughout her various roles in academia and engineering. “Follow your heart and your passions,” she says. “Without that, even the smartest person can’t excel.”

The post Prepped for re-entry appeared first on CERN Courier.

Its large, cryogenically-cooled mirror and infrared instruments let it capture the faint, redshifted ultraviolet light from the universe’s earliest stars and galaxies. Since its launch, the JWST has collected unprecedented samples of astrophysical sources within the first 500 million years of the Big Bang, utterly transforming our understanding of early galaxy formation.

Stellar observations

Tantalisingly, JWST’s observations hint at an excess of galaxies very bright in the ultra-violet (UV) within the first 400 million years, as compared to expectations from early formation within the standard Lambda Cold Dark matter model. Given that UV photons are a key indicator of young star formation, these observations seem to imply that early galaxies in any given volume of space were overly efficient at forming stars in the infancy of the universe.

However, extraordinary claims demand extraordinary evidence. These puzzling observations have come under immense scrutiny in confirming that the sources lie at the inferred redshifts, and do not just probe over-dense regions that might preferentially host galaxies with high star-formation rates. It could still be the case that the apparent excess of bright galaxies is cosmic variance – a statistical fluctuation caused by the relatively small regions of the sky probed by the JWST so far.

Such observational caveats notwith­standing, theorists have developed a number of distinct “families” of explanations.

UV photons are readily attenuated by dust at low redshifts. If, however, these early galaxies had ejected all of their dust, one might be able to observe almost all of the intrinsic UV light they produced, making them brighter than expected based on lower-redshift benchmarks.

Bias may also arise from detecting only those sources powered by rapid bursts of star formation that briefly elevate galaxies to extreme luminosities.

Extraordinary claims demand extraordinary evidence

Several explanations focus on modifying the physics of star formation itself, for example regarding “stellar feedback” – the energy and momentum that newly formed stars inject back into their surrounding gas, that can heat, ionise or expel gas, and slow or shut down further star formation. Early galaxies might have high star-formation rates because stellar feedback was largely inefficient, allowing them to retain most of their gas for further star formation, or perhaps because a larger fraction of gas was able to form stars in the first place.

While the relative number of low- and high-mass stars in a newly formed stellar population – the initial mass function (IMF) – has been mapped out in the local universe to some extent, its evolution with redshift remains an open question. Since the IMF crucially determines the total UV light produced per unit mass of star formed, a “top-heavy” IMF, with a larger fraction of massive stars compared to that in the local universe, could explain the observations.

Alternatively, the striking ultraviolet light may not arise solely from ordinary young stars – it could instead be powered by accretion onto black holes, which JWST is finding in unexpected numbers.

Alternative cosmologies

Finally, a number of works also appeal to alternative cosmologies to enhance structure formation at such early epochs, invoking an evolving dark-energy equation of state, primordial magnetic fields or even primordial black holes.

A key caveat involved in these observations is that redshifts are often inferred purely from broadband fluxes in different filters – a technique known as photometry. Spectroscopic data are urgently required, not only to verify their exact distances but also to distinguish between different physical scenarios such as bursty star formation, an evolving IMF or contamination by active galactic nuclei, where supermassive black holes accrete gas. Upcoming deep observations with facilities such as the Atacama Large Millimeter/submillimeter Array (ALMA) and the Northern Extended Millimeter Array (NOEMA) will be crucial for constraining the dust content of these systems and thereby clarifying their intrinsic star-formation rates. Extremely large surveys with facilities such as Euclid, the Nancy Grace Roman Space Telescope and the Extremely Large Telescope will also be crucial in surveying early galaxies over large volumes and sampling all possible density fields.

Combining these datasets will be critical in shedding light on this unexpected puzzle unearthed by the JWST.

The post The puzzle of an excess of bright early galaxies appeared first on CERN Courier.

A defining yet unobserved property of the Higgs boson is its ability to couple to itself. The ATLAS collaboration has now set new bounds on this interaction, by probing the rare production of Higgs-boson pairs. Since the self-coupling strength directly connects to the shape of the Higgs potential, any departure from the Standard Model (SM) prediction would have direct implications for electroweak symmetry breaking and the early history of the universe. This makes its measurement one of the most important objectives of modern particle physics.

Higgs-boson pair production is a thousand times less frequent than single-Higgs events, roughly corresponding to a single occurrence every three trillion proton–proton collisions at the LHC. Observing such a rare process demands both vast datasets and highly sophisticated analysis techniques, along with the careful choice of a sensitive probe. Among the most effective is the HH → bbγγ channel, where one Higgs boson decays into a bottom quark–antiquark pair and the other into two photons. This final state balances the statistical reach of the dominant Higgs decay to bottom quarks with the exceptionally clean signature offered by photon-pair measurements. Despite the small signal branching ratio of about 0.26%, the decay to two photons benefits from the excellent di-photon mass resolution and offers the highest efficiency among the leading HH channels. This provides the HH → bbγγ channel with an excellent sensitivity to variations in the trilinear self-coupling modifier κ_λ, defined as the ratio of the measured Higgs-boson self-coupling to the SM prediction.

In its new study, the ATLAS collaboration relied on Run 3 data collected between 2022 and 2024, and on the full Run 2 dataset, reaching an integrated luminosity of 308 fb^–1. Events were selected with two high-quality photons and at least two b-tagged jets, identified using the latest and most performant ATLAS b-tagging algorithm. To further distinguish signal from background, dominated by non-resonant γγ+jets and single-Higgs production with H → γγ, a set of machine-learning classifiers called “multivariate analysis discriminants” were trained and used to filter genuine HH → bbγγ signals.

The collaboration reported an HH → bbγγ signal significance of 0.84σ  under the background-only hypothesis, compared to a SM expectation of 1.01σ (see figure 1). At the 95% confidence level, the self-coupling modifier was constrained to –1.7 < κ_λ < 6.6. These results extend previous Run 2 analyses and deliver a substantially improved sensitivity, comparable to the observed (expected) significance of 0.4σ (1σ) in the combined Run 2 results across all channels. The improvement is primarily due to the adoption of advanced b-tagging algorithms, refined analysis techniques yielding better mass resolution and a larger dataset, more than double that of previous studies.

This result marks significant progress in the search for Higgs self-interactions at the LHC and highlights the potential of Run 3 data. With the full Run 3 dataset and the High-Luminosity LHC on the horizon, ATLAS is set to extend these measurements – improving our understanding of the Higgs boson and searching for possible signs of physics beyond the SM.

The post A step towards the Higgs self-coupling appeared first on CERN Courier.

The ALICE collaboration recently obtained the first direct measurement of the attraction between a proton and a ρ⁰ meson – a particle of particular interest due to its fleeting lifetime and close link to chiral symmetry breaking. The result establishes a technique known as femtoscopy as a new method for studying interactions between vector mesons and baryons, and opens the door to a systematic exploration of how short-lived hadrons behave.

Traditionally, interactions between baryons and vector mesons have been studied indirectly at low-energy facilities, using decay patterns or photoproduction measurements. These were mostly interpreted through vector–meson–dominance models developed in the 1960s, in which photons fluctuate into vector mesons to interact with hadrons. While powerful, these methods provide only partial information and cannot capture the full dynamics of the interaction. Direct measurements have long been out of reach, mainly because the extremely short lifetime of vector mesons – of the order of 1–10 fm/c – renders conventional scattering experiments impossible.

At the hadronic level, the strong force can be described as arising from the exchange of massive mesons, with the lightest among them, the pion, setting the interaction range to about 1.4 fm. For such a short-range effect to influence the products of a pp collision, the particles must be created close together and with low relative momentum, ensuring sufficient interaction time and a significant wavefunction overlap.

The ALICE collaboration has now studied this mechanism in high-multiplicity proton–proton (pp) collisions, at a centre-of-mass energy of 13 TeV, through femtoscopy, which examines correlations in the relative momentum (k^*) of particle pairs in their rest frame. These were expected to carry information on the size and shape of the particle-emitting source at k^* below about 200 MeV, with any deviations from unity indicating the presence of short-ranged forces.

To study the interaction between protons and ρ⁰ vector mesons, candidates were reconstructed via the hadronic decay channel ρ⁰ → π⁺π^–, identified from π⁺π^– pairs within the 0.70–0.85 GeV invariant mass window. Since the ρ⁰ decays almost instantly into pions, only about 3% of the candidates were genuine ρ⁰ mesons. Background corrections were therefore essential to extract the ρ⁰–proton correlation function, defined as the ratio of the relative-momentum distribution of same-event pairs to that of mixed-event pairs. The result is consistent with unity at large relative momenta (k^* > 200 MeV), as expected in the absence of strong forces. At lower values, however, a suppression with significance of about four standard deviations clearly signals ρ⁰–proton final-state interactions (see figure 1).

To interpret these results, ALICE used an effective field model based on chiral perturbation theory, which predicted two resonance states consistent with the formation of excited nucleon states. Because some pairs linger in these quasi-bound states instead of flying out freely, fewer emerge with nearly the same momentum. This results in a correlation suppression at low k^* consistent with observations. Unlike photoproduction experiments and QCD sum rules, femtoscopy delivers the complete phase information of the ρ⁰–proton interaction. By analysing both ρ–proton and φ–proton pairs, ALICE extracted precise scattering parameters that can now be incorporated into theoretical models.

This measurement sets a benchmark for vector-meson–dominance models and establishes femtoscopy as a tool to probe interactions involving the shortest-lived hadrons, while providing essential input for understanding ρ–nucleon interactions in vacuum and describing the meson’s properties in heavy-ion collisions. Pinning down how the ρ meson behaves is crucial for interpreting dilepton spectra and the restoration of chiral symmetry, as differences between light quark masses become negligible at high energies. For example, the mass gap between the ρ and its axial counterpart, a₁, comes from spontaneous chiral-symmetry breaking.

The post ALICE observes ρ–proton attraction appeared first on CERN Courier.

The Copenhagen interpretation remains the most widely held view, placing the act of measurement at the core of quantum theory well into the 2020s. Epistemic or QBist approaches, where the quantum state expresses an observer’s knowledge or belief, form the next most common group, followed by Everett’s many-worlds framework, in which all quantum outcomes continue to coexist without collapse (CERN Courier July/August 2025 p26). Other views maintain small but steady followings, including pilot-wave theory, spontaneous-collapse models and relational quantum mechanics (CERN Courier July/August 2025 p21).

Fewer than 10% of physicists surveyed declined to express a view. Though this cohort purports to include proponents of the “shut up and calculate” school of thought, an apparently dwindling cohort of disinterested working physicists may simply be undersampled.

Crucially, confidence is modest. Most respondents view their preferred interpretation as an adequate placeholder or a useful conceptual tool. Only 24% are willing to describe their preferred interpretation as correct, leaving ample room for manoeuvre in the very foundations of fundamental physics.

The post The measurement problem, measured appeared first on CERN Courier.

The LHCb collaboration has developed a new inclusive flavour-tagging algorithm for neutral B-mesons. Compared to standard approaches, it can correctly identify 35% more B⁰ and 20% more B⁰_s decays, expanding the dataset available for analysis. This increase in tagging power will allow for more accurate studies of charge–parity (CP) violation and B-meson oscillations.

In the Standard Model (SM), neutral B-mesons oscillate between particle and antiparticle states via second-order weak interactions involving a pair of W-bosons. Flavour-tagging techniques determine whether a neutral B-meson was initially produced as a B⁰ or its antiparticle B⁰, thereby enabling the measurement of time-dependent CP asymmetries. As the initial flavour can only be inferred indirectly from noisy, multi-particle correlations in the busy hadronic environment of the LHC, mistag rates have traditionally been high.

Until now, the LHCb collaboration has relied on two complementary flavour-tagging strategies. One infers the signal meson’s flavour by analysing the decay of the other b-hadron in the event, whose existence follows from bb– pair production in the original proton-proton collision. Since the two hadrons originate from oppositely-charged, early-produced bottom quarks, the method is known as “opposite-side” (OS) tagging. The other strategy, or “same-side” (SS) tagging, uses tracks from the fragmentation process that produced the signal meson. Each provides only part of the picture, and their combination defined the state of the art in previous analyses.

The new algorithm adopts a more comprehensive approach. Using a deep neural network based on the “DeepSets” architecture, it incorporates information from all reconstructed tracks associated with the hadronisation process, rather than preselecting a subset of candidates. By considering the global structure of the event, the algorithm builds a more detailed inference of the meson’s initial flavour. This inclusive treatment of the available information increases both the sensitivity and the statistical reach of the tagging procedure.

The model was trained and calibrated using well-established B⁰ and B⁰_s meson decay channels. When compared with the combination of opposite-side and same-side taggers, the inclusive algorithm displayed a 35% increase in tagging power for B⁰ mesons and 20% for B⁰_s mesons (see figure 1). The improvement stems from gains in both the fraction of events that receive a flavour tag and how often the tag is correct. Tagging power is a critical figure of merit, as it determines the effective amount of usable data. Therefore, even modest gains can dramatically reduce statistical uncertainties in CP-violation and B-oscillation measurements, enhancing the experiment’s precision and discovery potential.

This development illustrates how algorithmic innovation can be as important as detector upgrades in pushing the boundaries of precision. The improved tagging power effectively expands the usable data sample without requiring additional collisions, enhancing the experiment’s capacity to test the SM and seek signs of new physics within the flavour sector. The timing is particularly significant as LHCb enters Run 3 of the LHC programme, with higher data rates and improved detector components. The new algorithm is designed to integrate smoothly with existing reconstruction and analysis frameworks, ensuring immediate benefits while providing scalability for the much larger datasets expected in future runs.

As the collaboration accumulates more data, the inclusive flavour-tagging algorithm is likely to become a central tool in data analysis. Its improved performance is expected to reduce uncertainties in some of the most sensitive measurements carried out at the LHC, strengthening the search for deviations from the SM.

The post Neural networks boost B-tagging appeared first on CERN Courier.

In particle physics, searches for new phenomena have traditionally been guided by theory, focusing on specific signatures predicted by models beyond the Standard Model. Machine learning offers a different way forward. Instead of targeting known possibilities, it can scan the data broadly for unexpected patterns, without assumptions about what new physics might look like. CMS analysts are now using these techniques to conduct model-independent searches for short-lived particles that could escape conventional analyses.

Dynamic graph neural networks operate on graph-structured data, processing both the attributes of individual nodes and the relationships between them. One such model is ParticleNet, which represents large-radius-jet constituents as networks to identify N-prong hadronic decays of highly boosted particles, predicting their parent’s mass. The tool recently aided a CMS search for the single production of a heavy vector-like quark (VLQ) decaying into a top quark and a scalar boson, either the Higgs or a new scalar particle. Alongside ParticleNet, a custom deep neural network was trained to identify leptonic top-quark decays by distinguishing them from background processes over a wide range of momenta. With this approach, the analysis achieved sensitivity to VLQ production cross-sections as small as 0.15 fb. Emerging methods such as transformer networks can provide even more sensitivity in future searches (see figure 1).

Another novel approach combined two distinct machine-learning tools in the search for a massive scalar X decaying into a Higgs boson and a second scalar Y. While ParticleNet identified Higgs-boson decays to two bottom quarks, potential Y signals were assigned an “anomaly score” by an autoencoder – a neural network trained to reproduce its input and highlight atypical features in the data. This technique provided sensitivity to a wide range of unexpected decays without relying on specific theoretical models. By combining targeted identification with model-independent anomaly detection, the analysis achieved both enhanced performance and broad applicability.

Searches at the TeV scale sit at the frontier where not only more and more data but also algorithmic innovation drives experimental discovery. Tools such as targeted deep neural networks, parametric neural networks (PNNs) – which efficiently scan multi-dimensional mass landscapes (see figure 2) – and model-independent anomaly detection, are opening new ways to search for deviations from the Standard Model. Analyses of the full LHC Run 2 dataset have already revealed intriguing hints, with several machine-learning studies reporting local excesses – including a 3.6σ excess in a search for V′ → VV or VH → jets, and deviations up to 3.3σ in various X → HY searches. While no definitive signal has yet emerged, the steady evolution of neural-network techniques is already changing how new phenomena are sought, and anticipation is high for what they may reveal in the larger Run 3 dataset.

The post Machine learning and the search for the unknown appeared first on CERN Courier.

“There’s never been this type of report before,” says Maxim Titov (CEA Saclay), who co-chairs the LDG Sustainability Working Group. “The LDG Working Group consisted of representatives with technical expertise in sustainability evaluation from large institutions including CERN, DESY, IRFU, INFN, NIKHEF and STFC, as well as experts from future collider projects who signed off on the numbers.”

The report argues that carbon assessment cannot be left to the end of a project. Instead, facilities must evaluate their lifecycle footprint starting from the early design phase, all the way through construction, operation and decommissioning. Studies already conducted on civil-engineering footprints of large accelerator projects outline a reduction potential of up to 50%, says Titov.

In terms of accelerator technology, the report highlights cooling, ventilation, cryogenics, the RF cavities that accelerate charged particles and the klystrons that power them, as the largest sources of inefficiency. The report places particular emphasis on klystrons, and identifies three high-efficiency designs currently under development that could boost the energy efficiency of RF cavities from 60 to 90% (CERN Courier May/June 2025 p30).

Carbon assessment cannot be left to the end of a project

The report also addresses the growing footprint of computing and AI. Training algorithms on more efficient hardware and adapting trigger systems to reduce unnecessary computation are identified as ways to cut energy use without compromising scientific output.

“You need to perform a life-cycle assessment at every stage of the project in order to understand your footprint, not just to produce numbers, but to optimise design and improve it in discussions with policymakers,” emphasises Titov. “Conducting sustainability assessments is a complex process, as the criteria have to be tailored to the maturity of each project and separately developed for scientists, policymakers, and society applications.”

Established by the CERN Council, the LDG is an international coordination body that brings together directors and senior representatives of the world’s major accelerator laboratories. Since 2021, the LDG has been composed of five expert panels: high-field magnets, RF structures, plasma and laser acceleration, muon colliders and energy-recovery linacs. The Sustainability Working Group was added in January 2024.

The post Standardising sustainability: step one appeared first on CERN Courier.

Neutrino physics has come a long way since the discovery of neutrino oscillations in 1998. Experiments now measure oscillation parameters with a precision of a few per cent. At NuFact 2025, the IceCube collaboration reported new oscillation measurements using atmospheric neutrinos from 11 years of observations at the South Pole. The measurements achieve world-leading sensitivity on neutrino mixing angles, alongside new constraints on the unitarity of the neutrino mixing matrix. Meanwhile, the JUNO experiment in China celebrated the start of data-taking with its liquid-scintillator detector (see “JUNO takes aim at neutrino-mass hierarchy”). JUNO will determine the neutrino mass ordering by observing the fine oscillation patterns of antineutrinos produced in nuclear reactors.

Neutrino scattering

Beyond oscillations, a major theme of the conference was neutrino scattering. Although neutrinos are the most abundant massive particles in the universe, their interactions with matter remain poorly understood. Measuring and modelling these processes is essential: they probe nuclear structure and hadronic physics in a novel way, while also providing the foundation for oscillation analyses in current and next-generation experiments. Exciting advances were reported across the field. The SBND experiment at Fermilab announced the collection of around three million neutrino interactions using the Booster Neutrino Beam. ICARUS presented its first neutrino–argon cross-section measurement. MicroBooNE, MINERvA and T2K showcased new results on neutrino–nucleus interaction and compared them with theoretical models. The e4ν collaboration highlighted electron beams as potential sources of data to refine neutrino-scattering models, supporting efforts to achieve the detailed interaction picture needed for the coming precision era of oscillation physics. At higher energies, FASER and SND@LHC showcased their LHC neutrino observations with both emulsion and electronic detectors.

Neutrino physics is one of the most vibrant and global areas of particle physics today

CERN’s role in neutrino physics was on display throughout the conference. Beyond the results from ICARUS, FASER and SND@LHC, other contributions included the first observation of neutrinos in the ProtoDUNE detectors, the status of the MUonE experiment – aimed at measuring the hadronic contribution to the muon anomalous magnetic moment – and the latest results from NA61. The role of CERN’s Neutrino Platform was also highlighted in contributions about the T2K ND280 near-detector upgrade and the WAGASCI–BabyMIND detector, both of which were largely assembled and tested at CERN. Discussions featured the results of the Water Cherenkov Test Experiment, which operated in the T9 beamline to prototype technology for Hyper-Kamio­kande, and other novel CERN-based ideas, such as nuSCOPE – a proposal for a short-baseline experiment that would “tag” individual neutrinos at production, formed from the merging of ENUBET and NuTag. Building on a proof-of-principle result from NA62, which identified a neutrino candidate via its parent kaon decay, this technique could represent a paradigm shift in neutrino beam characterisation.

NuFact 2025 reinforced the importance of diversity and inclusion in science. The IDEEO working group led discussions on how varied perspectives and equitable participation strengthen collaboration, improve problem solving and attract the next generation of researchers. Dedicated sessions on education and outreach also highlighted innovative efforts to engage wider communities and ensure that the future of neutrino physics is both scientifically robust and socially inclusive. From precision oscillation measurements to ambitious new proposals, NuFact 2025 demonstrated that neutrino physics is one of the most vibrant and global areas of particle physics today.

The post NuFact prepares for a precision era appeared first on CERN Courier.

CERN’s current NA62 effort was nevertheless present in force. Eight presentations spanned its wide-ranging programme, from precision studies of rare kaon decays to searches for lepton-flavour and lepton-number violation, and explorations beyond the Standard Model (SM). Complementary perspectives came from Japan’s KOTO experiment at J-PARC, from multipurpose facilities such as KLOE-2, Belle II and CERN’s LHCb experiment, as well as from a large and engaged theoretical community. Together, these contributions underscored the vitality of kaon physics: a field that continues to test the SM at the highest levels of precision, with a strong potential to uncover new physics.

NA62 reported a big success on the so-called “golden mode” ultra-rare decay K⁺ → π⁺νν, a process that is highly sensitive to new physics (CERN Courier July/August 2024 p30). NA62 has already delivered remarkable progress in this domain: by analysing data up to 2022, the collaboration more than doubled its sample from 20 to 51 candidate events, achieving the first 5σ observation of the decay (CERN Courier November/December 2024 p11). This is the smallest branching fraction ever measured, and, intriguingly, shows a mild 1.7σ tension with the Standard Model prediction, which itself is known with a 2% theoretical uncertainty. With the experiment continuing to collect data until CERN’s next long shutdown (LS3), NA62’s final dataset is expected to triple the current statistics, sharpening what is already one of the most stringent tests of the SM.

Another major theme was the study of rare B-meson decays where kaons often appear in the final state, for example B → K^* (→ Kπ) ℓ⁺ℓ^–. Such processes are central to the long-debated “B anomalies,” in which certain branching fractions of rare semileptonic B decays show persistent tensions between experimental results and SM predictions (CERN Courier January/February 2025 p14). On the experimental front, CERN’s LHCb experiment continues to lead the field, delivering branching-fraction measurements with unprecedented precision. Progress is also being made on the theoretical side, though significant challenges remain in matching this precision. The conference highlighted new approaches reducing uncertainties and biases, based both on phenomenological techniques and lattice QCD.

Kaon physics is in a particularly dynamic phase. Theoretical predictions are reaching unprecedented precision, and two dedicated experiments are pushing the frontiers of rare kaon decays. At CERN, NA62 continues to deliver impactful results, even though plans for a next-stage European successor did not advance this year. Momentum is building in Japan, where the proposed KOTO-II upgrade, if approved, would secure the long-term future of the programme. Just after the conference, the KOTO-II collaboration held its first in-person meeting, bringing together members from both KOTO and NA62 – a promising sign for continued cross-fertilisation. Looking ahead, sustaining two complementary experimental efforts remains highly desirable: independent cross-checks and diversified systematics. Both will be essential to fully exploit the discovery potential of rare kaon decays.

The post Mainz muses on future of kaon physics appeared first on CERN Courier.

While the ICFA is neither a decision-making body nor a representation of funding agencies, its mandate assigns to the committee the important task of promoting international collaboration and coordination in all phases of the construction and exploitation of very-high-energy accelerators. This role is especially relevant in today’s context of strategic planning and upcoming decisions – with the ongoing European Strategy update, the Chinese decision process on CEPC in full swing, and the new perspectives emerging on the US–American side with the recent National Academy of Sciences report (CERN Courier September/October 2025 p10).

Consequently, the ICFA heard presentations on these important topics and discussed priorities and timelines. In addition, the theme of “physics beyond colliders” – and with it, the question of maintaining scientific diversity in an era of potentially vast and costly flagship projects – featured prominently. In this context, the importance of national laboratories capable of carrying out mid-sized particle-physics experiments was underlined. This also featured in the usual ICFA regional reports.

An important part of the work of the committee is carried out by the ICFA panels – groups of experts in specific fields of high relevance. The ICFA heard reports from the various panel chairs at the Wisconsin meeting, with a focus on the Instrumentation, Innovation and Development panel, where Stefan Söldner-Rembold (Imperial College London) recently took over as chair, succeeding the late Ian Shipsey. Among other things, the panel organises several schools and training events, such as the EDIT schools, as well as prizes that increase recognition for senior and early-career researchers working in the field of instrumentation.

Maintaining scientific diversity in an era of potentially vast and costly flagship projects  featured prominently

Another focus was the recent work of the Data Lifecycle panel chaired by Kati Lassila-Perini (University of Helsinki). This panel, together with numerous expert stakeholders in the field, recently published recommendations for best practices for data preservation and open science in HEP, advocating the application of the FAIR principles of findability, accessibility, interoperability and reusability at all levels of particle-physics research. The document provides guidance for researchers, experimental collaborations and organisations on implementing best-practice routines. It will now be distributed as broadly as possible and will hopefully contribute to the establishment of open and FAIR science practices.

Formally, the ICFA is a working group of the International Union for Pure and Applied Physics (IUPAP) and is linked to Commission C11, Particles and Fields. IUPAP has recently begun a “rejuvenation” effort that also involves rethinking the role of its working groups. Reflecting the continuity and importance of the ICFA’s work, Marcelo Gameiro Munhoz, chair of C11, presented a proposal to transform the ICFA into a standing committee under C11 – a new type of entity within IUPAP. This would allow ICFA to overcome its transient nature as a working group.

Finally, there were discussions on plans for a new set of ICFA seminars – triennial events in different world regions that assemble up to 250 leaders in the field. Following the 13th ICFA Seminar on Future Perspectives in High-Energy Physics, hosted by DESY in Hamburg in late 2023, the baton has now passed to Japan, which is finalising the location and date for the next edition, scheduled for late 2026.

The post ICFA meets in Madison appeared first on CERN Courier.

Marzia Bordone (University of Zurich) highlighted central questions in flavour physics, such as the tensions in the determinations of quark flavour-mixing parameters and the anomalies in leptonic and semileptonic B-meson decays (CERN Courier January/February 2025 p14). She showed that new bosons beyond the Standard Model that primarily interact with the heaviest quarks are theoretically well motivated and could be responsible for these flavour anomalies. Bordone emphasised that collaboration between experiment and theory, as well as data from future colliders like FCC-ee, will be essential to understand whether these effects are genuine signs of new physics.

Lina Necib (MIT) shared impressive new results on the distribution of galactic dark matter. Though invisible, dark matter interacts gravitationally and is present in all galaxies across the universe. Her team used exquisite data from the ESA Gaia satellite to track stellar trajectories in the Milky Way and determine the local dark-matter distribution to within 20–30% precision – which means about 300,000 dark-matter particles per cubic metre assuming they have mass similar to that of the proton. This is a huge improvement over what could be done just one decade ago, and will aid experiments in their direct search for dark matter in laboratories worldwide.

The most quoted dark-matter candidates at Invisibles25 were probably axions: particles once postulated to explain why the strong interactions that bind protons and neutrons behave in the same way for particles and antiparticles. Nicole Righi (King’s College London) discussed how these particles are ubiquitous in string theory. According to Righi, their detection may imply a hot Big Bang, with a rather late thermal stage, or hint at some special feature of the geometry of ultracompact dimensions related to quantum gravity.

The most intriguing talk was perhaps the CERN colloquium given by the 2011 Nobel laureate Adam Riess (Johns Hopkins University). By setting up an impressive system of distance measurements to extragalactic systems, Riess and his team have measured the expansion rate of the universe – the Hubble constant – with per cent accuracy. Their results indicate a value about 10% higher than that inferred from the cosmic microwave background within the standard ΛCDM model, a discrepancy known as the “Hubble tension”. After more than a decade of scrutiny, no single systematic error appears sufficient to account for it, and theoretical explanations remain tightly constrained (CERN Courier March/April 2025 p28). In this regard, Julien Lesgourgues (RWTH Aachen University) pointed out that, despite the thousands of papers written on the Hubble tension, there is no compelling extension of ΛCDM that could truly accommodate it.

While 95% of the universe’s energy density is invisible, the community studying it is very real. Invisibles now has a long history and is based on three innovative training networks funded by the European Union, as well as two Marie Curie exchange networks. The network includes more than 100 researchers and 50 PhD students spread across key beneficiaries in Europe, as well as America, Asia and Africa – CERN being one of their long-term partners. The energy and enthusiasm of the participants at this conference were palpable, as nature continues to offer deep mysteries that the Invisibles community strives to unravel.

The post Invisibles, in sight appeared first on CERN Courier.

One of the highlights concerned the Higgs boson’s coupling to the charm quark, with the CMS collaboration presenting a new search using Higgs production in association with a top–antitop pair. The analysis, targeting Higgs decays into charm–quark pairs, reached a sensitivity comparable to the best existing direct constraints on this elusive interaction. New ATLAS analyses showcased the impact of the large Run 3 dataset, hinting at great potential for Higgs physics in the years to come – for example, Run 3 data has reduced the uncertainties on the coupling of the Higgs boson to muons and Zγ by 30% and 38%, respectively. On the di-Higgs front, the expected upper limit on the signal-strength modifier, measured in the bbγγ final state only, has now surpassed in sensitivity the combination of all Run 2 HH channels (see “A step towards the Higgs self-coupling”). The sensitivity to di-Higgs production is expected to improve significantly during Run 3, raising hopes of seeing a signal before the next long shutdown, from mid-2026 to the end of 2029.

Juan Rojo (Vrije Universiteit Amsterdam) discussed parton distribution functions for Higgs processes at the LHC, while Thomas Gehrmann (University of Zurich) reviewed recent developments in general Higgs theory. Mathieu Pellen (University of Freiburg) provided a review of vector-boson fusion, Jose Santiago Perez (University of Granada) summarised the effective field theory framework and Oleksii Matsedonskyi (University of Cambridge) reviewed progress on electroweak phase transitions. In his “vision” talk, Alfredo Urbano (INFN Rome) discussed the interplay between Higgs physics and early-universe cosmology. Finally, Benjamin Fuks (LPTHE, Sorbonne University) presented a toponium model, bringing the elusive romance of top–quark pairs back into the spotlight (CERN Courier September/October 2025 p9).

After a cruise on the Seine in the light of the Olympic Cauldron, participants were propelled toward the future during the European Strategy for Particle Physics session. The ESPPU secretary Karl Jakobs (University of Freiburg) and various session speakers set the stage for spirited and vigorous discussions of the options before the community – in particular, the scenarios to pursue should the FCC programme, the clear plan A, not be realised. The next Higgs Hunting workshop will be held in Orsay and Paris from 16 to 18 September 2026.

The post Higgs hunters revel in Run 3 data appeared first on CERN Courier.

The first Workshop on the Impact of Higgs Studies on New Theories of Fundamental Interactions, which took place on the Island of Capri, Italy, from 6 to 10 October 2025, gathered around 40 experimentalists and theorists to explore the pivotal role of the Higgs boson in exploring BSM physics. Participants discussed the implications of extended scalar sectors and the latest ATLAS and CMS searches, including current potential anomalies in LHC data.

“The Higgs boson has moved from the realm of being just a new particle to becoming a tool for searches for BSM particles,” said Greg Landsberg (Brown University) in an opening talk.

An extended scalar sector can address several mysteries in the SM. For example, it could serve as a mediator to a hidden sector that includes dark-matter particles, or play a role in generating the observed matter–antimatter asymmetry during an electroweak phase transition. Modified or extended Higgs sectors also arise in supersymmetric and other BSM models that address why the 125 GeV Higgs boson is so light compared to the Planck mass – despite quantum corrections that should drive it to much higher scales – and might shed light on the perplexing pattern of fermion masses and flavours.

One way to look for new physics in the scalar sector is modifications in the decay rates, coupling strengths and CP-properties of the Higgs boson. Another is to look for signs of additional neutral or charged scalar bosons, such as those predicted in longstanding two-Higgs-doublet or Higgs-triplet models. The workshop saw ATLAS and CMS researchers present their latest limits on extended Higgs sectors, which are based on an increasing number of model-independent or signature-based searches. While the data so far are consistent with the SM, a few mild excesses have attracted the attention of some theorists.

In diphoton final states, a slight excess of events persists in CMS data at a mass of 95 GeV. Hints of a small excess at a mass of 152 GeV are also present in ATLAS data, while a previously reported excess at 650 GeV has faded after full examination of Run 2 data. Workshop participants also heard suggestions that the Brout–Englert–Higgs potential could allow for a second resonance at 690 GeV.

The High-Luminosity LHC will enable us to explore the scalar sector in detail

“We haven’t seen concrete evidence for extended Higgs sectors, but intriguing features appear in various mass scales,” said CMS collaborator Sezen Sekmen (Kyungpook National University). “Run 3 ATLAS and CMS searches are in full swing, with improved triggering, object reconstruction and analysis techniques.”

Di-Higgs production, the rate of which depends on the strength of the Higgs boson’s self-coupling, offers a direct probe of the shape of the Brout–Englert–Higgs potential and is a key target of the LHC Higgs programme. Multiple SM extensions predict measurable effects on the di-Higgs production rate. In addition to non-resonant searches in di-Higgs production, ATLAS and CMS are pursuing a number of searches for BSM resonances decaying into a pair of Higgs bosons, which were shown during the workshop.

Rich exchanges between experimentalists and theorists in an informal setting gave rise to several new lines of attack for physicists to explore further. Moreover, the critical role of the High-Luminosity LHC to probe the scalar sector of the SM at the TeV scale was made clear.

“Much discussed during this workshop was the concern that people in the field are becoming demotivated by the lack of discoveries at the LHC since the Higgs, and that we have to wait for a future collider to make the next advance,” says organiser Andreas Crivellin (University of Zurich). “Nothing could be further from the truth: the scalar sector is not only the least explored of the SM and the one with the greatest potential to conceal new phenomena, but one that the High-Luminosity LHC will enable us to explore in detail.”

The post All aboard the scalar adventure appeared first on CERN Courier.

Uncovering Quantum Field Theory and the Standard Model by Wolfgang Bietenholz of the National Autonomous University of Mexico and Uwe-Jens Wiese from the University of Bern, explains the foundations of quantum field theory in great depth, from classical field theory and canonical quantisation to regularisation and renormalisation, via path integrals and the renormalisation group. What really makes the book special are frequently discussed relations to statistical mechanics and condensed-matter physics.

Riding a wave

The section on particles and “wavicles” is highly original. In quantum field theory, quantised excitations of fields cannot be interpreted as point-like particles. Unlike massive particles in non-relativistic quantum mechanics, these excitations have non-trivial localisation properties, which apply to photons and electrons alike. To emphasise the difference between non-relativistic particles and wave excitations in a relativistic theory, one may refer to them as “wavicles”, following Frank Wilczek. As discussed in chapter 3, an intuitive understanding of wavicles can be gained by the analogy to phonons in a crystal. Another remarkable feature of charged fields is the infinite extension of their excitations due to their Coulomb field. This means that any charged state necessarily includes an infrared cloud of soft gauge bosons. As a result, they cannot be described by ordinary one-particle states and are referred to as “infra­particles”. Their properties, along with the related “superselection sectors,” are explained in the section on scalar quantum electrodynamics. 

The SM can be characterised as a non-abelian chiral gauge theory. Bietenholz and Wiese explain the various aspects of chirality in great detail. Anomalies in global and local symmetries are carefully discussed in the continuum as well as on a space–time lattice, based on the Ginsparg–Wilson relation and Lüscher’s lattice chiral symmetry. Confinement of quarks and gluons, the hadron spectrum, the parton model and hard processes, chiral perturbation theory and deconfinement at high temperatures uncover perturbative and non-perturbative aspects of quantum chromodynamics (QCD), the theory of strong interactions. Numerical simulations of strongly coupled lattice Yang–Mills theories are very demanding. During the past four decades, much progress has been made in turning lattice QCD into a quantitative reliable tool by controlling statistical and systematic uncertainties, which is clearly explained to the critical reader. The treatment of QCD is supplemented by an introduction to the electroweak theory covering the Higgs mechanism, electroweak symmetry breaking and flavour physics of quarks and leptons.

The number of quark colours, which is three in nature, plays a prominent role in this book. At the quantum level, gauge symmetries can fail due to anomalies, rendering a theory inconsistent. The SM is free of anomalies, but this only works because of a delicate interplay between quark and lepton charges and the number of colours. An important example of this interplay is the decay of the neutral pion into two photons. The subtleties of this process are explained in chapter 24.

The number of quark colours, which is three in nature, plays a prominent role in this book

Most remarkably, the SM predicts baryon-number-violating processes. This arises from the vacuum structure of the weak SU(2) gauge fields, which involves topologically distinct field configurations. Quantum tunnelling between them, together with the anomaly in the baryon–number current, leads to baryon–number violating transitions, as discussed in chapter 26. Similarly, in QCD a non-trivial topology of the gluon field leads to an explicit breaking of the flavour-singlet axial symmetry and, subsequently, to the mass of the η′ meson. Moreover, the gauge field topology gives rise to an additional parameter in QCD, the vacuum-angle θ. Since this parameter induces an electric dipole moment of the neutron that satisfies a strong upper bound, this confronts us with the strong-CP problem: what constrains θ to be so tiny that the experimental upper bound on the neutron dipole moment is satisfied? A solution may be provided by the Peccei–Quinn symmetry and axions, as discussed in a dedicated chapter.

By analogy with the QCD vacuum angle, one can introduce a CP-violating electromagnetic parameter θ into the SM – even though it has no physical effect in pure QED. This brings us to a gem of the book: its discussion of the Witten effect. In the presence of such a θ, the electric charge of a magnetic monopole becomes θ/2π plus an integer. This leads to the remarkable conclusion that for non-zero θ, all monopoles become dyons, carrying both electric and magnetic charge.

The SM is an effective low-energy theory and we do not know at what energy scale elements of a more fundamental theory will become visible. Its gauge structure and quark and lepton content hint at a possible unification of the interactions into a larger gauge group, which is discussed in the final chapter. Once gravity is included, one is confronted with a hierarchy problem: the question of why the electroweak scale is so small compared to the Planck mass, at which the Compton wavelength of a particle and its Schwarzschild radius coincide. Hence, at Planck energies quantum gravitational effects cannot be ignored. Perhaps, solving the electroweak hierarchy puzzle requires working with supersymmetric theories. For all students and scientists struggling with the SM and exploring possible extensions, the nine appendices will be a very valuable source of information for their research.

The post Subtleties of quantum fields appeared first on CERN Courier.

The magic of entanglement

In an entangled particle system, some properties have to be assigned to the system itself and not to individual particles. When a neutral pion decays into two photons, for example, conservation of angular momentum requires their total spin to be zero. Since the photons travel in opposite directions in the pion’s rest frame, in order for their spins to cancel they must share the same “helicity”. Helicity is the spin projection along the direction of motion, and only two states are possible: left- or right-handed. If one photon is measured to be left-handed, the other must be left-handed as well. The entangled photons must be thought of as a single quantum object: neither do the individual particles have predefined spins nor does the measurement performed on one cause the other to pick a spin orientation. Experiments in more complicated systems have ruled these possibilities out, at least in their simplest incarnations, and this is exactly where the magic of entanglement begins.

Quantum entanglement is the main topic of Einstein’s Entanglement by William Stuckey, Michael Silberstein and Timothy McDevitt, all currently teaching at Elizabethtown College, Pennsylvania. The trio have complementary expertise in physics, philosophy and maths, and this is not their first book on the foundations of physics. They aim to explain why entanglement is so puzzling to physicists and the various ways that have been employed over the years to explain (or even explain away) the phenomenon. They also want to introduce the readers to their own idea on how to solve the riddle and argue about its merits.

Why is entanglement so puzzling to physicists, and what has been employed to explain the phenomenon?

General readers may struggle in places. The book does have accessible chapters, for example one at the start with a quantum-gloves experiment – a nice way to introduce the reader to the problem – as well as a chapter on special relativity. Much of the discussion about quantum mechanics, however, uses advanced concepts such as Hilbert space and the Bloch sphere, that belong to an undergraduate course in quantum mechanics. Philosophical terminology, such as “wave-function realism”, is also used copiously. The explanations and the discussion provided are of good quality and an interested reader in the interpretations of quantum mechanics with some background in physics has a lot to gain. The authors quote copiously from a superb list of references and include many interesting historical facts that make reading the book very entertaining.

In general, the book criticises constructive approaches to interpreting quantum mechanics that explicitly postulate physical phenomena. In the example of neutral-pion decay that I gave previously, the case in which the measurement of one photon causes the other photon to pick a spin would require a constructive explanation. These can be contrasted with principle explanations, which may involve, for example, invoking an overarching symmetry. To quote an example that is used many times in the book, the relativity principle can be used to explain Lorentz length contraction without the need for a physical mechanism to contract the bodies, which would require a constructive explanation.

The authors make the claim that the conceptual issues with entanglement can be solved by sticking to principle explanations and, in particular, with the demand that Planck’s constant is measured to be the same in all inertial reference frames. Whether this simple suggestion is adequate to explain the mysteries of quantum mechanics, I will leave to the reader. Seneca wrote in his Natural Questions that “our descendants will be astonished at our ignorance of what to them is obvious”. If the authors are correct, entanglement may prove to be a case in point.

The post Einstein’s entanglement appeared first on CERN Courier.

John Peoples, the third director of Fermilab, who guided the lab through one of the most critical periods in its history, passed away on 25 June 2025. Born in New York City on 22 January 1933, John received his bachelor’s degree in electrical engineering from the Carnegie Institute of Technology (now Carnegie Mellon University) in 1955. After several years at the Glen L. Martin Company, John entered Columbia University where he received his PhD in physics in 1966 for the measurement of the Michel parameter in muon decay under the direction of Allan Sachs. This was followed by a teaching and research position at Cornell University and relocation to Fermilab, initially on sabbatical, in 1971.

John officially joined the Fermilab staff in 1975 as head of the Research Division. His tenure included the discovery of the upsilon particle (b-quark bound state) by Leon Lederman’s team in 1977. He also held responsibilities for the upgrading of the experimental areas to accept beams of up to 1 TeV in anticipation of the completion of the Fermilab Tevatron.

In 1981, following Lederman’s decision to utilise the Tevatron as a proton–antiproton collider, John was appointed head of the TeV-I Project, with responsibility for the construction of the Antiproton Source and the collision hall for the CDF detector. Under John’s leadership, a novel design was developed, building on the earlier pioneering work done at CERN for antiproton accumulation based on stochastic cooling, and proton–antiproton collisions were achieved in the Tevatron four years later, in 1985.

Tireless commitment

John succeeded Lederman to become Fermilab’s third director in July 1989, shortly after the decision to locate the Superconducting Super Collider (SSC) in Waxahachie, Texas, creating immense challenges to Fermilab’s future. John guided the US community to a plan for a new accelerator, the Main Injector (and ultimately the Recycler), that could support a high-luminosity collider programme for the decade of SSC construction while simultaneously providing high-intensity extracted beams for a future neutrino programme that could sustain Fermilab well beyond the SSC’s startup. The cancellation of the SSC in 1993 was a seismic event for US and global high-energy physics, and ensured the Tevatron’s role as the highest energy collider in the world for the next almost two decades. John was asked to lead the termination phase of the SSC lab. In 1994/1995, as director of both Fermilab and the SSC, he worked on this painful task with a special emphasis on helping the many suddenly unemployed people find new career paths.

During John’s tenure as director, Fermilab produced many important physics results. In 1995, the Tevatron Collider experiments, CDF and D∅, announced the discovery of the top quark, the final quark predicted in the Standard Model of particle physics at the mass of more than 175 times that of the proton. To ensure that the experiments could analyse their data quickly and efficiently, John supported replacing costly mainframe computers with “clusters” of inexpensive microprocessors developed in industry for personal computers and later laptops and phones. The final fixed-target run with 800 GeV extracted beam in 1997 and 1998 helped resolve an important and long-standing problem in CP violation in kaon decays and discovered the tau neutrino.

His leadership both enhanced international collaboration and retained a prominent role for Fermilab in collider physics

From 1993–1997, John served as chair of the International Committee for Future Accelerators (ICFA). He stepped down after two terms as Fermilab director in 1999. In 2010, he received the Robert R. Wilson Prize for Achievement in the Physics of Particle Acceleration from the American Physical Society.

Under John’s influence, there were frequent personnel exchanges between Fermilab and CERN throughout the 1980s, as Fermilab staff benefited from CERN’s experience with antiproton production and CERN benefited from Fermilab’s experience with the operations of a superconducting accelerator. These exchanges extended into the 1990s, and following the termination of the SSC, John was instrumental in securing support for US participation in the LHC accelerator and detector projects. His leadership both enhanced international collaboration and retained a prominent role for Fermilab in collider physics after the Tevatron completed operations in 2011.

During the 1980s, astrophysics became an important contributor to our knowledge of particle physics and required more ambitious experiments with strong synergies with the latest round of HEP experiments. In 1991, John formed the Experimental Astrophysics Group at Fermilab. This led to its strong participation in the Sloan Digital Sky Survey (SDSS), the Pierre Auger Cosmic Ray Observatory, the Cryogenic Dark Matter Search (CDMS) and the Dark Energy Survey (DES), of which John became director in 2003. John’s vision of a vibrant community of particle physicists, astrophysicists and cosmologists exploring the inner space-outer space connection is now reality.

Those of us who had the privilege of knowing and working with John were challenged by his intense work ethic and by the equally intense flood of new ideas for running and improving our programmes. He was a gifted and dedicated experimental physicist, skilled in accelerator science, an expert in superconducting magnet design and technology, a superb manager, and a great recruiter and mentor of young engineers and scientists, including the authors of this article. We will miss him!

The post John Peoples 1933–2025 appeared first on CERN Courier.

Together with Ove Nathan, Ole oversaw a proposal to build a large tandem accelerator at the Niels Bohr Institute department located at Risø, near Roskilde. The government and research authorities had supported the costly project, but it was scrapped on an afternoon in August 1978 as a last-minute saving to help establish a coalition between the two parties across the centre of Danish politics. Ole’s disappointment was enormous: he decided to take up an offer at Brookhaven National Laboratory (BNL) to continue his nuclear work there, while Nathan threw himself into university politics and later became rector of the University of Copenhagen.

Deep exploration

Ole sent his resignation as a professor at the University of Copenhagen to the Queen – a civil servant had to do so at the time – but was almost immediately confronted with demands for cutbacks at BNL, which would stop the research programme with the tandem accelerator there. Ole did not withdraw his resignation, but together with US colleagues proposed a research programme at very high energies by injecting ions from the tandem into the existing particle accelerator, AGS, thereby achieving energies in the nucleon–nucleon centre-of-mass system of up to 5 GeV. This was the start of the exploration of the deeper structure of nuclear matter, which is revealed as a system consisting of quarks and gluons at temperatures of billions of degrees. This later led to the construction of the first atomic nucleus collision machine, the Relativistic Heavy Ion Collider (RHIC) in the US. Ole himself participated in the E802 and E866 experiments at BNL/AGS, and in the BRAHMS experiment at RHIC.

Ole will be remembered as the first director of the unified Niels Bohr Institute and for establishing the Danish National Research Foundation

Ole will also be remembered as the first director, called back from the US, of the unified Niels Bohr Institute, which was established in 1993 as a fusion of the physics, astronomy and geophysics departments surrounding the Fælledparken commons in Copenhagen after an international panel chaired by him had recommended a merger. Ole realised the necessity of merging the departments in order to create the financial room for manoeuvre needed to be able to hire new and younger researchers again. He left his mark on the construction, which initially had to deal with the very different cultures of the Blegdamsvej, Ørsted and Geophysics institutes. He approached the task efficiently but with a good understanding and respect for the scientific perspectives and the individual researchers.

Back in Denmark, Ole played a significant role in the establishment of the competitive research system we know today, including the establishment of the Danish National Research Foundation (DNRF), of which he was vice-chair in the first years, and with the streamlining of the institute’s research and the establishment of several new areas.

Strong interests

Despite the scale of all his administrative tasks, Ole maintained a lively interest in research and actively supported the establishment of the Centre for CERN Research (now the NICE National Instrument Center) together with the author of this obituary. He was also a member of the CERN Council during the exciting period when the LHC took shape.

Ole will be remembered as an open-minded, energetic and visionary man with an irreverent sense of humour that some feared but others greatly appreciated. Despite his modest manner, he influenced his colleagues with his strong interest in new physics and his sharp scepticism. If consulted, he would probably turn his nose up at the word “loyal”, but he was ever a good and loyal friend. He is survived by his wife, Ruth, and four children.

The post Ole Hansen 1934–2025 appeared first on CERN Courier.

Born in Turin in 1959, Michele graduated in physics from the University of Torino in 1982. He was awarded a Fulbright Fellowship to pursue graduate studies at Princeton University, where he received his MA in 1985 and his PhD in 1992. He began his career as a staff researcher at INFN Torino, before moving to academia as an associate professor at the University of Calabria and then, from 1995, at the University of Piemonte Orientale in Novara, where he became full professor in 2002.

Michele’s research career began with the European Muon Collaboration (NA2 and NA9) and the New Muon Collaboration (NA37) at CERN, investigating the structure of nucleons through the deep inelastic scattering of muons. He went on to play a leading role in the ZEUS experiment at DESY’s HERA collider, focusing on the diffractive physics programme, coordinating groups in Torino and Novara, and overseeing the operation of the Leading Proton Spectrometer. Awarded an Alexander von Humboldt fellowship, he worked at DESY between 1996 and 1999.

With the start of the LHC era, Michele devoted his efforts to CMS, becoming a central figure in diffractive physics and the relentless force behind the construction of the CMS Precision Proton Spectrometer (PPS) and the subsequent merging of the TOTEM and CMS collaborations. He was convener of the diffractive physics group, served on the CMS Publication and Style committees, and from 2014 chaired the Institution Board of the CMS PPS, where he was also resource manager and INFN national coordinator. He had been appointed as chairperson of the CMS Collaboration Board, a role that he was due to begin this year.

A central figure in diffractive physics and the relentless force behind the construction of the Precision Proton Spectrometer

Teaching was central to Michele’s vocation. At the University of Piemonte Orientale, he developed courses on radiation physics for medical students and radiology specialists, building bridges between particle physics and medical applications. He was also widely recognised as a dedicated mentor, always attentive to the careers of younger collaborators.

We will remember Michele as a very talented physicist and a genuinely kind person, who had the style and generosity of a bygone era. Always approachable, he could be found with a smile, a sincere interest in others’ well-being, and a delicate sense of humour that brought lightness to professional exchanges. His students and collaborators valued his constant encouragement and his passion for transmitting enthusiasm for physics and science.

While leaving a lasting mark on physics and on the institutions he served, Michele also cultivated enduring friendships and dedicated himself fully to his family, to whom the thoughts of the CMS and wider CERN communities go at this difficult time.

Michele, “Rest forever here in our hearts”.

The post Michele Arneodo 1959–2025 appeared first on CERN Courier.

Originally an employee of the Italian National Committee for Nuclear Energy (CNEN), Miro had a long career as a key figure in the INFN institutions.

He made his mark at the pioneering ADONE collider in the 1970s, optimising its performance, developing an innovative luminosity monitor, and improving the machine optics and injection system. Later he served as the director of ADONE, participating in all second-generation experiments, colliding beams for particle physics and producing synchrotron radiation and gamma rays for nuclear physics.

Beyond LNF, Miro played an important role in the design of the Italian synchrotron radiation source ELETTRA in Trieste, and the ESRF in Grenoble; he also collaborated on many other accelerator projects, including CTF3 and CLIC at CERN.

Miro made outstanding contributions to the DAΦNE collider project, leading the realisation of the electron–positron injection system

Miro held many institutional roles, and as head of the Accelerator Physics Service, he taught the art and science of accelerators to many young scientists, with clarity, patience and dedication. As a mentor, he leaves a legacy of accelerator experts who have ensured the success of many LNF initiatives.

Miro made outstanding contributions to the DAφNE collider project from the beginning, leading the design and realisation of the entire electron–positron injection system. He was deeply involved in the very challenging commissioning and achieving the high luminosity that was required by the experiments.

Besides his characteristic dynamism, one of Miro’s distinctive traits was his ability to foster harmonious collaboration among technicians, technologists and researchers.

Away from physics, Miro was an excellent tennis player and skier, along with being a skilled sailor, activities that he often shared with colleagues.

The post Miro Preger 1946–2025 appeared first on CERN Courier.

In October, the Circular Electron–Positron Collider (CEPC) study group completed its full suite of technical design reports, marking a key step for China’s Higgs-factory proposal. However, CEPC will not be considered for inclusion in China’s next five-year plan (2026–2030).

“Although our proposal that CEPC be included in the next five-year plan was not successful, IHEP will continue this effort, which an international collaboration has developed for the past 10 years,” says study leader Wang Yifang, of the Institute of High Energy Physics (IHEP) in Beijing. “We plan to submit CEPC for consideration again in 2030, unless FCC is officially approved before then, in which case we will seek to join FCC, and give up CEPC.”

Electroweak precision

CEPC has been under development at IHEP since shortly after the discovery of the Higgs boson at CERN in 2012. To enable precision studies of the new particle, Chinese physicists formally proposed a dedicated electron–positron collider in September 2012. Sharing a concept similar to the Future Circular Collider (FCC) proposed in parallel at CERN, CEPC’s high-luminosity collisions would greatly improve precision in measuring Higgs and electroweak processes.

“CEPC is designed as a multi-purpose particle factory,” explains Wang. “It would not only serve as an efficient Higgs factory but would also precisely study other fundamental particles, and its tunnel can be re-used for a future upgrade to a more powerful super proton–proton collider.”

Following completion of the Conceptual Design Report in 2018, which defined the physics case and baseline layout, the CEPC collaboration entered a detailed technical phase to validate key technologies and complete subsystem designs. The accelerator Technical Design Report (TDR) was released in 2023, followed in October 2025 by the reference detector TDR, providing a mature blueprint for both components.

Although our proposal that CEPC be included in the next five-year plan was not successful, IHEP will continue this effort

Wang Yifang

Compared to the 2018 detector concept, the new technical report proposes several innovations. An electromagnetic calorimeter based on orthogonally oriented crystal bars and a hadronic calorimeter based on high-granularity scintillating glass have been optimised for advanced particle-flow algorithms, improving their energy resolution by a factor of 10 and a factor of two, respectively. A tracking detector employing AC-coupled low-gain avalanche-diode technology will enable simultaneous 10 µm position and 50 ps time measurements, enhancing vertex and flavour tagging. Meanwhile, a readout chip developed in 55 nm technology will achieve state-of-the-art performance at 65% power consumption, enabling better resolution, large-scale integration and reduced cooling-pipe materials. Among other advances, a new type of high-density, high-yield scintillating glass forms the possibility for a full absorption hadronic calorimeter.

To ensure the scientific soundness and feasibility of the design, the CEPC Study Group established an International Detector Review Committee in 2024, chaired by Daniela Bortoletto of the University of Oxford.

Design consolidation

“After three rounds of in-depth review, the committee concluded in September 2025 that the Reference Detector TDR defines a coherent detector concept with a clearly articulated physics reach,” says Bortoletto. “The collaboration’s ambitious R&D programme and sustained technical excellence have been key to consolidating the major design choices and positioning the project to advance from conceptual design into integrated prototyping and system validation.”

CEPC’s technical advance comes amid intense international interest in participating in a Higgs factory. Alongside the circular FCC concept at CERN, Higgs factories with linear concepts have been proposed in Europe and Japan, and both Europe and the US have named constructing or participating in a Higgs factory as a strategic priority. Following China’s decision to defer CEPC, attention now turns to Europe, where the ongoing update of the European Strategy for Particle Physics will prioritise recommendations for the laboratory’s flagship collider beyond the HL-LHC. Domestically, China will consider other large science projects for the 2026 to 2030 period, including a proposed Super Tau–Charm Facility to succeed the Beijing Electron–Positron Collider II.

With completion of its core technical designs, CEPC now turns to engineering design.

“The newly released detector report is the first dedicated to a circular electron–positron Higgs factory,” says Wang. “It showcases the R&D capabilities of Chinese scientists and lays the foundation for turning this concept into reality.”

The post CEPC matures, but approval is on hold appeared first on CERN Courier.

A community-driven process is building consensus

CERN Council president Costas Fountas sums up the vision of CERN’s Member States.

In March 2024, the CERN Council called on the particle-physics community to develop a visionary and concrete plan that greatly advances human knowledge in fundamental physics through the realisation of the next flagship project at CERN. This community-driven strategy will be submitted to the CERN Council in March 2026, leading to discussions among CERN Member States. The CERN Council will update the European strategy for particle physics (ESPP) based on these deliberations, with a view to approving CERN’s next flagship collider in 2028.

This third update to the ESPP builds on a process initiated by the CERN Council in 2006 and updated in 2013 and 2020. It is designed to convey to the CERN Council the views of the community on strategic questions that are key to the future of high-energy physics (HEP). The process involves all CERN Member States and Associate Member States, with the goal of developing a roadmap for the field for many years to come. The CERN Council asked that the newly updated ESPP should take into account the status of implementation of the 2020 ESPP, recent accomplishments at the LHC and elsewhere, progress in the construction of the High-Luminosity LHC (HL-LHC), the outcome of the Future Circular Collider (FCC) Feasibility Study, recent technological developments in accelerator, detector and computing technology, and the international landscape of the field. Scientific inputs were requested from across the community.

On behalf of the CERN Council, I would like to thank the high-energy community for understanding that this is a critical time for our field and participating very actively. Throughout this time, the various national groups have held a large number of meetings to debate which would be the best accelerator to be hosted at CERN after the HL-LHC. They also discussed and proposed alternative options as requested by the CERN Council, which followed the process closely.

By June 2025 we were delighted to hear from the ESPP secretariat that the participation of the community had been overwhelming and that a very large number of proposals had been submitted (CERN Courier May/June 2025 p8). These submissions show a broad consensus that CERN should be maintained as the global centre for collider physics through the realisation of a new flagship project. Europe’s strategy should be ambitious, innovative and forward looking. An overwhelming majority of the communities from CERN Member States express their strong support for the FCC programme, starting with an electron–positron collider (FCC-ee) as a first stage. Their strong support is largely based on its superb physics potential and its long-term prospects, given the potential to explore the energy frontier with a hadron collider (FCC-hh) following a precision era at FCC-ee.

This strategy coherently develops the vision of ESPP 2020, which recommended to the CERN Council that an electron–positron Higgs factory be the highest-priority next collider. The 2020 ESPP update further recommended that Europe, together with its international partners, should investigate the technical and financial feasibility of a future hadron collider at CERN with a centre-of-mass energy of at least 100 TeV and with an electron–positron Higgs and electro­weak factory as a possible first stage. Such a feasibility study of the colliders and related infrastructure should be established as a global endeavour and be completed on the timescale of the next strategy update.

Based on ESPP 2020, the CERN Council mandated the CERN management to undertake a feasibility study for the FCC and approved an initial budget of CHF 100 million over a five-year period. Throughout the past five years, the FCC feasibility study was undertaken by CERN management under the oversight of the CERN Council. Council heard presentations on its progress at every session and carefully scrutinised a very successful mid-term review (CERN Courier March/April 2024 p25). The FCC collaboration completed the FCC feasibility study ahead of schedule and summarised the results of the study in a three-volume report that was released in March 2025 (CERN Courier May/June 2025 p8). The results are currently under review by panels which will scrutinise both the scientific aspects of the project as well as its budget estimates. The project will be presented to the Scientific Policy and Finance committees in September 2025 and to the CERN Council in November 2025.

It is rewarding to see that the scientific opinion of the community is in sync with ESPP 2020, the decision of the CERN Council to initiate the FCC feasibility study, and the efforts of CERN management to steer and complete it. This is a sign of the strength of the HEP community. While respecting a healthy diversity of opinion, a clear consensus has emerged across the community that the FCC is the highest priority project.

Crucially, however, the CERN Council requested that the community provide not only the scientifically most attractive option, but also hierarchically ordered alternative options. Specifically, the Council requested that the strategy update should include the preferred option for the next collider at CERN and prioritised alternative options to be pursued if the chosen preferred plan turns out not to be feasible or competitive. No consensus has yet been reached here, however two projects have the required readiness to be candidates for alternative programmes: the Linear Collider Facility (LCF, 250 GeV) and the Compact Linear Collider (CLIC, 380 GeV), with additional R&D required in the latter case. A third proposal, LEP3, also requires further study, but could be a promising candidate for a Higgs factory in the existing LEP/LHC tunnel, albeit at a significantly reduced luminosity relative to FCC-ee.

On behalf of the CERN Council, I would like to thank the high-energy community for understanding that this is a critical time for our field and participating very actively

The R&D for several of these projects has been supported by CERN for a long time. Research on linear colliders has been an active programme for the past 30 years and has received significant support, not only ensuring their readiness for consideration as future HEP facilities, but also sparking an exceptional R&D programme in the applications of fundamental research, for example in accelerators for cancer treatment (CERN Courier July/August 2024 p46). Over the past five years, CERN has also invested in muon colliders and hosts the International Muon Collider Collaboration. CERN also leads research into the application of plasma-wakefield acceleration for fundamental physics, having supported the AWAKE experiment for 10 years now (CERN Courier May/June 2024 p25).

The next milestone for updating the ESPP is 14 November: the deadline for submission of the final national inputs. The final drafting session of the strategy update will then take place from 1 to 5 December 2025 at Monte Verità Ascona, where the community recommendations will be finalised. These will be presented to the CERN Council in March 2026 and discussed at a dedicated meeting of the CERN Council in May 2026 in Budapest.

Meanwhile, a key milestone for community deliberations recently passed. The full spectrum of community inputs was presented and debated at an Open Symposium held in Venice in June. As strategy secretary Karl Jakobs reports on the following pages, the symposium was a smashing success with lively discussions and broad participation from our community. On behalf of Council, I would like to convey my sincere thanks to the Italian delegation for the superb organisation of the symposium.

Costas Fountas has served as president of the CERN Council since his appointment in January this year, and as the Greek scientific delegate to the Council since 2016. A professor of physics at the University of Ioannina and longstanding member of the CMS collaboration, he previously served as vice-president of the Council from 2022 to 2024. (Image credit: M Brice, CERN)

Venice symposium debates decades of collider strategy

Strategy secretary Karl Jakobs reports from a vibrant Open Symposium in Venice.

The Open Symposium of the European Strategy for Particle Physics (ESPP) brought together more than 600 physicists from almost 40 countries in Venice, Italy, from 23 to 27 June, to debate the future of European particle physics. In the focus was the discussion on the next large-scale accelerator project at CERN to follow the HL-LHC, which is scheduled to operate until the end of 2041. The strategy update should – according to the remit defined by the CERN Council – define a preferred option for the next collider and prioritised alternative options to be pursued if the preferred plan turns out not to be feasible or competitive. In addition, the strategy update should indicate areas of priority for exploration complementary to colliders and other experiments to be considered at CERN and at other European laboratories, as well as for participation in projects outside Europe.

The Open Symposium is an important step in the strategy process. The aim is to involve the full community in discussions of the 266 scientific contributions that had been submitted by the community to the ESPP process before the symposium (CERN Courier May/June 2025 p8).

In the opening session of the symposium CERN Director-General Fabiola Gianotti summarised the impressive achievements of the CERN community in the implementation of the recommendations from the 2020 update to the ESPP. Eric Laenen (Nikhef) stressed that the outstanding questions in particle physics require a broad and diverse experimental programme, including the HL-LHC, a new flagship collider, and a wide variety of other experiments including those in neighbouring fields. A broad consensus emerged that a future collider programme should be realised that can fully leverage both precision and energy, covering the widest range of observables at different energy scales. To match experimental precision, significant progress on the theoretical side is also required, in particular regarding higher-order calculations.

An important part of the symposium was devoted to presentations of possible future large-scale accelerator projects. Detailed presentations were given on the FCC-ee and FCC-hh colliders, either in the integrated FCC programme or proceeding directly to FCC-hh as a standalone realisation at an earlier time. Linear colliders were presented as alternative options, with a Linear Collider Facility (LCF) based on the design of the International Linear Collider (ILC) and CLIC both considered. In addition, smaller collider options were presented, based on re-using the LHC/LEP tunnel. A first proposal, LEP3, suggests accelerating electrons and positrons up to energies of 230 GeV, while a second proposal, LHeC, proposes the realisation of electron–proton collisions in one interaction point of the LHC. LHeC would require the construction of an additional new energy-recovery linac for the acceleration of electrons.

Moving focus from the precision frontier to the energy frontier, several ways to reach the 10 TeV “parton scale” were presented. (Comparisons between the energy reach of hadron and lepton colliders must discuss parton–parton centre-of-mass energies, where partons refer to the pointlike constituents of hadrons, as only a fraction of the energy of collisions between composite particles can be used to probe the existence of new particles and fields.) If FCC-ee is realised, a natural path is to proceed with proton-proton collisions with proton–proton centre-of-mass energies in the range of 85 to 120 TeV, depending on the available high-field magnet technology. As an alternative, a muon collider could provide a path towards high-energy lepton collisions, however, demonstrations of how to address the significant technological challenges, such as six-dimensional cooling in transverse and longitudinal phase space, and other items associated with the various acceleration steps, need to be achieved. Likewise, plasma-based acceleration techniques for electrons and positrons capable of exceeding the 1 TeV energy scale are yet to be demonstrated.

A broad consensus emerged that a future collider programme should be realised that can fully leverage both precision and energy

The symposium was organised to foster strong engagement by the community in discussion sessions. Six physics topics – covering electroweak physics, strong interactions, flavour physics, physics beyond the Standard Model, neutrino physics and cosmic messengers, and dark matter and the dark sector, as well as the three technology areas on accelerators, detectors and computing, were summarised in rapporteur talks, followed by 45-minute discussions, where the people present in Venice strongly engaged.

For the study of precision Higgs measurements, the performance of all the considered electron–positron (e⁺e^–) colliders is comparable. While a sub-percent precision can be reached in several measurements of Higgs couplings to fermions and bosons, HL-LHC measurements would prevail for rare processes. On the determination of the important Higgs-boson (H) self-coupling, the precision obtained at the HL-LHC will prevail until either e⁺e^– linear colliders can improve it in direct HH production measurements at collision energies above 500 GeV, or before precisions at the level of a few percent can be reached at FCC-hh or a muon collider. It was further stressed that precision measurements in the Higgs, electroweak (Z, W, top) and flavour physics constitute three facets for indirect discoveries and that their synergy is essential to maximise the discovery potential of future colliders. Due to its high luminosity at low energies and its four experiments, the FCC-ee shows a superior physics performance in the electroweak programme.

In flavour physics, a lot of progress will be achieved in the coming decade by the LHCb and Belle-II experiments. While the tera-Z production at a future FCC-ee would provide a major step forward, the giga-Z data samples available at linear colliders do not seem to be a good option for flavour physics. The FCC-ee and LHeC would also achieve high precision on QCD measurements, leading, for example, to a per-mille level determination of the strong coupling constant α_s. The important investigations of the quark–gluon plasma at the HL-LHC could be continued in parallel to an e⁺e^– collider operation at CERN at the SPS fixed target programme, before FCC-hh would eventually allow for novel studies in the high-temperature QCD domain.

Keeping diversity in the particle-physics programme was also felt to be essential: the next collider project should not come at the expense of a diverse scientific programme in Europe. Given that we do not know where new physics will show up, ensuring a diverse and comprehensive physics programme is vital, including fixed-target, neutrino, flavour, astroparticle and nuclear-physics experiments. Experiments in these areas have the potential for groundbreaking discoveries.

The discussions in Venice revealed a community united in its desire for a future flagship collider at CERN

At the technology frontier, essential work on accelerator R&D, such as on high-field and high-temperature superconducting magnets and RF systems, remain a high priority and appropriate investments must be made. R&D on advanced acceleration concepts should continue with adequate effort to prepare future projects. In the detector area, the establishment of the Detector Research & Development (DRD) collaborations as a result of the implementation of the recommendations of the 2020 ESPP update were considered to provide a solid basis to tackle the challenges related to the developments for high-performing detectors for future colliders and beyond. It is also expected that the required software and computing challenges for future colliders can be mastered, provided that adequate person power and funding are available and adaptations to new technologies, in particular GPUs, AI and – on a longer timescale – quantum computing, can be made.

The discussions in Venice revealed a community united in its desire for a future flagship collider at CERN. Over the past years, very significant progress has been made in this direction, and the discussions on the prioritisation of collider options will continue over the next months. In addition to the FCC-ee, linear colliders (LCF, CLIC) present mature options for a Higgs factory at CERN. LEP3 and LHeC could alternatively be considered as intermediate collider projects, followed by a larger accelerator capable of exploring the 10 TeV parton scale.

The differences in the physics potential between the various collider options will be documented in the Physics Briefing Book that will be released by the Physics Preparatory Group by the end of September. In parallel, the technical readiness, risks, timescales and costs will be reviewed by the European Strategy Group (ESG). Alongside the final national inputs, these assessments will provide the foundation for the final recommendations to be drafted by the ESG in early December 2025.

Karl Jakobs is the secretary of the 2026 update to the European strategy for particle physics. A professor at the University of Freiburg, Jakobs served as spokesperson of the ATLAS collaboration from 2017 to 2021 and as chairman of the European Committee for Future Accelerators from 2021 to 2023. (Image credit: K Jakobs)

The post Europe’s collider strategy takes shape appeared first on CERN Courier.

This new “charmonium” state was the first example of quarkonium: a heavy quark bound to an antiquark of the same flavour. It was named by analogy to positronium, a bound state of an electron and a positron, which decays by mutual annihilation into two or three photons. Composed of unstable quarks, bound by gluons rather than photons, and decaying mainly via the annihilation of their constituent quarks, quarkonia have fascinated particle physicists ever since.

The charmonium interpretation of the J/ψ was cemented by the subsequent discovery of a spectrum of related cc–states, and ultimately by the observation of charmed particles in 1976. The discovery of charmonium was followed in 1977 by the identification of bottomonium mesons and particles containing bottom quarks. While toponium – a bound state of a top quark and antiquark – was predicted in principle, most physicists thought that its observation would have to wait for the innate precision of a next-generation e⁺e^– collider following the LHC, in view of the top quark’s large mass and exceptionally rapid decay, more than 10¹² times quicker than the bottom quark. The complex environment at a hadron collider, where the composite nature of protons precludes knowledge of the initial collision energy of pairs of colliding partons within them, would make toponium particularly difficult to identify at the LHC.

However, in the second half of 2024, the CMS collaboration reported an enhancement near the threshold for tt production at the LHC, which is now most plausibly interpreted as the lowest-lying toponium state. The existence of this enhancement has recently been corroborated by the ATLAS collaboration (see”ATLAS confirms top–antitop excess“).

Here are the personal memories of an eyewitness who followed these 50 years of quarkonium discoveries firsthand.

Strangeonium?

In hindsight, the quarkonium story can be thought to have begun in 1963 with the discovery of the φ meson. The φ was an unexpectedly stable and narrow resonance, decaying mainly into kaons rather than the relatively light pions, despite lying only just above the KK threshold. Heavier quarkonia cannot decay into a pair of mesons containing single heavy quarks, as their masses lie below the energy threshold for such “open flavour” decays.

The preference of the φ to decay into kaons was soon interpreted by Susumu Okubo as a consequence of approximate SU(3) flavour symmetry, developing mathematical ideas based on unitary 3 × 3 matrices with a determinant one. At the beginning of 1964, quarks were proposed and George Zweig suggested that the φ was a bound state of a strange quark and a strange anti-quark (or aces as he termed them). After 1974, the portmanteau word “strangeonium” was retrospectively applied to the φ and similar heavier ss bound states, but the name has never really caught on.

In the year or so prior to the discovery of the J/ψ in November 1974, there was much speculation about data from the Cambridge Electron Accelerator (CEA) at Harvard and the Stanford Positron–Electron Asymmetric Ring (SPEAR) at SLAC. Data from these e⁺e^– colliders indicated a rise in the ratio, R, of cross-sections for hadron and μ⁺μ^– production (see “Why is R rising?” figure). Was this a failure of the parton model that had only recently found acceptance as a model for the apparently scale-invariant internal structure of hadrons observed in deep-inelastic scattering experiments? Did partons indeed have internal structure? Or were there “new” partons that had not been seen previously, such as charm or coloured quarks? I was asked on several occasions to review the dozens of theoretical suggestions on the market, including at the ICHEP conference in the summer of 1974. In preparation, I toted a large Migros shopping bag filled with dozens of theoretical papers around Europe. Playing the part of an objective reviewer, I did not come out strongly in favour of any specific interpretation, however, during talks that autumn in Copenhagen and Dublin, I finally spoke out in favour of charm as the best-motivated explanation of the increase in R.

November revolution

Then, on 11 November 1974, the news broke that two experimental groups, one working at BNL under the leadership of Sam Ting and the other at SLAC led by Burt Richter, had discovered, in parallel, the narrow vector boson that bears the composite name J/ψ (see “Charmonium” figure). The worldwide particle-physics community went into convulsions (CERN Courier November/December 2024 p41) – and the CERN Theory Division was no exception. We held informal midnight discussion sessions around an open-mic phone with Fred Gilman in the SLAC theory group, who generously shared with us the latest J/ψ news. Away from the phone, like many groups around the world, we debated the merits and demerits of many different theoretical ideas. Rather than write a plethora of rival papers about these ideas, we decided to bundle our thoughts into a collective preprint. Instead of taking individual responsibility for our trivial thoughts, the preprint was anonymous, the place of the authors’ names being taken by a mysterious “CERN Theory Boson Workshop”. Eagle eyes will spot that the equations were handwritten by Mary K Gaillard (CERN Courier July/August 2025 p47). Informally, we called ourselves Co-Co, for communication collective. With “no pretentions to originality or priority,” we explored five hypotheses: a hidden charm vector meson, a coloured vector meson, an intermediate vector boson, a Higgs meson and narrow resonances in strong interactions.

My immediate instinct was to advocate the charmonium interpretation of the J/ψ, and this was the first interpretation to be described in our paper. This was on the basis of the Glashow–Iliopoulos–Maiani (GIM) mechanism, which accounted for the observed suppression of flavour-changing neutral currents by postulating the existence a charm quark with a mass around 2 GeV (see CERN Courier July/August 2024 p30), and the Zweig rule, which suggested phenomenologically that quarkonia do not easily decay by quark–antiquark annihilation via gluons into other flavours of quarks. So I was somewhat surprised when one of the authors of the GIM paper wrote a paper proposing that it might be an intermediate electroweak vector boson. A few days after the J/ψ discovery came the news of the (almost equally narrow) ψ′ discovery, which I was told as I was walking along the theory corridor to my office one morning. My informant was a senior theorist who was convinced that this discovery would kill the charmonium interpretation of the J/ψ. However, before I reached my office I realised that an extension of the Zweig rule would also suppress ψ′ → J/ψ + light meson decays, so the ψ′ could also be narrow.

Keen competition

The charmonium interpretation of the J/ψ and ψ′ states predicted that there should be intermediate P-wave states (with one unit of orbital angular momentum) that could be detected in radiative decays of the ψ′. In the first half of 1975 there was keen competition between teams at SLAC and DESY to discover these states. That summer I was visiting SLAC, where I discovered one day under the cover of a copying machine, before their discovery was announced, a sheet of paper with plots showing clear evidence for the P-wave states. I made a copy, went to Burt Richter’s office and handed him the sheet of paper. I also asked whether he wanted my copy. He graciously allowed me to keep it, as long as I kept quiet about it, which I did until the discovery was officially announced a few weeks later.

The story of quarkonium can be thought to have begun in 1963 with the discovery of the φ meson

Discussion about the interpretation of the new particles, in particular between advocates of charm and Han–Nambu coloured quarks – a different way to explain the new particles’ astounding stability by giving them a new quantum number – rumbled on for a couple of years until the discovery of charmed particles in 1976. During this period we conducted some debates in the main CERN auditorium moderated by John Bell. I remember one such debate in particular, during which a distinguished senior British theorist spoke for coloured quarks and I spoke for charm. I was somewhat taken aback when he described me as representing the “establishment”, as I was under 30 at the time.

Over the following year, my attention wandered to grand unified theories, and my first paper on the subject was with Michael Chanowitz and Mary K Gaillard, which we completed in May 1977. We realised while writing this paper that simple grand unified theories – which unify the electroweak and strong interactions – would relate the mass of the τ heavy lepton that had been discovered in 1975 to the mass of the bottom quark, which was confidently expected but whose mass was unknown. Our prediction was m_b/m_τ = 2 to 5, but we did not include it in the abstract. Shortly afterwards, while our paper was in proof, the discovery of the ϒ state (or states) by a group at Fermilab led by Leon Lederman (see “Bottomonium” figure) became known, implying that m_b ~ 4.5 GeV. I added our successful mass prediction by hand in the margin of the corrected proof. Unfortunately, the journal misunderstood my handwriting and printed our prediction as m_b/m_τ = 2605, a spectacularly inaccurate postdiction! It remains to be seen whether the idea of a grand unified theory is correct: it also predicted successfully the electroweak mixing angle θ_W and suggested that neutrinos might have mass, but direct evidence, such as the decay of the proton, has yet to be found.

Peak performance

Meanwhile, buoyed by the success of our prediction for m_b, Mary K Gaillard, Dimitri Nanopoulos, Serge Rudaz and I set to work on a paper about the phenomenology of the top and bottom quarks. One of our predictions was that the first two excited states of the ϒ, the ϒ′ and ϒ′′, should be detectable by the Lederman experiment because the Zweig rule would suppress their cascade decays to lighter bottomonia via light-meson emission. Indeed, the Lederman experiment found that the ϒ bump was broader than the experimental resolution, and the bump was eventually resolved into three bottomonium peaks.

It was in the same paper that we introduced the terminology of “penguin diagrams”, wherein a quark bound in a hadron changes flavour not at tree level via W-boson exchange but via a loop containing heavy particles (like W bosons or top quarks), emitting a gluon, photon or Z boson. Similar diagrams had been discussed by the ITEP theoretical school in Moscow, in connection with K decays, and we realised that they would be important in B-hadron decays. I took an evening off to go to a bar in the Old Town of Geneva, where I got involved in a game of darts with the experimental physicist Melissa Franklin. She bet me that if I lost the game I had to include the word “penguin” in my next paper. Melissa abandoned the darts game before the end, and was replaced by Serge Rudaz, who beat me. I still felt obligated to carry out the conditions of the bet, but for some time it was not clear to me how to get the word into the b-quark paper that we were writing at the time. Then, another evening, after working at CERN, I stopped to visit some friends on my way back to my apartment, where I inhaled some (at that time) illegal substance. Later, when I got home and continued working on our paper, I had a sudden inspiration that the famous Russian diagrams look like penguins. So we put the word into our paper, and it has now appeared in almost 10,000 papers.

What of toponium, the last remaining frontier in the world of quarkonia? In the early 1980s there were no experimental indications as to how heavy the top quark might be, and there were hopes that it might be within the range of existing or planned e⁺e^– colliders such as PETRA, TRISTAN and LEP. When the LEP experimental programme was being devised, I was involved in setting “examination questions” for candidate experimental designs that included asking how well they could measure the properties of toponium. In parallel, the first theoretical papers on the formalism for toponium production in e⁺e^– and hadron–hadron collisions appeared.

Toponium will be a very interesting target for future e⁺e^– colliders

But the top quark did not appear until the mid-1990s at the Tevatron proton–antiproton collider at Fermilab, with a mass around 175 GeV, implying that toponium measurements would require an e⁺e^– collider with an energy much greater than LEP, around 350 GeV. Many theoretical studies were made of the cross section in the neighbourhood of the e⁺e^– → tt threshold, and how precisely the top quark mass, electroweak and Higgs couplings could be measured.

Meanwhile, a smaller number of theorists were calculating the possible toponium signal at the LHC, and the LHC experiments ATLAS and CMS started measuring tt production with high statistics. CMS and ATLAS embarked on programmes to search for quantum-mechanical correlations in the final-state decay products of the top quarks and antiquarks, as should occur if the tt state were to be produced in a specific spin-parity state. They both found decay correlations characteristic of tt production in a pseudoscalar state: it was the first time such a quantum correlation had been observed at such high energies.

The CMS collaboration used these studies to improve the sensitivities of dedicated searches they were making for possible heavy Higgs bosons decaying into tt final states, as would be expected in many extensions of the Standard Model. Intriguingly, hints of a possible excess of events around the tt threshold with the type of correlation expected from a pseudoscalar tt state began to emerge in the CMS data, but initially not with high significance.

Pseudoscalar states

I first heard about this excess at an Asia–CERN physics school in Thailand, and started wondering whether it could be due to the lowest-lying toponium state, which would decay predominantly into unstable top quarks and antiquarks rather than via their annihilation, or to a heavy pseudoscalar Higgs boson, and how one might distinguish between these hypotheses. A few years previously, Abdelhak Djouadi, Andrei Popov, Jérémie Quevillon and I had studied in detail the possible signatures of heavy Higgs bosons in tt final states at the LHC, and shown that they would have significant interference effects that would generate dips in the cross-section as well as bumps.

The significance of the CMS signal subsequently increased to over 5σ, showing up in a tailored search for new pseudoscalar states decaying into tt pairs with specific spin correlations, and recently this CMS discovery has been confirmed by the ATLAS Collaboration, with a significance over 7σ. Unfortunately, the experimental resolution in the tt invariant mass is not precise enough to see any dip due to pseudoscalar Higgs production, and Djouadi, Quevillon and I have concluded that it is not yet possible to discriminate between the toponium and Higgs hypotheses on purely experimental grounds.

However, despite being a fan of extra Higgs bosons, I have to concede that toponium is the more plausible interpretation of the CMS threshold excess. The mass is consistent with that expected for toponium, the signal strength is consistent with theoretical calculations in QCD, and the tt spin correlations are just what one expects for the lowest-lying pseudoscalar toponium state that would be produced in gluon–gluon collisions.

Caution is still in order. The pseudoscalar Higgs hypothesis cannot (yet) be excluded. Nevertheless, it would be a wonderful golden anniversary present for quarkonium if, some 50 years after the discovery of the J/ψ, the appearance of its last, most massive sibling were to be confirmed.

Toponium will be a very interesting target for future e⁺e^– colliders, which will be able to determine its properties with much greater accuracy than a hadron collider could achieve, making precise measurements of the mass of the top quark and its electroweak couplings possible. The quarkonium saga is far from over.

The post Memories of quarkonia appeared first on CERN Courier.

In 2009, the JADE experiment had been inoperational for 23 years. The PETRA electron–positron collider that served it had already completed a second life as a pre-accelerator for the HERA electron–proton collider and was preparing for a third life as an X-ray source. JADE and the other PETRA experiments were a piece of physics history, well known for seminal measurements of three-jet quark–quark-gluon events, and early studies of quark fragmentation and jet hadronisation. But two decades after being decommissioned, the JADE collaboration was yet to publish one of its signature measurements.

At high energies and short distances, the strong force becomes weaker. Quarks behave almost like free particles. This “asymptotic freedom” is a unique hallmark of QCD. In 2009, as now, JADE’s electron–positron data was unique in the low-energy range, with other data sets lost to history. When reprocessed with modern next-to-next-to-leading-order QCD and improved simulation tools, the DESY experiment was able to rival experiments at CERN’s higher-energy Large Electron–Positron (LEP) collider for precision on the strong coupling constant, contributing to a stunning proof of QCD’s most fundamental behaviour. The key was a farsighted and original initiative by Siggi Bethke to preserve JADE’s data and analysis software.

New perspectives

This data resurrection from JADE demonstrated how data can be reinterpreted to give new perspectives decades after an experiment ends. It was a timely demonstration. In 2009, HERA and SLAC’s PEP-II electron–positron collider had been recently decommissioned, and Fermilab’s Tevatron proton–antiproton collider was approaching the end of its operations. Each facility nevertheless had a strong analysis programme ahead, and CERN’s Large Hadron Collider (LHC) was preparing for its first collisions. How could all this data be preserved?

The uniqueness of these programmes, for which no upgrade or followup was planned for the coming decades, invited the consideration of data usability at horizons well beyond a few years. A few host labs risked a small investment, with dedicated data-preservation projects beginning, for example, at SLAC, DESY, Fermlilab, IHEP and CERN (see “Data preservation” dashboard). To exchange data-preservation concepts, methodologies and policies, and to ensure the long-term preservation of HEP data, the Data Preservation in High Energy Physics (DPHEP) group was created in 2014. DPHEP is a global initiative under the supervision of the International Committee for Future Accelerators (ICFA), with strong support from CERN from the beginning. It actively welcomes new collaborators and new partner experiments, to ensure a vibrant and long-term future for the precious data sets being collected at present and future colliders.

At the beginning of our efforts, DPHEP designed a four-level classification of data abstraction. Level 1 corresponds to the information typically found in a scientific publication or its associated HEPData entry (a public repository for high-energy physics data tables). Level 4 includes all inputs necessary to fully reprocess the original data and simulate the experiment from scratch.

The concept of data preservation had to be extended too. Simply storing data and freezing software is bound to fail as operating systems evolve and analysis knowledge disappears. A sensible preservation process must begin early on, while the experiments are still active, and take into account the research goals and available resources. Long-term collaboration organisation plays a crucial role, as data cannot be preserved without stable resources. Software must adapt to rapidly changing computing infrastructure to ensure that the data remains accessible in the long term.

Return on investment

But how much research gain could be expected for a reasonable investment in data preservation? We conservatively estimate that for dedicated investments below 1% of the cost of the construction of a facility, the scientific output increases by 10% or more. Publication records confirm that scientific outputs at major experimental facilities continue long after the end of operations (see “Publications per year, during and after data taking” panel). Publication rates remain substantial well beyond the “canonical” five years after the end of the data taking, particularly for experiments that pursued dedicated data-preservation programmes. For some experiments, the lifetime of the preservation system is by now comparable with the data-taking period, illustrating the need to carefully define collaborations for the long term.

Publication records confirm that scientific outputs at major experimental facilities continue long after the end of operations

The most striking example is BaBar, an electron–positron-collider experiment at SLAC that was designed to investigate the violation of charge-parity symmetry in the decays of B mesons, and which continues to publish using a preservation system now hosted outside the original experiment site. Aging infrastructure is now presenting challenges, raising questions about the very-long-term hosting of historical experiments – “preservation 2.0” – or the definitive end of the programme. The other historical b-factory, Belle, benefits from a follow-up experiment on site.

Publications per year, during and after data taking

The publication record at experiments associated with the DPHEP initiative. Data-taking periods of the relevant facilities are shaded, and the fraction of peer-reviewed articles published afterwards is indicated as a percentage for facilities that are not still operational. The data, which exclude conference proceedings, were extracted from Inspire-HEP on 31 July 2025.

HERA, an electron– and positron–proton collider that was designed to study deep inelastic scattering (DIS) and the structure of the proton, continues to publish and even to attract new collaborators as the community prepares for the Electron Ion Collider (EIC) at BNL, nicely demonstrating the relevance of data preservation for future programmes. The EIC will continue studies of DIS in the regime of gluon saturation (CERN Courier January/February 2025 p31), with polarised beams exploring nucleon spin and a range of nuclear targets. The use of new machine-learning algorithms on the preserved HERA data has even allowed aspects of the EIC physics case to be explored: an example of those “treasures” not foreseen at the end of collisions.

IHEP in China conducts a vigorous data-preservation programme around BESIII data from electron–positron collisions in the BEPCII charm factory. The collaboration is considering using artificial intelligence to rank data priorities and user support for data reuse.

Remarkably, LEP experiments are still publishing physics analyses with archived ALEPH data almost 25 years after the completion of the LEP programme on 4 November 2000. The revival of the CERNLIB collection of FORTRAN data-analysis software libraries has also enabled the resurrection of the legacy software stacks of both DELPHI and OPAL, including the spectacular revival of their event displays (see “Data resurrection” figure). The DELPHI collaboration revised their fairly restrictive data-access policy in early 2024, opening and publishing their data via CERN’s Open Data Portal.

Some LEP data is currently being migrated into the standardised EDM4hep (event data model) format that has been developed for future colliders. As well as testing the format with real data, this will ensure data preservation and support software development, analysis training and detector design for the electron–positron collider phase of the proposed Future Circular Collider using real events.

The future is open

In the past 10 years, data preservation has grown in prominence in parallel with open science, which promotes free public access to publications, data and software in community-driven repositories, and according to the FAIR principles of findability, accessibility, interoperability and reusability. Together, data preservation and open science help maximise the benefits of fundamental research. Collaborations can fully exploit their data and share its unique benefits with the international community.

The two concepts are distinct but tightly linked. Data preservation focuses on maintaining data integrity and usability over time, whereas open data emphasises accessibility and sharing. They have in common the need for careful and resource-loaded planning, with a crucial role played by the host laboratory.

Data preservation and open science both require clear policies and a proactive approach. Beginning at the very start of an experiment is essential. Clear guidelines on copyright, resource allocation for long-term storage, access strategies and maintenance must be established to address the challenges of data longevity. Last but not least, it is crucially important to design collaborations to ensure smooth international cooperation long after data taking has finished. By addressing these aspects, collaborations can create robust frameworks for preserving, managing and sharing scientific data effectively over the long term.

Today, most collaborations target the highest standards of data preservation (level 4). Open-source software should be prioritised, because the uncontrolled obsolescence of commercial software endangers the entire data-preservation model. It is crucial to maintain all of the data and the software stack, which requires continuous effort to adapt older versions to evolving computing environments. This applies to both software and hardware infrastructures. Synergies between old and new experiments can provide valuable solutions, as demonstrated by HERA and EIC, Belle and Belle II, and the Antares and KM3NeT neutrino telescopes.

From afterthought to forethought

In the past decade, data preservation has evolved from simply an afterthought as experiments wrapped up operations into a necessary specification for HEP experiments. Data preservation is now recognised as a source of cost-effective research. Progress has been rapid, but its implementation remains fragile and needs to be protected and planned.

In the past 10 years, data preservation has grown in prominence in parallel with open science

The benefits will be significant. Signals not imagined during the experiments’ lifetime can be searched for. Data can be reanalysed in light of advances in theory and observations from other realms of fundamental science. Education, training and outreach can be brought to life by demonstrating classic measurements with real data. And scientific integrity is fully realised when results are fully reproducible.

The LHC, having surpassed an exabyte of data, now holds the largest scientific data set ever accumulated. The High-Luminosity LHC will increase this by an order of magnitude. When the programme comes to an end, it will likely be the last data at the energy frontier for decades. History suggests that 10% of the LHC’s scientific programme will not yet have been published when collisions end, and a further 10% not even imagined. While the community discusses its strategy for future colliders, it must therefore also bear in mind data preservation. It is the key to unearthing hidden treasures in the data of the past, present and future.

The post Hidden treasures appeared first on CERN Courier.

In the history of elementary particle physics, 1964 was truly an annus mirabilis. Not only did the quark hypothesis emerge – independently from two theo­rists half a world apart – but a multiplicity of theorists came up with the idea of spontaneous symmetry breaking as an attractive method to generate elementary particle masses. And two pivotal experiments that year began to alter the way astronomers, cosmologists and physicists think about the universe.

Shown on the left is a timeline of the key 1964 milestones; discoveries that laid the groundwork for the Standard Model of particle physics and continue to be actively studied and refined today (images: N Eskandari, A Epshtein).

Some of the insights published in 1964 were first conceived in 1963. Caltech theorist Murray Gell-Mann had been ruminating about quarks ever since a March 1963 luncheon discussion with Robert Serber at Columbia University. Serber was exploring the possibility of a triplet of fundamental particles that in various combinations could account for mesons and baryons in Gell-Mann’s SU(3) symmetry scheme, dubbed “the Eightfold Way”. But Gell-Mann summarily dismissed his suggestion, showing him on a napkin how any such fundaments would have to have fractional charges of –2/3 or 1/3 the charge on an electron, which seemed absurd.

From the ridiculous to the sublime

Still, he realised, such ridiculous entities might be allowable if they somehow never materialised outside of the hadrons. For much of the year, Gell-Mann toyed with the idea in his musings, calling such hypothetical entities by the nonsense word “quorks”, until he encountered the famous line in Finnegans Wake by James Joyce, “Three quarks for Muster Mark.” He even discussed it with his old MIT thesis adviser, then CERN Director-General Victor Weisskopf, who chided him not to waste their time talking about such nonsense on an international phone call.

In late 1963, Gell-Mann finally wrote the quark idea up for publication and sent his paper to the newer European journal Physics Letters rather than the (then) more prestigious Physical Review Letters, in part because he thought it would be rejected there. “A schematic model of baryons and mesons”, published on 1 February 1964, is brief and to the point. After a few preliminary remarks, he noted that “a simpler, more elegant scheme can be constructed if we allow non-integral values for the charges … We then refer to the members u(2/3), d(–1/3) and s(–1/3) of the triplet as ‘quarks’.” But toward the end, he hedged his bets, warning readers not to take the existence of these quarks too seriously: “A search for stable quarks of charge +2/3 or –1/3 … at the highest-energy accelerators would help to reassure us of the non-existence of real quarks.”

As often happens in the history of science, the idea of quarks had another, independent genesis – at CERN in 1964. George Zweig, a CERN postdoc who had recently been a Caltech graduate student with Richard Feynman and Gell-Mann, was wondering why the φ meson lived so long before decaying into a pair of K mesons. A subtle conservation law must be at work, he figured, which led him to consider a constituent model of the hadrons. If the φ were somehow composed of two more fundamental entities, one with strangeness +1 and the other with –1, then its great preference for kaon decays over other, energetically more favourable possibilities, could be explained. These two strange constituents would find it difficult to “eat one another,” as he later put it, so two individual, strange kaons would be required to carry each of them away.

Late in the fall of 1963, Zweig discovered that he could reproduce the meson and baryon octets of the Eightfold Way from such constituents if they carried fractional charges of 2/3 and –1/3. Although he at first thought this possibility artificial, it solved a lot of other problems, and he began working feverishly on the idea, day and night. He wrote up his theory for publication, calling his fractionally charged particles “aces” – in part because he figured there would be four of them. Mesons, built from pairs of these aces, formed the “deuces” and baryons the “treys” in his deck of cards. His theory first appeared as a long CERN report in mid-January 1964, just as Gell-Mann’s quark paper was awaiting publication at Physics Letters.

As chance would have it, there was an intensive activity going on in parallel that January – an experimental search for the Ω^– baryon that Gell-Mann had predicted just six months earlier at a Geneva particle-physics conference. With negative charge and a mass almost twice that of the proton, it had to have strangeness –3 and would sit atop a 10-fold decuplet of heavy baryons predicted in his Eightfold Way. Brookhaven experimenter Nick Samios was eagerly seeking evidence of this very strange particle in the initial run of the 80 inch bubble chamber that he and colleagues had spent years planning and building. On 31 January 1964, he finally found a bubble-chamber photograph with just the right signatures. It might be the “gold-plated event” that could prove the existence of the Ω^– baryon.

After more detailed tests to make sure of this conclusion, the Brookhaven team delivered a paper with the unassuming title “Observation of a hyperon with strangeness minus three” to Physical Review Letters. With 33 authors, it reported only one event. But with that singular event, any remaining doubt about SU(3) symmetry and Gell-Mann’s Eightfold Way evaporated.

A fourth quark for Muster Mark?

Later in spring 1964, James Bjorken and Sheldon Glashow crossed paths in Copenhagen, on leave from Harvard and Stanford, working at Niels Bohr’s Institute for Theoretical Physics. Seeking to establish lepton–hadron symmetry, they needed a fourth quark because a fourth lepton – the muon neutrino – had been discovered in 1962 at Brookhaven. Bjorken and Glashow were early adherents of the idea that hadrons were made of quarks, but based their arguments on SU(4) symmetry rather than SU(3). “We called the new quark flavour ‘charm,’ completing two weak doublets of quarks to match two weak doublets of leptons, and establishing lepton–quark symmetry, which holds to this day,” recalled Glashow (CERN Courier January/February 2025 p35). Their Physics Letters article appeared that summer, but it took another decade before solid evidence for charm turned up in the famous J/ψ discovery at Brookhaven and SLAC. The charm quark they had predicted in 1964 was the central player in the so-called November Revolution a decade later that led to widespread acceptance of the Standard Model of particle physics.

In the same year, Oscar Greenberg at the University of Maryland was wrestling with the difficult problem of how to confine three supposedly identical quarks within a volume hardly larger than a proton. According to the sacrosanct Pauli exclusion principle, identical spin–1/2 fermions could never occupy the exact same quantum state. So how, for example, could one ever cram three strange quarks inside an Ω^– baryon?

One possible solution, Greenberg realised, was that quarks carry a new physical property that distinguished them from one another so they were not in fact identical. Instead of a single quark triplet, that is, there could be three distinct triplets of what he dubbed “paraquarks”, publishing his ideas in November 1964, and capping an extraordinary year of insights into hadrons. We now recognise his insight as anticipating the existence of “coloured” quarks, where colour is the source of the relentless QCD force binding them within mesons and baryons.

The origin of mass

Although it took more than a decade for experiments to verify them, these insights unravelled the nature of hadrons, revealing a new family of fermions and hinting at the nature of the strong force. Yet they were not necessarily the most important ideas developed in particle physics in 1964. During that summer, three theorists – Robert Brout, François Englert and Peter Higgs – formulated an innovative technique to generate particle masses using spontaneous symmetry breaking of non-Abelian Yang–Mills gauge theories – a class of field theories that would later describe the electroweak and strong forces in the Standard Model.

Inspired by successful theories of superconductivity, symmetry-breaking ideas had been percolating among those few still working on quantum field theory, then in deep decline in particle physics, but they foundered whenever masses were introduced “by hand” into the theories. Or, as Yoichiro Nambu and Peter Goldstone realised in the early 1960s, massless bosons appeared in the theories that did not correspond to anything observed in experiments.

If they existed, the W (and later, Z) bosons carrying the short-range weak force had to be extremely massive (as is now well known). Brout and Englert – and independently Higgs – found they could generate the masses of such vector bosons if the gauge symmetry governing their behaviour was instead spontaneously broken, preserving the underlying symmetry while allowing for distinctive, asymmetric particle states. In solid-state physics, for example, magnetic domains will spontaneously align along a single direction, breaking the underlying symmetry of the electromagnetic field. Brout and Englert published their solution in June 1964, while Higgs followed suit a month later (after his paper was rejected by Physics Letters). Higgs subsequently showed that this symmetry breaking required a scalar boson to exist that was soon named after him. Dubbed the “Higgs mechanism,” this mass-generating process became a crucial feature of the unification of the weak and electromagnetic forces a few years later by Steven Weinberg and Abdus Salam. And after their electroweak theory was shown in 1971 to be renormalisable, and hence calculable, the theoretical floodgates opened wide, leading to today’s dominant Standard Model paradigm.

Surprise, surprise!

Besides the quark model and the Higgs mechanism, 1964 witnessed two surprising discoveries that would light up almost any other year in the history of science. That summer saw the publication of an epochal experiment leading to the discovery of CP violation in the decays of long-lived neutral mesons. Led by Princeton physicists Jim Cronin and Val Fitch, their Brookhaven experiment had discerned a small but non-negligible fraction – 0.2% – of two-body decays into a pair of pions, instead of into the dominant CP-conserving three-body decays. For months, the group wrestled with trying to understand this surprising result before publishing it that July in Physical Review Letters.

It took almost another decade before Japanese theorists Makoto Kobayashi and Toshihide Maskawa proved that such a small amount of CP violation was the natural result of the Standard Model if there were three quark-lepton families instead of the two then known to exist. Whether this phenomenon has any causal relation to the dominance of matter in the universe is still up for grabs decades later. “Indeed, it is almost certain that the CP violation observed in the K-meson system is not directly responsible for the matter dominance of the universe,” wrote Cronin in the early 1990s, “but one would wish that it is related to whatever the mechanism was that created [this] matter dominance.”

Another epochal 1964 observation was not published until 1965, but it deserves mention here because of its tremendous significance for the subsequent marriage of particle physics and cosmology. That summer, Arno Penzias and Robert W Wilson of Bell Telephone Labs were in the process of converting a large microwave antenna in Holmdel, NJ, for use in radio astronomy. Shaped like a giant alpenhorn lying on its side, the device had been developed for early satellite communications. But the microwave signals that it was receiving included a faint, persistent “hiss” no matter the direction in which the horn was pointed; they at first interpreted the hiss as background noise – possibly due to some smelly pigeon droppings that had accumulated inside, which they removed. Still it persisted. Penzias and Wilson were at a complete loss to explain it.

Cosmological consequences

It so happened that a Princeton group led by Robert Dicke and James Peebles was just then building a radiometer to search for the uniform microwave radiation that should suffuse the universe had it begun in a colossal fireball, as a few cosmologists had been arguing for decades. In the spring of 1965, Penzias read a preprint of a paper by Peebles on the subject and called Dicke to suggest he come to Holmdel to view their results. After arriving and realising they had been scooped, the Princeton physicists soon confirmed the Bell Labs results using their own rooftop radiometer.

Besides the quark model and the Higgs mechanism, 1964 witnessed two surprising discoveries that would light up almost any other year in the history
of science

The results were published as back-to-back letters in the Astrophysical Journal on 7 May 1965. The Princeton group wrote extensively about the cosmological consequences of the discovery, while Penzias and Wilson submitted just a brief, dry description of their work, “A measurement of excess antenna temperature at 4080 Mc/s” – ruling out other possible interpretations of the uniform signal corresponding to the radiation expected from a 3.5 K blackbody.

Subsequent measurements at many other frequencies have established that this is indeed the cosmic background radiation expected from the Big Bang birth of the universe, confirming that it had in fact occurred. That was an incredibly brief, hot, dense phase of its existence, which has prodded many particle physicists to take up the study of its evolution and remnants. This discovery of the cosmic background radiation therefore serves as a fitting capstone on what was truly a pivotal year for particle physics.

The post Nineteen sixty-four appeared first on CERN Courier.

Almost a decade after ATOMKI researchers reported an unexpected peak in electron–positron pairs from beryllium nuclear transitions, the case for a new “X17” particle remains open. Proposed as a light boson with a mass of about 17 MeV and very weak couplings, it would belong to the sometimes-overlooked low-energy frontier of physics beyond the Standard Model. Two recent results now pull in opposite directions: the MEG II experiment at the Paul Scherrer Institute found no signal in the same transition, while the PADME experiment at INFN Frascati reports a modest excess in electron–positron scattering at the corresponding mass.

The story of the elusive X17 particle began at the Institute for Nuclear Research (ATOMKI) in Debrecen, Hungary, where nuclear physicist Attila János Krasznahorkay and colleagues set out to study the de-excitation of a beryllium-8 state. Their target was the dark photon – a particle hypothesised to mediate interactions between ordinary and dark matter. In their setup, a beam of protons strikes a lithium-7 target, producing an excited beryllium nucleus that releases a proton or de-excites to the beryllium-8 ground state by emitting an 18.1 MeV gamma ray – or, very rarely, an electron–positron pair.

Controversial anomaly

In 2015, ATOMKI claimed to have observed an excess of electron–positron pairs with a statistical significance of 6.8σ. Follow-up measurements with different nuclei were also reported to yield statistically significant excess at the same mass. The team claimed the excess was consistent with the creation of a short-lived neutral boson with a mass of about 17 MeV. Given that it would be produced in nuclear transitions and decay into electron–positron pairs, the X17 should couple to nucleons, electrons and positrons. But many relevant constraints squeeze the parameter space for new physics at low energies, and independent tests are essential to resolve an unexpected and controversial anomaly that is now a decade old.

In November 2024, MEG II announced a direct cross-check of the anomaly, publishing their results in July 2025. Designed for high-precision tracking and calorimetry, the experiment combines dedicated background monitors with a spectrometer based on a lightweight, single-volume drift chamber that records the ionisation trails of charged particles. The detector is designed to search for evidence of the rare lepton-flavour-violating decay μ⁺ → e⁺γ, with the collaboration recently reporting world-leading limits at EPS-HEP (see “High-energy physics meets in Marseille”). It is also well suited to probing electron–positron final states, and has the mass resolution required to test the narrow-resonance interpretation of the ATOMKI anomaly.

Motivated by interest in X17, the collaboration directed a proton beam with energy up to 1.1 MeV onto a lithium-7 target, to study the same nuclear process as ATOMKI. Their data disfavours the ATOMKI hypothesis and imposes an upper limit on the branching ratio of 1.2 × 10^–5 at 90% confidence.

“While the result does not close the case,” notes Angela Papa of INFN, the University of Pisa and the Paul Scherrer Institute, “it weakens the simplest interpretations of the anomaly.”

But MEG II is not the only cross check in progress. In May, the PADME collaboration reported an independent test that doesn’t repeat the ATOMKI experiment, but seeks to disentangle the X17 question from the complexities of nuclear physics.

For theorists, X17 is an awkward fit

Initially designed to search for evidence of states that decay invisibly, like dark photons or axion-like particles, PADME collides a positron beam with energies reaching 550 MeV with a 100 µm-thick active diamond target. Annihilations of positrons with electrons bound in the target material are reconstructed by detecting the resulting photons, with any peak in the missing-mass spectrum signalling an unseen product. The photon energy and impact position is measured by a finely segmented electromagnetic calorimeter with crystals refurbished from the L3 experiment at LEP.

“The PADME approach relies only on the suggested interaction of X17 with electrons and positrons,” remarks spokesperson Venelin Kozhuharov of Sofia University and INFN Frascati. “Since the ATOMKI excess was observed in electron–positron final states, this is the minimal possible assumption that can be made for X17.”

Instead of searching for evidence of unseen particles, PADME varied the beam energy to look for an electron-positron resonance in the expected X17 mass range. The collaboration claims that the combined dataset displays an excess near 16.90 MeV with a local significance of 2.5σ.

For theorists, X17 is an awkward fit. Most consider dark photons and axions to be the best motivated candidates for low mass, weakly coupled new physics states, says Claudio Toni of LAPTh. Another possibility, he says, is a bound state of known particles, though QCD states such as pions are about eight times heavier, and pure QED effects usually occur at much lower scales than 17 MeV.

“We should be cautious,” says Toni. “Since X17 is expected to couple to both protons and electrons, the absence of signals elsewhere forces any theoretical proposal to respect stringent constraints. We should focus on its phenomenology.”

The post Mixed signals from X17 appeared first on CERN Courier.

At the LHC, almost all top–antitop pairs are produced in a smooth invariant-mass spectrum described by perturbative QCD. In March, the CMS collaboration announced the discovery of an additional 1% localised near the energy threshold to produce a top quark and its antiquark (CERN Courier May/June 2025 p7). The ATLAS collaboration has now confirmed this observation.

“The measurement was challenging due to the small cross section and the limited mass resolution of about 20%,” says Tomas Dado of the ATLAS collaboration and CERN. “Sensitivity was achieved by exploiting high statistics, lepton angular variables sensitive to spin correlations, and by carefully constraining modelling uncertainties.”

Toponium

The simplest explanation for the excess appears to be a spectrum of “quasi-bound” states of a top quark and its antiquark that are often collectively referred to as toponium, by reference to the charmonium and bottomonium states discovered in the November Revolution of 1974 (see “Memories of quarkonia“). But there the similarities end. Thanks to the unique properties of the most massive fundamental particle yet discovered, toponium is expected to be exceptionally broad rather than exceptionally narrow in energy spectra, and to disintegrate via the weak decay of its constituent quarks rather than via their mutual annihilation.

“Historically, it was assumed that the LHC would never reach the sensitivity required to probe such effects, but ATLAS and CMS have shown that this expectation was too pessimistic,” says Benjamin Fuks of the Sorbonne. “This regime corresponds to the production of a slowly moving top–antitop pair that has time to exchange multiple gluons before one of the top quarks decays. The invariant mass of the system lies slightly below the open top–antitop threshold, which implies that at least one of the top quarks is off-shell. This contrasts with conventional top–antitop production, where the tops are typically produced far above threshold, move relativistically and do not experience significant non-relativistic gluon dynamics.”

While CMS fitted a pseudo-scalar resonance that couples to gluons and top quarks – the essential features of the ground state of toponium – the new ATLAS analysis employs a model recently published by Fuks and his collaborators that additionally includes all S-wave excitations. ATLAS reports a cross-section for such quasi-bound excitations of 9.0 ± 1.3 pb, consistent with CMS’s measurement of 8.8 ± 1.3 pb. ATLAS’s measurement rises to 13.9 ± 1.9 pb when applying the same signal model as CMS.

Future measurements of top quark–antiquark pairs will compare the threshold excess to the expectations of non-relativistic QCD, search for the possible presence of new fields beyond the Standard Model, and study the quantum entanglement of the top and antitop quarks.

“At the High-Luminosity LHC, the main objective is to exploit the much larger dataset to go beyond a single-bin description of the sub-threshold top–antitop invariant mass distribution,” says Fuks. “At a future electron–positron collider, the top–antitop threshold scan has long been recognised as a cornerstone measurement, with toponium contributions playing an essential role.”

For Dado, this story reflects a satisfying interplay between theorists and the LHC experiments.

“Theorists proposed entanglement studies, ATLAS demonstrated entangled top–antitop pairs and CMS applied spin-sensitive observables to reveal the quasi-bound-state effect,” he says. “The next step is for theory to deliver a complete description of the top–antitop threshold.”

The post ATLAS confirms top–antitop excess appeared first on CERN Courier.

Big science requires long-term planning. In June, the US National Academies of Sciences, Engineering, and Medicine published an unprecedented 40-year strategy for US particle physics titled Elementary Particle Physics: The Higgs and Beyond. Its recommendations include participating in the proposed Future Circular Collider at CERN and hosting the world’s highest-energy elementary particle collider around the middle of the century (see “Eight recommendations” panel). The report assesses that a 10 TeV muon col­lider would complement the discovery potential of a 100 TeV proton collider.

“The shift to a 40-year horizon in the new report reflects a recognition that modern particle-physics projects and scientific questions are of unprecedented scale and complexity, demanding a much longer-term strategic commitment, international cooperation and investment for continued leadership,” says report co-chair Maria Spiropulu of the California Institute of Technology. “A staggered approach towards large research-infrastructure projects, rich in scientific advancement, technological breakthroughs and collaboration, can shield the field from stagnation.”

The report is authored by a committee of leading scientists selected by the National Academies. Its mandate complements the grassroots-led Snowmass process and the budget-conscious P5 process (CERN Courier January/February 2024 p7). The previous report in this series, Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics was published in 2006. It called for the full exploitation of the LHC, a strategic focus on linear-collider R&D, expanding particle astrophysics, and pursuing an internationally coordinated, staged programme in neutrino physics.

Two conclusions underpin the new report’s recommendations. The first identifies three workforce issues currently threatening the future of particle physics: the morale of early-career scientists, a shortfall in the number of accelerator scientists, and growing barriers to international exchanges. The second urges US leadership in elementary particle physics, citing benefits to science, the nation and humanity.

The post US publishes 40-year vision for particle physics appeared first on CERN Courier.

“I am convinced that we are seeing the beginning of a new field in neutrino physics based on CEνNS observations,” says Manfred Lindner (Max Planck Institute for Nuclear Physics in Heidelberg), the spokesperson for the CONUS+ experiment, which reported the first evidence for fully coherent CEνNS in July. “The technology of CONUS+ is mature and seems scalable. I believe that we are at the beginning of precision neutrino physics with CEνNS and CONUS+ is one of the door openers!”

Act of hubris

Daniel Z Freedman is not best known for CEνNS, but in 1974 the future supergravity architect suggested that experimenters search for evidence of neutrinos interacting not with nucleons but “coherently” with entire nuclei. This process should dominate when the de Broglie wavelength of the neutrino is the diameter of the nucleus or larger. The question of which specific neutron exchanged a Z boson with the incoming neutrino would sum in the quantum amplitude rather than the probability, leading to an N² dependence on the number of neutrons. As a result, CEνNS cross sections are typically enhanced by a factor of between 100 and 1000.

Freedman noted that his proposal may have been an “act of hubris”, because the interaction rate, detector resolution and backgrounds would all pose grave experimental difficulties. His caveat was perspicacious. It took until 2017 for indisputable evidence for CEνNS to emerge at Oak Ridge National Laboratory in the US, where the COHERENT experiment observed CEνNS by neutrinos with a maximum energy of 52 MeV, emerging from pion decays at rest (CERN Courier October 2017 p8). At these energies, the coherence condition is only partially fulfilled, and nuclear structure still plays a role.

The CONUS+ collaboration now presents evidence for CEνNS in the fully coherent regime. The experiment – one of many launched at nuclear reactors following the COHERENT demonstration – uses reactor electron anti-neutrinos with energies below 10 MeV generated across 119 days at the Leibstadt Nuclear Power Plant in Switzerland. The team observed 395 ± 106 neutrinos compared to a Standard Model expectation of 347 ± 59 events, corresponding to a statistical significance for the observation of CEνNS of 3.7σ.

I am convinced that we are seeing the beginning of a new field in neutrino physics based on CEνNS observations

It is no wonder that detection took 50 years. The only signal of CEνNS is a gentle nuclear recoil – an effect often compared to the effect of a ping-pong ball on a tanker. In CONUS+, the nuclear recoils of the CEνNS interactions are detected using the ionisation signal of point-contact high-purity germanium detectors with ultra-low energy thresholds as low as 160 eV.

The team has now increased the mass of their four semiconductor detectors from 1 to 2.4 kg to provide better statistics and potentially a lower threshold energy. CONUS+ is highly sensitive to physics beyond the Standard Model, says the team, including non-standard interaction parameters, new light mediators and electromagnetic properties of the neutrino such as electrical millicharges or neutrino magnetic moments. Lindner estimates that the CONUS+ technology could be scaled up to 100 kg, potentially yielding 100,000 CEνNS events per year of operation.

Into the neutrino fog

One researcher’s holy grail is another’s curse. In 2024, dark-matter experiments reported entering the “neutrino fog”, as their sensitivity to nuclear recoils crossed the threshold to detect a background of solar-neutrino CEνNS interactions. The PandaX-4T and XENONnT collaborations reported 2.6σ and 2.7σ evidence for CEνNS interactions in their liquid–xenon time projection chambers, based on estimated signals of 79 and 11 interactions, respectively. These were the first direct measurements of nuclear recoils from solar neutrinos with dark-matter detectors. Boron-8 solar neutrinos have slightly higher energies than those detected by CONUS+, and are also in the fully coherent regime.

CEνNS has promise for nuclear-reactor monitoring

“The neutrino flux in CONUS+ is many orders of magnitude bigger than in dark-matter detectors,” notes Lindner, who is also co-spokesperson of the XENON collaboration. “This is compensated by a much larger target mass, a larger CEνNS cross section due to the larger number of neutrons in xenon versus germanium, a longer running time and differences in detection efficiencies. Both experiments have in common that all backgrounds of natural or imposed radioactivity must be suppressed by many orders of magnitude such that the CEνNS process can be extracted over backgrounds.”

The current experimental frontier for CEνNS is towards low energy thresholds, concludes COHERENT spokesperson Kate Scholberg of Duke University. “The coupling of recoil energy to observable energy can be in the form of a dim flash of light picked up by light sensors, a tiny zap of charge collected in a semiconductor detector, or a small thermal pulse observed in a bolometer. A number of collaborations are pursuing novel technologies with sub-keV thresholds, among them cryogenic bolometers. A further goal is measurement over a range of nuclei, as this will test the SM prediction of an N² dependence of the CEνNS cross section. And for higher-energy neutrino sources, for which the coherence is not quite perfect, there are opportunities to learn about nuclear structure. Another future possibility is directional recoil detection. If we are lucky, nature may give us a supernova burst of CEνNS recoils. As for societal applications, CEνNS has promise for nuclear-reactor monitoring for nonproliferation purposes due to its large cross section and interaction threshold below that for inverse-beta-decay of 1.8 MeV.”

The post Full coherence at fifty appeared first on CERN Courier.

After the detection of over 10,000 gamma-ray components of GRBs by dedicated gamma-ray satellites, the most common models associate the longer ones with supernovae. This has been confirmed thanks to detections of afterglow emission coinciding with supernova events in other galaxies. The exact characteristics that cause some heavy stars to produce a GRB remain, however, poorly understood. Furthermore, many open questions remain regarding the nature and origin of the relativistic jets and how the gamma rays are produced within them.

While the emission has been studied extensively in gamma rays, detections at soft X-ray energies are limited. This changed in early 2024 with the launch of the Einstein Probe (EP) satellite. EP is a novel X-ray telescope, developed by the Chinese Academy of Sciences (CAS) in collaboration with ESA, the Max Planck Institute for Extraterrestrial Physics and the Centre National d’Études Spatiales. EP is unique in its wide field of view (1/11th of the sky) in soft X-rays, made possible thanks to complex X-ray optics. As GRBs occur at random positions in the sky at random times, the large field of view increases its chance to observe them. Within its first year EP detected several GRB events, most of which challenge our understanding of them.

One of these occurred on 14 April 2024. It consisted of a bright flash of X-rays lasting about 2.5 minutes. The event was also observed by ground-based optical and radio telescopes that were alerted to its location in the sky by EP. These observations at lower photon energies were consistent with a weak afterglow together with the signatures from a relatively standard supernova-like event. The supernova emission showed it to originate from a star which, prior to its death, had already shed its outer layers of hydrogen and helium. Along with the spectrum detected by EP, the detection of an afterglow indicates the existence of a relativistic jet. The overall picture is therefore consistent with a GRB. However, a crucial part was missing: a gamma-ray component.

In addition, the emission spectrum observed by EP looks significantly softer as it peaks at keV rather than the 100s of keV energies typical for GRBs. The results hint at this being at an explosion that produced a relativistic jet which – for unknown reasons – was not energetic enough to produce the standard gamma-ray emission. The progenitor star therefore appears to bridge the stellar population which causes a “simple” core collapse supernova and those that produce GRBs.

Another event, detected on 15 March 2024, produced soft X-rays consisting of six separate epochs spread out over 17 minutes. Here, a gamma-ray component was detected by NASA’s Swift BAT instrument, confirming it to be a GRB. However, unlike any other GRB, the gamma-ray emission started long after the onset of the X-ray emission. This lack of gamma-ray emission in the early stages is difficult to reconcile with standard emission models. There, the emission comes from a single uniform jet where the highest energies are emitted at the start when the jet is at its most energetic.

In their publication in Nature Astronomy, the EP collaboration suggests the possibility that the early X-ray emission comes from either shocks from the supernova explosion itself or from weaker relativistic jets preceding the main powerful jet. Other proposed explanations include complex jet structures and pose that EP observed the jet far away from its centre. In this explanation, the matter in the jet moves faster in the centre while at the edges its Lorentz factor (or velocity) is significantly slower, thereby producing a lower-energy longer-lasting emission, undetectable before the launch of EP.

Overall, the two detections appear to indicate that the GRBs detected over the last 60 years, where the emission was dominated by gamma rays, were only a subset of a more complex phenomenon. At a time where two of the most important instruments in GRB astronomy from the last two decades, NASA’s Fermi and Swift missions, are proposed to be switched off, EP is taking over an important role and opening the window to soft X-ray observations.

The post Einstein Probe detects exotic gamma-ray bursts appeared first on CERN Courier.

In addition to giving mass to elementary particles, the Brout–Englert–Higgs mechanism provides a testing ground for the fundamental symmetries of nature. In a recent analysis, the CMS collaboration searched for violations of charge–parity (CP) symmetry in the decays of Higgs bosons into two photons. The results set some of the strongest limits to date on anomalous Higgs-boson couplings that violate CP symmetry.

CP symmetry is particularly interesting as violations reveal fundamental differences in the behaviour of matter and antimatter, potentially explaining why the former appears to be much more abundant in the observed universe. While the Standard Model predicts that CP symmetry should be violated, the effect is not sufficient to account for the observed imbalance, motivating searches for additional sources of CP violation. CP symmetry requires that the laws of physics remain the same when particles are replaced by their corresponding antiparticles (C symmetry) and their spatial coordinates are reflected as in a mirror (P symmetry). In 1967, Andrei Sakharov established CP violation as one of three necessary requirements for a cosmic imbalance between matter and antimatter.

The CMS collaboration probed Higgs-boson interactions with electro­weak bosons and gluons, using decays into two energetic photons. This final state is particularly precise: photons are well reconstructed thanks to the energy resolution of the CMS electromagnetic calorimeter and backgrounds can be accurately estimated. The analysis employed 138 fb^–1 of proton–proton collision data at a centre-of-mass energy of 13 TeV and focused on two main channels. Electroweak production of the Higgs boson, via vector boson fusion (VBF) or in association with a W or Z boson (VH), tests the Higgs boson’s couplings to electroweak gauge bosons. Gluon fusion, which occurs through loops dominated by the top quark, is sensitive to possible CP-violating interactions with fermions. A full angular analysis was performed to separate different coupling hypotheses, exploiting both the kinematic properties of the photons from the Higgs boson decay and the particles produced alongside it.

The matrix element likelihood approach (MELA) was used to minimise the number of observables, while retaining all essential information. Deep neural networks and boosted decision trees classified events based on their topology and kinematic properties, isolating signal-like events from background or alternative new-physics scenarios. Events were then grouped into analysis categories, each optimised to enhance sensitivity to anomalous couplings for a specific production mode.

The data favour the Standard Model configuration, with no significant deviation from its predictions (see figure 1). By placing some of the most stringent constraints yet on CP-violating interactions between the Higgs boson and vector bosons, the study highlights how precise measurements in simple final states can yield insights into the symmetries governing particle physics. With the upcoming data from Run 3 of the LHC and the High-Luminosity LHC, CMS is well positioned to push these limits further and potentially uncover hidden aspects of the Higgs sector.

The post CP symmetry in diphoton Higgs decays appeared first on CERN Courier.

Narrow sprays of particles called jets erupt from high-energy quarks and gluons. The ALICE collaboration has now measured so-called energy–energy correlators (EECs) of charm-quark jets for the first time – revealing new details of the elusive “dead cone” effect.

Unlike in quantum electrodynamics, the quantum chromodynamics (QCD) coupling constant gets weaker at higher energies – a feature known as asymptotic freedom. This allows high-energy partons to scatter and radiate additional partons, forming showers. As their energy splits between more and more products, decreasing toward the characteristic QCD confinement scale, interactions grow strong enough to bind partons within colour-neutral hadrons. The structure, energy profile and angular distribution of particles within the jets bear traces of the initial collision and the parton-to-hadron transitions, making them powerful probes of both perturbative and non-perturbative QCD effects. To understand the interplay between these two regimes, researchers track how jet properties vary with the mass and colour of the initiating partons.

Due to the gluon’s larger colour charge, QCD predicts gluon-initiated jets to be broader and contain more low-momentum particles than those from quarks. Additionally, the significant mass of heavy quarks should suppress collinear gluon emission, inducing the so-called “dead-cone” effect at small angles. These expectations can be tested by comparing jet substructure across flavours. A key observable for this purpose is the EEC, which measures how energy is distributed within a jet as a function of the angular separation R_L between particle pairs. The large-R_L region is dominated by early partonic splittings, reflecting perturbative dynamics, while a small R_L value corresponds to later radiation shaped by final-state hadrons. The intermediate-R_L region captures the transition where hadronisation begins to affect the jet structure. This characteristic shape enables the separation of perturbative and non-perturbative regimes, revealing flavour-dependent dynamics of jet formation and hadronisation.

The ALICE Collaboration measured the EEC for charm–quark jets tagged with D⁰ mesons, reconstructed via the D⁰ → K^– π⁺ decay mode (branching ratio 3.93 ± 0.04%), in proton–proton collisions at centre-of-mass energy 13 TeV. Jets are inferred from charged-particle tracks using the anti-k_T algorithm, clustering products in momentum space with a resolution parameter R = 0.4.

At low transverse momentum, where the effect of the charm-quark mass is most prominent, the EEC amplitude is found to be significantly suppressed for charm jets relative to inclusive jets initiated by light-quarks and gluons. The difference is more pronounced at small angles due to the dead-cone effect (see figure 1). Despite the sizable charm–quark mass, the distribution peak position remains similar across the two populations, pointing to a complex mix of parton flavour effects in the shower evolution and enhanced non-perturbative contributions such as hadronisation. Perturbative QCD calculations reproduce the general shape at large R_L but show tension near the peak, indicating the need for theoretical improvements for heavy-quark jets. The upward trend in the ratio of charm to inclusive jets as a function of R_L, reproduced with PYTHIA 8, suggests that they deviate in fragmentation.

This first measurement of the heavy-flavour jet EEC helps disentangle perturbative and non-perturbative QCD effects in jet formation, constraining theoretical models. Furthermore, it provides an essential vacuum baseline for future studies in heavy-ion collisions, where the quark–gluon plasma is expected to alter jet properties.

The post Charming energy–energy correlators appeared first on CERN Courier.

Rare, unobserved decays of the Higgs boson are natural places to search for new physics. At the EPS-HEP conference, the ATLAS collaboration presented new improved measurements of two highly suppressed Higgs decays: into a pair of muons; and into a Z boson accompanied by a photon. Producing a single event of either H → μμ or H → Zγ → (ee/μμ) γ at the LHC requires, on average, around 10 trillion proton–proton collisions. The H → μμ and H → Zγ signals appear as narrow resonances in the dimuon and Zγ invariant mass spectra, atop backgrounds some three orders of magnitude larger.

In the Standard Model, the Brout–Englert–Higgs mechanism gives mass to the muon through its Yukawa coupling to the Higgs field, which can be tested via the rare H → μμ decay. An indirect comparison with the well-known muon mass, determined to 22 parts per billion, provides a stringent test of the mechanism in the second fermion generation and is a powerful probe of new physics. With a branching ratio of just 0.02%, and a large background dominated by the Drell–Yan production of muon pairs through virtual photons or Z bosons, the inclusive signal-over-background ratio plunges to the level of one part in a thousand. To single out its decay signature, the ATLAS collaboration employed machine-learning techniques for background suppression and generated over five billion Drell–Yan Monte Carlo events at next-to-leading-order accuracy in QCD, all passed through the full detector simulation. This high-precision sample provides templates to refine the background model and minimise bias on the tiny H → μμ signal.

The Higgs boson can decay into a Z boson and a photon via loop diagrams involving W bosons and heavy charged fermions, like the top quark. Detecting this rare process would complete the suite of established decays into electroweak boson pairs and offer a window on physics beyond the Standard Model. To reduce QCD background and improve sensitivity, the ATLAS analysis focused on Z bosons further decaying into electron or muon pairs, with an overall branching fraction of 7%. This additional selection reduces the event rate to about one in 10,000 Higgs decays, with an inclusive signal-over-background ratio at the per-mille level. The low momenta of final-state particles, combined with the high-luminosity conditions of LHC Run 3, pose additional challenges for signal extraction and suppression of Z + jets backgrounds. To enhance signal significance, the ATLAS collaboration improved background modelling techniques, optimised event categorisation by Higgs production mode, and employed machine learning to boost sensitivity.

The two ATLAS searches are based on 165 fb^–1 of LHC Run 3 proton–proton collision data collected between 2022 and 2024 at √s = 13.6 TeV, with a rigorous blinding procedure in place to prevent biases. Both channels show excesses at the Higgs-boson mass of 125.09 GeV, with observed (expected) 2.8σ (1.8σ) significance for H to μμ and 1.4σ (1.5σ) for H to Zγ. These results are strengthened by combining them with 140 fb^–1 of Run-2 data collected at √s = 13 TeV, updating the H → μμ and H → Zγ observed (expected) significances to 3.4σ (2.5σ) and 2.5σ (1.9σ), respectively (see figure 1). The measured signal strengths are consistent with the Standard Model within uncertainties.

These results mark the ATLAS collaboration’s first evidence for the H → μμ decay, following the earlier claim by CMS based on Run-2 data (see CERN Courier September/October 2020 p7). Meanwhile, the H → Zγ search achieves a 19% increase in expected significance with respect to the combined ATLAS–CMS Run-2 analysis, which first reported evidence for this process. As Run 3 data-taking continues, the LHC experiments are closing in on establishing these two rare Higgs decay channels. Both will remain statistically limited throughout the LHC’s lifetime, with ample room for discovery in the high-luminosity phase.

The post Mapping rare Higgs-boson decays appeared first on CERN Courier.

Axion-like particles (ALPs) are some of the most promising candidates for physics beyond the Standard Model. At the LHC, searches for ALPs that couple to gluons and photons have so far been limited to masses above 10 GeV due to trigger requirements that reduce low-energy sensitivity. In its first ever analysis on purely neutral final states, the LHCb collaboration has now extended this experimental reach and set new bounds on the ALP parameter space.

When a global symmetry is spontaneously broken, it gives rise to massless excitations called Goldstone bosons, which reflect the system’s freedom to transform continuously without changing its energy. It is thought that ALPs may arise via a similar mechanism, acquiring a small mass though, as they originate from symmetries that are only approximate. Depending on the underlying theory, they could contribute to dark matter, solve the strong-CP problem, or mediate interactions with a hidden sector. Their coupling to known particles varies across models, leading to a range of potential experimental signatures. Among the most compelling are those involving gluons and photons.

Thanks to the magnitude of the strong coupling constant, even a small interaction with gluons can dominate the production and decay of ALPs. This makes searches at the LHC challenging since low-energy jets in proton–proton collisions are often indistinguishable from the expected ALP decay signature. In this environment, a more effective approach is to focus on the photon channel and search for ALPs that are produced in proton–proton collisions – mostly via gluon–gluon fusion – and that decay into photon pairs. These processes have been investigated at the LHC, but previous searches were limited by trigger thresholds requesting photons with large momentum components transverse to the beam. This is particularly restrictive for low-mass ALPs, whose decay products are often too soft to pass these thresholds.

The new search, based on Run-2 data collected in 2018, overcomes this limitation by leveraging the LHCb detector’s flexible software-based trigger system, lower pile-up and forward geometry. The latter enhances sensitivity to products with a small momentum component transverse to the beam, making it well suited to probe resonances in the 4.9 to 19.4 GeV mass region. This is the first LHCb analysis of a purely neutral final state, hence requiring a new trigger and selection strategy, as well as a dedicated calibration procedure. Candidate photon pairs are identified from two high-energy calorimeter clusters, produced in isolation from the rest of the event, which could not originate from charged particles or neutral pions. ALP decays are then sought using maximum likelihood fits that scan the photon-pair invariant mass spectrum for peaks.

No photon-pair excess is observed over the background-only hypothesis, and upper limits are set on the ALP production cross-section times decay branching. These results constrain the ALP decay rate and its coupling to photons, probing a region of parameter space that has so far remained unexplored (see figure 1). The investigated mass range is also of interest beyond ALP searches. Alongside the main analysis, the study targeted two-photon decays of B⁰_(s) and the little-studied η_b meson, almost reaching the sensitivity required for its detection.

The upgraded LHCb detector, which began operations with Run 3 in 2022, is expected to deliver another boost in sensitivity. This will allow future analyses to benefit from the extended flexibility of its purely software trigger, significantly larger datasets and a wider energy coverage of the upgraded calorimeter.

The post Closing the gap on axion-like particles appeared first on CERN Courier.

Mounting evidence

From the point of view of fundamental theory, there are at least four good reasons to believe that dark energy must be time-dependent and cannot be a cosmological constant.

The first piece of evidence is well known: if there is a cosmological constant induced by a particle-physics description of matter, then its value should be 120 orders of magnitude larger than observations indicate. This is the famous cosmological constant problem.

A second argument is the “infrared instability” of a spacetime induced by a cosmological constant. Alexander Polyakov (Princeton) has forcefully argued that inhomogeneities on very large length scales would gradually mask a preexisting cosmological constant, making it appear to vary over time.

Recently, other arguments have been put forwards indicating that dark energy must be time-dependent. Since quantum matter generates a large cosmological constant when treated as an effective field theory, it should be expected that the cosmological constant problem can only be addressed in a quantum theory of all forces. The best candidate we have is superstring theory. There is mounting evidence that – at least in the regions of the theory under mathematical control – it is impossible to obtain a positive cosmological constant corresponding to the observed accelerating expansion. But one can obtain time-dependent dark energy, for example in quintessence toy models.

Recent observations provide tantalising evidence that dark energy might be time-dependent

The final reason is known as the trans-Planckian censorship conjecture. As the nature of dark energy remains a complete mystery, it is often treated as an effective field theory. This means that one expands all fields in Fourier modes and quantises each field as a harmonic oscillator. The modes one uses have wavelengths that increase in proportion to the scale of space. This creates a theoretical headache at the highest energies. To avoid infinities, an “ultraviolet cutoff” is required at or below the Planck mass. This must be at a fixed physical wavelength. In order to maintain this cutoff in an expanding space, it is necessary to continuously create new modes at the cutoff scale as the wavelength of the previously present modes increases. This implies a violation of unitarity. If dark energy were a cosmological constant, then modes with wavelength equal to the cutoff scale at the present time would become classical at some time in the future, and the violation of unitarity would be visible in hypothetical future observations. To avoid this problem, we conclude that dark energy must be time-dependent.

Because of its deep implications for fundamental physics, we are eagerly awaiting new observational results that will shine more light on the issue of the time-dependence of dark energy.

The post Four reasons dark energy should evolve with time appeared first on CERN Courier.

The 2025 European Physical Society Conference on High Energy Physics (EPS-HEP), held in Marseille from 7 to 11 July, took centre stage in this pivotal year for high-energy physics as the community prepares to make critical decisions on the next flagship collider at CERN to enable major leaps at the high-precision and high-energy frontiers. The meeting showcased the remarkable creativity and innovation in both experiment and theory, driving progress across all scales of fundamental physics. It also highlighted the growing interplay between particle, nuclear, astroparticle physics and cosmology.

Advancing the field relies on the ability to design, build and operate increasingly complex instruments that push technological boundaries. This requires sustained investment from funding agencies, laboratories, universities and the broader community to support careers and recognise leadership in detectors, software and computing. Such support must extend across construction, commissioning and operation, and include strategic and basic R&D. The implementation of detector R&D (DRD) collaborations, as outlined in the 2021 ECFA roadmap, is an important step in this direction.

Physics thrives on precision, and a prime example this year came from the Muon g–2 collaboration at Fermilab, which released its final result combining all six data runs, achieving an impressive 127 parts-per-billion precision on the muon anomalous magnetic moment (CERN Courier July/August 2025 p7). The result agrees with the latest lattice–QCD predictions for the leading hadronic–vacuum-polarisation term, albeit within a four times larger theoretical uncertainty than the experimental one. Continued improvements to lattice QCD and to the traditional dispersion-relation method based on low-energy e⁺e^– and τ data are expected in the coming years.

Runaway success

After the remarkable success of LHC Run 2, Run 3 has now surpassed it in delivered luminosity. Using the full available Run-2 and Run-3 datasets, ATLAS reported 3.4σ evidence for the rare Higgs decay to a muon pair, and a new result on the quantum-loop mediated decay into a Z boson and a photon, now more consistent with the Standard Model prediction than the earlier ATLAS and CMS Run-2 combination (see “Mapping rare Higgs-boson decays”). ATLAS also presented an updated study of Higgs pair production with decays into two b-quarks and two photons, whose sensitivity was increased beyond statistical gains thanks to improved reconstruction and analysis. CMS released a new Run-2 search for Higgs decays to charm quarks in events produced with a top-quark pair, reaching sensitivity comparable to the traditional weak-boson-associated production. Both collaborations also released new combinations of nearly all their Higgs analyses from Run 2, providing a wide set of measurements. While ATLAS sees overall agreement with predictions, CMS observes some non-significant tensions.

Advancing the field relies on the ability to design, build and operate increasingly complex instruments that push technological boundaries

A highlight in top-quark physics this year was the observation by CMS of an excess in top-pair production near threshold, confirmed at the conference by ATLAS (see “ATLAS confirms top–antitop excess”). The physics of the strong interaction predicts highly compact, colour-singlet, quasi-bound pseudoscalar top–antitop state effects arising from gluon exchange. Unlike bottomonium or charmonium, no proper bound state is formed due to the rapid weak decay of the top quark (see “Memories of quarkonia”). This “toponium” effect can be modelled with the use of non-relativistic QCD. Both experiments observed a cross section about 100 times smaller than for inclusive top-quark pair production. The subtle signal and complex threshold modelling make the analysis challenging, and warrant further theoretical and experimental investigation.

A major outcome of LHC Run 2 is the lack of compelling evidence for physics beyond the Standard Model. In Run 3, ATLAS and CMS continue their searches, aided by improved triggers, reconstruction and analysis techniques, as well as a dataset more than twice as large, enabling a more sensitive exploration of rare or suppressed signals. The experiments are also revisiting excesses seen in Run 2, for example, a CMS hint of a new resonance decaying into a Higgs and another scalar was not confirmed by a new ATLAS analysis including Run-3 data.

Hadron spectroscopy has seen a renaissance since Belle’s 2003 discovery of the exotic X(3872), with landmark advances at the LHC, particularly by LHCb. CMS recently reported three new four-charm-quark states decaying into J/ψ pairs between 6.6 and 7.1 GeV. Spin-parity analysis suggests they are tightly bound tetraquarks rather than loosely bound molecular states (CERN Courier November/December 2024 p33).

Rare observations

Flavour physics continues to test the Standard Model with high sensitivity. Belle-II and LHCb reported new CP violation measurements in the charm sector, confirming the expected small effects. LHCb observed, for the first time, CP violation in the baryon sector via Λ_b decays, a milestone in CP violation history. NA62 at CERN’s SPS achieved the first observation of the ultra-rare kaon decay K⁺ → π⁺νν with a branching ratio of 1.3 × 10^–10, matching the Standard Model prediction. MEG-II at PSI set the most stringent limit to date on the lepton-flavour-violating decay μ → eγ, excluding branching fractions above 1.5 × 10^–13. Both experiments continue data taking until 2026.

Heavy-ion collisions at the LHC provide a rich environment to study the quark–gluon plasma, a hot, dense state of deconfined quarks and gluons, forming a collective medium that flows as a relativistic fluid with an exceptionally low viscosity-to-entropy ratio. Flow in lead–lead collisions, quantified by Fourier harmonics of spatial momentum anisotropies, is well described by hydrodynamic models for light hadrons. Hadrons containing heavier charm and bottom quarks show weaker collectivity, likely due to longer thermalisation times, while baryons exhibit stronger flow than mesons due to quark coalescence. ALICE reported the first LHC measurement of charm–baryon flow, consistent with these effects.

Spin-parity analysis suggests the states are tightly bound tetraquarks

Neutrino physics has made major strides since oscillations were confirmed 27 years ago, with flavour mixing parameters now known to a few percent.  Crucial questions still remain: are neutrinos their own antiparticles (Majorana fermions)? What is the mass ordering – normal or inverted? What is the absolute mass scale and how is it generated? Does CP violation occur? What are the properties of the right-handed neutrinos? These and other questions have wide-ranging implications for particle physics, astrophysics and cosmology.

Neutrinoless double-beta decay, if observed, would confirm that neutrinos are Majorana particles. Experiments using xenon and germanium are beginning to constrain the inverted mass ordering, which predicts higher decay rates. Recent combined data from the long-baseline experiments T2K and NOvA show no clear preference for either ordering, but exclude vanishing CP violation at over 3σ in the inverted scenario. The KM3NeT detector in the Mediterranean, with its ORCA and ARCA components, has delivered its first competitive oscillation results, and detected a striking ~220 PeV muon neutrino, possibly from a blazar (CERN Courier March/April 2025 p7). The next-generation large-scale neutrino experiments JUNO (China), Hyper-Kamiokande (Japan) and LBNF/DUNE (USA) are progressing in construction, with data-taking expected to begin in 2025, 2028 and 2031, respectively. LBNF/DUNE is best positioned to determine the neutrino mass ordering, while Hyper-Kamiokande will be the most sensitive to CP violation. All three will also search for proton decay, a possible messenger of grand unification.

There is compelling evidence for dark matter from gravitational effects across cosmic times and scales, as well as indications that it is of particle origin. Its possible forms span a vast mass range, up to the ~100 TeV unitarity limit for a thermal relic, and may involve a complex, structured “dark sector”. The wide complementarity among the search strategies gives the field a unifying character. Direct detection experiments looking for tiny, elastic nuclear recoils, such as XENONnT (Italy), LZ (USA) and PandaX-4T (China), have set world-leading constraints on weakly interacting massive particles. XENONnT and PandaX-4T have also reported first signals from boron-8 solar neutrinos, part of the so-called “neutrino fog” that will challenge future searches. Axions, introduced theoretically to suppress CP violation in strong interactions, could be viable dark-matter candidates. They would be produced in the early universe with enormous number density, behaving, on galactic scales, as a classical, nonrelativistic, coherently oscillating bosonic field, effectively equivalent to cold dark matter. Axions can be detected via their conversion into photons in strong magnetic fields. Experiments using microwave cavities have begun to probe the relevant μeV mass range of relic QCD axions, but the detection becomes harder at higher masses. New concepts, using dielectric disks or wire-based plasmonic resonance, are under development to overcome these challenges.

Cosmological constraints

Cosmology featured prominently at EPS-HEP, driven by new results from the analysis of DESI DR2 baryon acoustic oscillation (BAO) data, which include 14 million redshifts. Like the cosmic microwave background (CMB), BAO also provides a “standard ruler” to trace the universe’s expansion history – much like supernovae (SNe) do as standard candles. Cosmological surveys are typically interpreted within the ΛCDM model, a six-parameter framework that remarkably accounts for 13.8 billion years of cosmic evolution, from inflation and structure formation to today’s energy content, despite offering no insight into the nature of dark matter, dark energy or the inflationary mechanism. Recent BAO data, when combined with CMB and SNe surveys, show a preference for a form of dark energy that weakens over time. Tensions also persist in the Hubble expansion rate derived from early-universe (CMB and BAO) and late-universe (SN type-Ia) measurements (CERN Courier March/April 2025 p28). However, anchoring SN Ia distances in redshift remains challenging, and further work is needed before drawing firm conclusions.

Cosmological fits also constrain the sum of neutrino masses. The latest CMB and BAO-based results within ΛCDM appear inconsistent with the lower limit implied by oscillation data for inverted mass ordering. However, firm conclusions are premature, as the result may reflect limitations in ΛCDM itself. Upcoming surveys from the Euclid satellite and the Vera C. Rubin Observatory (LSST) are expected to significantly improve cosmological constraints.

Cristinel Diaconu and Thomas Strebler, chairs of the local organising committee, together with all committee members and many volunteers, succeeded in delivering a flawlessly organised and engaging conference in the beautiful setting of the Palais du Pharo overlooking Marseille’s old port. They closed the event with a memorial phrase of British cyclist Tom Simpson: “There is no mountain too high.”

The post High-energy physics meets in Marseille appeared first on CERN Courier.

The international conference Dark Matter and Stars: Multi-Messenger Probes of Dark Matter and Modified Gravity (ICDMS) was held at Queen’s University in Kingston, Ontario, Canada, from 14 to 16 July. The meeting brought together around 70 researchers from across astrophysics, cosmology, particle physics and gravitational theory. The goal was to foster interdisciplinary dialogue on how observations of stellar systems, gravitational waves and cosmological data can help shed light on the dark sector. The conference was specifically dedicated to exploring how astrophysical and cosmological systems can be used to probe the nature of dark matter.

The first day centred on compact objects as natural laboratories for dark-matter physics. Giorgio Busoni (University of Adelaide) opened with a comprehensive overview of recent theoretical progress on dark-matter accumulation in neutron stars and white dwarfs, highlighting refinements in the treatment of relativistic effects, optical depth, Fermi degeneracy and light mediators – all of which have shaped the field in recent years. Melissa Diamond (Queen’s University) followed with a striking talk with a nod to Dr. Strangelove, exploring how accumulated dark matter might trigger thermonuclear instability in white dwarfs. Sandra Robles (Fermilab) shifted the perspective from neutron stars to white dwarfs, showing how they constrain dark-matter properties. One of the authors highlighted postmerger gravitational-wave observations as a tool to distinguish neutron stars from low-mass black holes, offering a promising avenue for probing exotic remnants potentially linked to dark matter. Axions featured prominently throughout the day, alongside extensive discussions of the different ways in which dark matter affects neutron stars and their mergers.

ICDMS continues to strengthen the interface between fundamental physics and astrophysical observations

On the second day, attention turned to the broader stellar population and planetary systems as indirect detectors. Isabelle John (University of Turin) questioned whether the anomalously long lifetimes of stars near the galactic centre might be explained by dark-matter accumulation. Other talks revisited stellar systems – white dwarfs, red giants and even speculative dark stars – with a focus on modelling dark-matter transport and its effects on stellar heat flow. Complementary detection strategies also took the stage, including neutrino emission, stochastic gravitational waves and gravitational lensing, all offering potential access to otherwise elusive energy scales and interaction strengths.

The final day shifted toward galactic structure and the increasingly close interplay between theory and observation. Lina Necib (MIT) shared stellar kinematics data used to map the Milky Way’s dark-matter distribution, while other speakers examined the reliability of stellar stream analyses and subtle anomalies in galactic rotation curves. The connection to terrestrial experiments grew stronger, with talks tying dark matter to underground detectors, atomic-precision tools and cosmological observables such as the Lyman-alpha forest and baryon acoustic oscillations. Early-career researchers contributed actively across all sessions, underscoring the field’s growing vitality and introducing a fresh influx of ideas that is expanding its scope.

The ICDMS series is now in its third edition. It began in 2018 at Instituto Superior Técnico, Portugal, and is poised to become an annual event. The next conference will take place at the University of Southampton, UK, in 2026, followed by the Massachusetts Institute of Technology in the US in 2027. With increasing participation and growing international interest, the ICDMS series continues to strengthen the interface between fundamental physics and astrophysical observations in the quest to understand the nature of dark matter.

The post Probing the dark side from Kingston appeared first on CERN Courier.

As higher experimental precision relies on new technologies, new theory results require better algorithms, both from the mathematical and computer-algebraic side, and new techniques in quantum field theory. The central software package for perturbative calculations, FORM, now has a new major release, FORM 5. Progress has also been achieved in integration-by-parts reduction, which is of central importance for reducing to a much smaller set of master integrals. New developments were also reported in analytic and numerical Feynman-diagram integration using Mellin–Barnes techniques, new compact function classes such as Feynman–Fox integrals, and modern summation technologies and methods to establish and solve gigantic recursions and differential equations of degree 4000 and order 100. The latest results on elliptic integrals and progress on the correct treatment of the γ₅-problem in real dimensions were also presented. These technologies allow the calculation of processes up to five loops and in the presence of more scales at two- and three-loop order. New results for single-scale quantities like quark condensates and the ρ-parameter were also reported.

In the loop

Measurements at future colliders will depend on the precise knowledge of parton distribution functions, the strong coupling constant α_s(M_Z) and the heavy-quark masses. Experience suggests that going from one loop order to the next in the massless and massive cases takes 15 years or more, as new technologies must be developed. By now, most of the space-like four-loop splitting functions governing scaling violations are known with a good precision, as well as new results for the three-loop time-like splitting functions. The massive three-loop Wilson coefficients for deep-inelastic scattering are now complete, requiring far larger and different integral spaces compared with the massless case. Related to this are the Wilson coefficients of semi-inclusive deep-inelastic scattering at next-to-next-to leading order (NNLO), which will be important to tag individual flavours at the EIC. For the α_s(M_Z) measurement at low-scale processes, the correct treatment of renormalon contributions is necessary. Collisions at high energies also allow the detailed study of scattering processes in the forward region of QCD. Other long-term projects concern NNLO corrections for jet-production at e⁺e^– and hadron colliders, and other related processes like Higgs-boson and top-quark production, in some cases with a large number of partons in the final state. This also includes the use of effective Lagrangians.

Many more steps lie ahead if we are to match the precision of measurements at high-luminosity colliders

The complete calculation of difficult processes at NNLO and beyond always drives the development of term-reduction algorithms and analytic or numerical integration technologies. Many more steps lie ahead in the coming years if we are to match the precision of measurements at high-luminosity colliders. Some of these will doubtless be reported at Loopsummit-3 in summer 2027.

The post Loopsummit returns to Cadenabbia appeared first on CERN Courier.

The 39th edition of the International Cosmic Ray Conference (ICRC), a key biennial conference in astroparticle physics, was held in Geneva from 15 to 24 July. Plenary talks covered solar, galactic and ultra-high-energy cosmic rays. A strong multi-messenger perspective combined measurements of charged particles, neutrinos, gamma rays and gravitational waves. Talks were informed by limits from the LHC and elsewhere on dark-matter particles and primordial black-holes. The bundle of constraints has improved very significantly over the past few years, allowing more meaningful and stringent tests.

Solar modelling

The Sun and its heliosphere, where the solar wind offers insights into magnetic reconnection, shock acceleration and diffusion, are now studied in situ thanks to the Solar Orbiter and Parker Solar Probe spacecraft. Long-term PAMELA and AMS data, spanning over an 11-year solar cycle, allow precise modelling of solar modulation of cosmic-ray fluxes below a few tens of GeV. AMS solar proton data show a 27-day periodicity up to 20 GV, caused by corotating interaction regions where fast solar wind overtakes slower wind, creating shocks. AMS has recorded 46 solar energetic particle (SEP) events, the most extreme reaching a few GV, from magnetic-reconnection flares or fast coronal mass ejections. While isotope data once suggested such extreme events occur every 1500 years, Kepler observations of Sun-like stars indicate they may happen every 100 years, releasing more than 10³⁴ erg, often during weak solar minima, and linked to intense X-ray flares.

The spectrum of galactic cosmic rays, studied with high-precision measurements from satellites (DAMPE) and ISS-based experiments (AMS-02, CALET, ISS-CREAM), is not a single power law but shows breaks and slope changes, signatures of diffusion or source effects. A hardening at about 500 GV, common to all primaries, and a softening at 10 TV, are observed in protons and He spectra by all experiments – and for the first time also in DAMPE’s O and C. As the hardening is detected in primary spectra scaling at the same rigidity (charge, not mass) as in secondary-to-primary ratios, they are attributed to propagation in the galaxy and not to source-related effects. This is supported by secondary (Li, Be, B) spectra with breaks about twice as strong as primaries (He, C, O). A second hardening at 150 TV was reported by ISS-CREAM (p) and DAMPE (p + He) for the first time, broadly consistent – within large hadronic-model and statistical uncertainties – with indirect ground-based results from GRAPES and LHAASO.

A strong multi-messenger perspective combined measurements of charged particles, neutrinos, gamma rays and gravitational waves

Ratios of secondary over primary species versus rigidity R (energy per unit charge) probe the ratio of the galactic halo size H to the energy-dependent diffusion coefficient D(R), and so measure the “grammage” of material through which cosmic rays propagate. Unstable/stable secondary isotope ratios probe the escape times of cosmic rays from the halo (H²/D(R)), so from both measurements H and D(R) can be derived. The flattening evidenced by the highest energy point at 10 to 12 GeV/nucleon of the ¹⁰Be/⁹Be ratio as a function of energy, hints at a possibly larger halo than previously believed beyond 5 kpc, to be tested by HELIX. AMS-02 spectra of single elements will soon allow separation of the primary and secondary fractions for each nucleus, also based on spallation cross-sections. Anomalies remain, such as a flattening at ~7 TeV/nucleon in Li/C and B/C, possibly indicating reacceleration or source grammage. AMS-02’s ⁷Li/⁶Li ratio disagrees with pure secondary models, but cross-section uncertainties preclude firm conclusions on a possible Li primary component, which would be produced by a new population of sources.

The muon puzzle

The dependency of ground-based cosmic-ray measurements on hadronic models has been widely discussed by Boyd and Pierog, highlighting the need for more measurements at CERN, such as the recent proton-O run being analysed by LHCf. The EPOS–LHC model, based on the core–corona approach, shows reduced muon discrepancies, producing more muons and a heavier composition, namely deeper shower maxima (+20 g/cm²) than earlier models. This clarifies the muon puzzle raised by Pierre Auger a few years ago of a larger muon content in atmospheric showers than simulations. A fork-like structure remains in the knee region of the proton spectrum, where the new measurements presented by LHAASO are in agreement with IceTop/IceCube, and could lead to a higher content of protons beyond the knee than hinted at by KASCADE and the first results of GRAPES. Despite the higher proton fluxes, a dominance of He above the knee is observed, which requires a special kind of close-by source to be hypothesised.

Multi-messenger approaches

Gamma-ray and neutrino astrophysics were widely discussed at the conference, highlighting the relevance of multi-messenger approaches. LHAASO produced impressive results on UHE astrophysics, revealing a new class of pevatrons: microquasars alongside young massive clusters, pulsar wind nebulae (PWNe) and supernova remnants.

Microquasars are gamma-ray binaries containing a stellar-mass black hole that drives relativistic jets while accreting matter from their companion stars. Outstanding examples include Cyg X-3, a potential PeV microquasar, from which the flux of PeV photons is 5–10 times higher than in the rest of the Cygnus bubble.

Five other microquasars are observed beyond 100 TeV: SS 433, V4641 Sgr, GRS 1915 + 105, MAXI J1820 + 070 and Cygnus X-1. SS 433 is a microquasar with two gamma-ray emitting jets nearly perpendicular to our line of sight, terminated at 40 pc from the black hole (BH) identified by HESS and LHAASO beyond 10 TeV. Due to the Klein–Nishina effect, the inverse Compton flux above ~10 TeV is gradually suppressed, and an additional spectral component is needed to explain the flux around 100 TeV.

Gamma-ray and neutrino astrophysics were widely discussed at the conference

Beyond 100 TeV, LHAASO also identifies a source coincident with a giant molecular cloud; this component may be due to protons accelerated close to the BH or in the lobes. These results demonstrate the ability to resolve the morphology of extended galactic sources. Similarly, ALMA has discovered two hotspots, both at 0.28° (about 50 pc) from GRS 1915 + 105 in opposite directions from its BH. These may be interpreted as two lobes, or the extended nature of the LHAASO source may instead be due to the spatial distribution of the surrounding gas, if the emission from GRS 1915 + 105 is dominated by hadronic processes.

Further discussions addressed pulsar halos and PWNe as unique laboratories for studying the diffusion of electrons and mysterious as-yet-unidentified pevatrons, such as MGRO J1908 + 06, coincident with a SNR (favoured) and a PSR. One of these sources may finally reveal an excess in KM3NeT or IceCube neutrinos, proving their cosmic-ray accelerator nature directly.

The identification and subtraction of source fluxes on the galactic plane is also important for the measurement of the galactic plane neutrino flux by IceCube. This currently assumes a fixed spectral index of E^–2.7, while authors like Grasso et al. presented a spectrum becoming as soft as E^–2.4, closer to the galactic centre. The precise measurements of gamma-ray source fluxes and the diffuse emission from galactic cosmic rays interacting in the interstellar matter lead to better constraints on neutrino observations and on cosmic ray fluxes around the knee.

Cosmogenic origins

KM3NeT presented a neutrino of energy well beyond the diffuse cosmic neutrino flux of IceCube, which does not extend beyond 10 PeV (CERN Courier March/April 2025 p7). Its origin was widely discussed at the conference. The large error on its estimated energy – 220 PeV, within a 1σ confidence interval of 110 to 790 PeV – makes it nevertheless compatible with the flux observed by IceCube, for which a 30 TeV break was first hypothesised at this conference. If events of this kind are confirmed, they could have transient or dark-matter origins, but a cosmogenic origin is improbable due to the IceCube and Pierre Auger limits on the cosmogenic neutrino flux.

The post Geneva witnesses astroparticle boom appeared first on CERN Courier.

Reconciling general relativity and quantum mechanics remains a central problem in fundamental physics. Though successful in their own domains, the two theories resist unification and offer incompatible views of space, time and matter. The field of quantum gravity, which has sought to resolve this tension for nearly a century, is still plagued by conceptual challenges, limited experimental guidance and a crowded landscape of competing approaches. Now in its third instalment, the “Quantum Gravity” conference series addresses this fragmentation by promoting open dialogue across communities. Organised under the auspices of the International Society for Quantum Gravity (ISQG), the 2025 edition took place from 21 to 25 July at Penn State University. The event gathered researchers working across a variety of frameworks – from random geometry and loop quantum gravity to string theory, holography and quantum information. At its core was the recognition that, regardless of specific research lines or affiliations, what matters is solving the puzzle.

One step to get there requires understanding the origin of dark energy, which drives the accelerated expansion of the universe and is typically modelled by a cosmological constant Λ. Yasaman K Yazdi (Dublin Institute for Advanced Studies) presented a case for causal set theory, reducing spacetime to a discrete collection of events, partially ordered to capture cause–effect relationships. In this context, like a quantum particle’s position and momentum, the cosmological constant and the spacetime volume are conjugate variables. This leads to the so-called “ever-present Λ” models, where fluctuations in the former scale as the inverse square root of the latter, decreasing over time but never vanishing. The intriguing agreement between the predicted size of these fluctuations and the observed amount of dark energy, while far from resolving quantum cosmology, stands as a compelling motivation for pursuing the approach.

In the spirit of John Wheeler’s “it from bit” proposal, Jakub Mielczarek (Jagiellonian University) suggested that our universe may itself evolve by computing – or at least admit a description in terms of quantum information processing. In loop quantum gravity, space is built from granular graphs known as spin networks, which capture the quantum properties of geometry. Drawing on ideas from tensor networks and holography, Mielczarek proposed that these structures can be reinterpreted as quantum circuits, with their combinatorial patterns reflected in the logic of algorithms. This dictionary offers a natural route to simulating quantum geometry, and could help clarify quantum theories that, like general relativity, do not rely on a fixed background.

Quantum clues

What would a genuine quantum theory of spacetime achieve, though? According to Esteban Castro Ruiz (IQOQI), it may have to recognise that reference frames, which are idealised physical systems used to define spatio-temporal distances, must themselves be treated as quantum objects. In the framework of quantum reference frames, notions such as entanglement, localisation and superposition become observer-dependent. This leads to a perspective-neutral formulation of quantum mechanics, which may offer clues for describing physics when spacetime is not only dynamical, but quantum.

The conference’s inclusive vocation came through most clearly in the thema­tic discussion sessions, including one on the infamous black-hole information problem chaired by Steve Giddings (UC Santa Barbara). A straightforward reading of Stephen Hawking’s 1974 result suggests that black holes radiate, shrink and ultimately destroy information – a process that is incompatible with standard quantum mechanics. Any proposed resolution must face sharp trade-offs: allowing information to escape challenges locality, losing it breaks unitarity and storing it in long-lived remnants undermines theoretical control. Giddings described a mild violation of locality as the lesser evil, but the controversy is far from settled. Still, there is growing consensus that dissolving the paradox may require new physics to appear well before the Planck scale, where quantum-gravity effects are expected to dominate.

Once the domain of pure theory, quantum gravity has become eager to engage with experiment

Among the few points of near-universal agreement in the quantum-gravity community has long been the virtual impossibility of detecting a graviton, the hypothetical quantum of the gravitational field. According to Igor Pikovski (Stockholm University), things may be less bleak than once thought. While the probability of seeing graviton-induced atomic transitions is negligible due to the weakness of gravity, the situation is different for massive systems. By cooling a macroscopic object close to absolute zero, Pikovski suggested, the effect could be amplified enough, with current interferometers simultaneously monitoring gravitational waves in the correct frequency window. Such a signal would not amount to a definitive proof of gravity’s quantisation, just as the photoelectric effect could not definitely establish the existence of photons, nor would it single out a specific ultraviolet model. However, it could constrain concrete predictions and put semiclassical theories under pressure. Giulia Gubitosi (University of Naples Federico II) tackled phenomenology from a different angle, exploring possible deviations from special relativity in models where spacetime becomes non-commutative. There, coordinates are treated like quantum operators, leading to effects like decoherence, modified particle speeds and soft departures from locality. Although such signals tend to be faint, they could be enhanced by high-energy astrophysical sources: observations of neutrinos corresponding to gamma-ray bursts are now starting to close in on these scenarios. Both talks reflected a broader, cultural shift: quantum gravity, once the domain of pure theory, has become eager to engage with experiment.

Quantum Gravity 2025 offered a wide snapshot of a field still far from closure, yet increasingly shaped by common goals, the convergence of approaches and cross-pollination. As intended, no single framework took centre stage, with a dialogue-based format keeping focus on the central, pressing issue at hand: understanding the quantum nature of spacetime. With limited experimental guidance, open exchange remains key to clarifying assumptions and avoiding duplication of efforts. Building on previous editions, the meeting pointed toward a future where quantum-gravity researchers will recognise themselves as part of a single, coherent scientific community.

The post Quantum gravity beyond frameworks appeared first on CERN Courier.

UPC studies have expanded significantly since the first UPC workshop in Mexico in December 2023. The opportunity to study scattering processes in a clean photon–nucleus environment at collider energies has inspired experimentalists to examine both inclusive and exclusive scattering processes, and to look for signals of collectivity and even the formation of quark–gluon plasma (QGP) in this unique environment.

For many years, experimental activity in UPCs was mainly focused on exclusive processes and QED phenomena including photon–photon scattering. This year, fresh inclusive particle-production measurements gained significant attention, as well as various signatures of QGP-like behaviour observed by different experiments at RHIC and at the LHC. The importance of having complementing experiments to perform similar measurements was also highlighted. In particular, the ATLAS experiment joined the ongoing activities to measure exclusive vector–meson photoproduction, finding a cross section that disagrees with the previous ALICE measurements by almost 50%. After long and detailed discussions, it was agreed that different experimental groups need to work together closely to resolve this tension before the next UPC workshop.

Experimental and theoretical developments very effectively guide each other in the field of UPCs. This includes physics within and beyond the Standard Model (BSM), such as nuclear modifications to the partonic structure of protons and neutrons, gluon-saturation phenomena predicted by QCD (CERN Courier January/February 2025 p31), and precision tests for BSM physics in photon–photon collisions. The expanding activity in the field of UPCs, together with the construction of the Electron Ion Collider (EIC) at Brookhaven National Laboratory in the US, has also made it crucial to develop modern Monte Carlo event generators to the level where they can accurately describe various aspects of photon–photon and photon–nucleus scatterings.

As a photon collider, the LHC complements the EIC. While the centre-of-mass energy at the EIC will be lower, there is some overlap between the kinematic regions probed by these two very different collider projects thanks to the varying energy spectra of the photons. This allows the theoretical models needed for the EIC to be tested against UPC data, thereby reducing theoretical uncertainty on the predictions that guide the detector designs. This complementarity will enable precision studies of QCD phenomena and BSM physics in the 2030s.

The post Ultra-peripheral physics in the ultraperiphery appeared first on CERN Courier.

For Heike Riel, IBM fellow and head of science and technology at IBM Research, successful careers in science are built not by choosing between academia and industry, but by moving fluidly between them. With a background in semiconductor physics and a leadership role in one of the world’s top industrial research labs, Riel learnt to harness the skills she picked up in academia, and now uses them to build real-world applications. Today, IBM collaborates with academia and industry partners on projects ranging from quantum computing and cybersecurity to developing semiconductor chips for AI hardware.

“I chose semiconductor physics because I wanted to build devices, use electronics and understand photonics,” says Riel, who spent her academic years training to be an applied physicist. “There’s fundamental science to explore, but also something that can be used as a product to benefit society. That combination was very motivating.”

Hands-on mindset

For experimental physicists, this hands-on mindset is crucial. But experiments also require infrastructure that can be difficult to access in purely academic settings. “To do experiments, you need cleanrooms, fabrication tools and measurement systems,” explains Riel. “These resources are expensive and not always available in university labs.” During her first industry job at Hewlett-Packard in Palo Alto, Riel realised just how much she could achieve if given the right resources and support. “I felt like I was then the limit, not the lab,” she recalls.

This experience led Riel to proactively combine academic and industrial research in her PhD with IBM, where cutting-edge experiments are carried out towards a clear, purpose-driven goal within a structured research framework, leaving lots of leeway for creativity. “We explore scientific questions, but always with an application in mind,” says Riel. “Whether we’re improving a product or solving a practical problem, we aim to create knowledge and turn it into impact.”

Shifting gears

According to Riel, once you understand the foundations of fundamental physics, and feel as though you have learnt all the skills you can leach from it, then it’s time to consider shifting gears and expanding your skills with economics or business. In her role, understanding economic value and organisational dynamics is essential. But Riel advises against independently pursuing an MBA. “Studying economics or an MBA later is very doable,” she says. “In fact, your company might even financially support you. But going the other way – starting with economics and trying to pick up quantum physics later – is much harder.”

Riel sees university as a precious time to master complex subjects like quantum mechanics, relativity and statistical physics – topics that are difficult to revisit later in life. “It’s much easier to learn theoretical physics as a student than to go back to it later,” she says. “It builds something more important than just knowledge: it builds your tolerance for frustration, and your capacity for deep logical thinking. You become extremely analytical and much better at breaking down problems. That’s something every employer values.”

In demand

High-energy physicists are even in high demand in fields like consulting, says Riel. A high-achieving academic has a really good chance at being hired, as long as they present their job applications effectively. When scouring applications, recruiters look for specific key words and transferable skills, so regardless of the depth or quality of your academic research, the way you present yourself really counts. Physics, Riel argues, teaches a kind of thinking that’s both analytical and resilient. With experimental physics, your application can be tailored towards hands-on experience and understanding tangible solutions to real-world problems. For theoretical physicists, your application should demonstrate logical problem-solving and thinking outside of the box. “The winning combination is having aspects of both,” says Riel.

On top of that, research in physics increases your “frustration tolerance”. Every physicist has faced failure at one point during their academic career. But their determination to persevere is what makes them resilient. Whether this is through constantly thinking on your feet, or coming up with new solutions to the same problems, this resilience is what can make a physicist’s application pierce through the others. “In physics, you face problems every day that don’t have easy answers, and you learn how to deal with that,” explains Riel. “That mindset is incredibly useful, whether you’re solving a semiconductor design problem or managing a business unit.”

Academic research is often driven by curiosity and knowledge gain, while industrial research is shaped by application

Riel champions the idea of the “T-shaped person”: someone with deep expertise in one area (the vertical stroke of the T) and broad knowledge across fields (the horizontal bar of the T). “You start by going deep – becoming the go-to person for something,” says Riel. This deep knowledge builds your credibility in your desired field: you become the expert. But after that, you need to broaden your scope and understanding.

That breadth can include moving between fields, working on interdisciplinary projects, or applying physics in new domains. “A T-shaped person brings something unique to every conversation,” adds Riel. “You’re able to connect dots that others might not even see, and that’s where a lot of innovation happens.”

Adding the bar on the T means that you can move fluidly between different fields, including through academia and industry. For this reason, Riel believes that the divide between academia and industry is less rigid than people assume, especially in large research organisations like IBM. “We sit in that middle ground,” she explains. “We publish papers. We work with universities on fundamental problems. But we also push toward real-world solutions, products and economic value.”

The difficult part is making the leap from academia to industry. “You need the confidence to make the decision, to choose between working in academia or industry,” says Riel. “At some point in your PhD, your first post-doc, or maybe even your second, you need to start applying your practical skills to industry.” Companies like IBM offer internships, PhDs, research opportunities and temporary contracts for physicists all the way from masters students to high-level post-docs. These are ideal ways to get your foot in the door of a project, get work published, grow your network and garner some of those industry-focused practical skills, regardless of the stage you are at in your academic career. “You can learn from your colleagues about economy, business strategy and ethics on the job,” says Riel. “If your team can see you using your practical skills and engaging with the business, they will be eager to help you up-skill. This may mean supporting you through further study, whether it’s an online course, or later an MBA.”

Applied knowledge

Riel notes that academic research is often driven by curiosity and knowledge gain, while industrial research is shaped by application. “US funding is often tied to applications, and they are much stronger at converting research into tangible products, whereas in Europe there is still more of a divide between knowledge creation and the next step to turn this into products,” she says. “But personally, I find it most satisfying when I can apply what I learn to something meaningful.”

That applied focus is also cyclical, she says. “At IBM, projects to develop hardware often last five to seven years. Software development projects have a much faster turnaround. You start with an idea, you prove the concept, you innovate the path to solve the engineering challenges and eventually it becomes a product. And then you start again with something new.” This is different to most projects in academia, where a researcher contributes to a small part of a very long-term project. Regardless of the timeline of the project, the skills gained from academia are invaluable.

For early-career researchers, especially those in high-energy physics, Riel’s message is reassuring: “Your analytical training is more useful than you think. Whether you stay in academia, move to industry, or float between both, your skills are always relevant. Keep learning and embracing new technologies.”

The key, she says, is to stay flexible, curious and grounded in your foundations. “Build your depth, then your breadth. Don’t be afraid of crossing boundaries. That’s where the most exciting work happens.”

The post Becoming T-shaped appeared first on CERN Courier.

Precision and rigour

Specht began his career in nuclear physics under the mentorship of Heinz Maier-Leibnitz at the Technische Universität München. His early work was grounded in precision measurements and experimental rigour. Among his most celebrated early achievements were the discoveries of superheavy quasi-molecules and quasi-atoms, where electrons can be bound for short times to a pair of heavy ions, and nuclear-shape isomerism, where nuclei exhibit long-lived prolate or oblate deformations. These milestones significantly advanced the understanding of atomic and nuclear structure. Around 1979, he shifted focus, joining the emerging efforts at CERN to explore the new frontier of ultra-relativistic heavy-ion collisions, which was started five years earlier at Berkeley by the GSI-LBL collaboration. It was Bill Willis, one of CERN’s early advocates for high-energy nucleus–nucleus collisions, who helped draw Specht into this developing field. That move proved foundational for both Specht and CERN.

From the early 1980s through to 2010, Specht played leading roles in four CERN nuclear-collision experiments: R807/808 at the Intersecting Storage Rings, and HELIOS, CERES/NA45 and NA60 at the Super Proton Synchrotron (SPS). As the book describes, he was instrumental, and not only in their scientific goals, namely to search for the highest temperatures of the newly formed hot, dense QCD matter, exceeding the well established Hagedorn limiting hadron fluid temperature of roughly 160 MeV. The overarching aim was to establish that quasi-thermalised gluon matter and even quark–gluon matter can be created at the SPS. Specht was also involved in the design and execution of these detectors. At the Universität Heidelberg, he built a heavy-ion research group and became a key voice in securing German support for CERN’s heavy-ion programme.

CERES was Specht’s brainchild, and stood out for its bold concept

As spokesperson of the HELIOS experiment from 1984 onwards, Specht gained recognition as a community leader. But it was CERES, his brainchild, that stood out for its bold concept: to look for thermal dileptons using a hadron-blind detector – a novel idea at the time that introduced the concept of heavy-ion collision experiments. Despite considerable scepticism, CERES was approved in 1989 and built in under two years. Its results on sulphur–gold collisions became some of the most cited of the SPS era, offering strong evidence for thermal lepton-pair production, potentially from a quark–gluon plasma – a hot and deconfined state of QCD matter then hypothesised to exist at high temperatures and densities, such as in the early universe. Such high temperatures, above the hadrons’ limiting Hagedorn temperature of 160 MeV, had not yet been experimentally demonstrated at LBNL’s Bevalac and Brookhaven’s Alternating Gradient Synchrotron.

Advising ALICE

In the early 1990s, while CERES was being upgraded for lead–gold runs, Specht co-led a European Committee for Future Accelerators working group that laid the groundwork for ALICE, the LHC’s dedicated heavy-ion experiment. His Heidelberg group formally joined ALICE in 1993. Even after becoming scientific director of GSI in 1992, Specht remained closely involved as an advisor.

Specht’s next major CERN project was NA60, which collided a range of nuclei in a fixed-target experiment at the SPS and pushed dilepton measurements to new levels of precision. The NA60 experiment achieved two breakthroughs: a nearly perfect thermal spectrum consistent with blackbody radiation of temperatures 240 to 270 MeV, some hundred MeV above the previous highest hadron Hagedorn temperature of 160 MeV. Clear evidence of in-medium modification of the ρ meson was observed, due to meson collisions with nucleons and heavy baryon resonances, showing that this medium is not only hot, but also that its net baryon density is high. These results were widely seen as strong confirmation of the lattice–QCD-inspired quark–gluon plasma hypothesis. Many chapter authors, some of whom were direct collaborators, others long-time interpreters of heavy-ion signals, highlight the impact NA60 had on the field. Earlier claims, based on competing hadronic signals for deconfinement, such as strong collective hydrodynamic flow, J/ψ melting and quark recombination, were often also described by hadronic transport theory, without assuming deconfinement.

Specht didn’t limit himself to fundamental research. As director of GSI, he oversaw Europe’s first clinical ion-beam cancer therapy programme using carbon ions. The treatment of the first 450 patients at GSI was a breakthrough moment for medical physics and led to the creation of the Heidelberg Ion Therapy centre in Heidelberg, the first hospital-based hadron therapy centre in Europe. Specht later recalled the first successful treatment as one of the happiest moments of his career. In their essays, Jürgen Debus, Hartmut Eickhoff and Thomas Nilsson outline how Specht steered GSI’s mission into applied research without losing its core scientific momentum.

Specht was also deeply engaged in institutional planning, helping to shape the early stages of the Facility for Antiproton and Ion Research, a new facility to study heavy ion collisions, which is expected to start operations at GSI at the end of the decade. He also initiated plasma-physics programmes, and contributed to the development of detector technologies used far beyond CERN or GSI. In parallel, he held key roles in international science policy, including within the Nuclear Physics Collaboration Committee, as a founding board member of the European Centre for Theoretical Studies in Nuclear Physics in Trento, and at CERN as chair of the Proton Synchrotron and Synchro-Cyclotron Committee, and as a decade-long member of the Scientific Policy Committee.

The book doesn’t shy away from more unusual chapters either. In later years, Specht developed an interest in the neuroscience of music. Collaborating with Hans Günter Dosch and Peter Schneider, he explored how the brain processes musical structure – an example of his lifelong intellectual curiosity and openness to interdisciplinary thinking.

Importantly, Scientist and Visionary is not a hagiography. It includes a range of perspectives and technical details that will appeal to both physicists who lived through these developments and younger researchers unfamiliar with the history behind today’s infrastructure. At its best, the book serves as a reminder of how much experimental physics depends not just on ideas, but on leadership, timing and institutional navigation.

That being said, it is not a typical scientific biography. It’s more of a curated mosaic, constructed through personal reflections and contextual essays. Readers looking for deep technical analysis will find it in parts, especially in the sections on CERES and NA60, but its real value lies in how it tracks the development of large-scale science across different fields, from high-energy physics to medical applications and beyond.

For those interested in the history of CERN, the rise of heavy-ion physics, or the institutional evolution of European science, this is a valuable read. And for those who knew or worked with Hans Specht, it offers a fitting tribute – not through nostalgia, but through careful documentation of the many ways Hans shaped the physics and the institutions we now take for granted.

The post The history of heavy ions appeared first on CERN Courier.

Such questions are strategically important for the future of fundamental science, which is increasingly often big science. Two new books tackle its socio-economic impacts head on, though with quite different approaches, one more qualitative in its research, and the other more quantitative. What are the pros and cons of qualitative versus quantitative analysis in social sciences? Personally, as an economist, at a certain point I would tend to say show me the figures! But, admittedly, when assessing the socio-economic impact of large-scale research infrastructures, if good statistical data is not available, I would always prefer a fine-grained qualitative analysis to quantitative models based on insufficient data.

Big Science, Innovation & Societal Contributions, edited by Shantha Liyanage (CERN), Markus Nordberg (CERN) and Marilena Streit-Bianchi (vice president of ARSCIENCIA), takes the qualitative route – a journey into mostly uncharted territory, asking difficult questions about the socio-economic impact of large-scale research infrastructures.

Some figures about the book may be helpful: the three editors were able to collect 15 chapters, with about 100 figures and tables, to involve 34 authors, to list more than 700 references, and to cover a wide range of scientific fields, including particle physics, astrophysics, medicine and computer science. A cursory reading of the list of about 300 acronyms, from AAI (Architecture Adaptive Integrator) to ZEPLIN (ZonEd Proportional scintillation in Liquid Noble gas detector), would be a good test to see how many research infrastructures and collaborations you already know.

After introducing the LHC, a chapter on new accelerator technologies explores a remarkable array of applications of accelerator physics. To name a few: CERN’s R&D in superconductivity is being applied in nuclear fusion; the CLOUD experiment uses particle beams to model atmospheric processes relevant to climate change (CERN Courier January/February 2025 p5); and the ELISA linac is being used to date Australian rock art, helping determine whether it originates from the Pleistocene or Holocene epochs (CERN Courier March/April 2025 p10).

A wide-ranging exploration of how large-scale research infrastructures generate socio-economic value

The authors go on to explore innovation with a straightforward six-step model: scanning, codification, abstraction, diffusion, absorption and impacting. This is a helpful compass to build a narrative. Other interesting issues discussed in this part of the book include governance mechanisms and leadership of large-scale scientific organisations, including in gravitational-wave astronomy. No chapter better illustrates the impact of science on human wellbeing than the survey of medical applications by Mitra Safavi-Naeini and co-authors, which covers three major domains of applications in medical physics: medical imaging with X-rays and PET; radio­therapy targeting cancer cells internally with radioactive drugs or externally using linacs; and more advanced but expensive particle-therapy treatments with beams of protons, helium ions and carbon ions. Personally, I would expect that some of these applications will be enhanced by artificial intelligence, which in turn will have an impact on science itself in terms of digital data interpretation and forecasting.

Sociological perspectives

The last part of the book takes a more sociological perspective, with discussions about cultural values, the social responsibility to make sure big data is open data, and social entrepreneurship. In his chapter on the social responsibility of big science, Steven Goldfarb stresses the importance of the role of big science for learning processes and cultural enhancement. This topic is particularly dear to me, as my previous work on the cost–benefit analysis of the LHC revealed that the value of human capital accumulation for early-stage researchers is among the biggest contributions to the machine’s return on investment.

I recommend Big Science, Innovation & Societal Contributions as a highly infor­mative, non-technical and updated introduction to the landscape of big science, but I would suggest complemen­ting it with another very recent book, The Economics of Big Science 2.0, edited by Johannes Gutleber and Panagiotis Charitos, both currently working at CERN. Charitos was also the co-editor of the volume’s predecessor, The Economics of Big Science, which focuses more on science policy, as well as public investment in science.

Why a “2.0” book? There is a shift of angle. The Economics of Big Science 2.0 builds upon the prior volume, but offers a more quantitative perspective on big science. Notably, it takes advantage of a larger share of contributions by economists, including myself as co-author of a chapter about the public’s perception of CERN.

It is worth clarifying that economics, as a domain within the paradigm of social sciences more generally, has its rules of the game and style. For example, the social sciences can be used as an umbrella encompassing sociology, political science, anthropology, history, management and communication studies, linguistics, psychology and more. The role of economics within sociology is to build quantitative models and to test them with statistical evidence, a field also known as econometrics.

Here, the authors excel. The Economics of Big Science 2.0 offers a wide-ranging exploration of how large-scale research infrastructures generate socio-economic value, primarily driven by quantitative analysis. The authors explore a diverse range of empirical methods, from forming cost–benefit analyses to evaluating econometric modelling, allowing them to assess the tangible effects of big science across multiple fields. There is a unique challenge for applied economics here, as big science centres by definition do not come in large numbers, however the authors involve large numbers of stakeholders, allowing for a statistical analysis of impacts, and the estimation of expected values, standard errors and confidence intervals.

Societal impact

The Economics of Big Science 2.0 examines the socio-economic impact of ESA’s space programmes, the local economic benefits from large-scale facilities and the efficiency benefits from open science. The book measures public attitudes toward and awareness of science within the context of CERN, offering insights into science’s broader societal impacts. It grounds its analyses in a series of focused case studies, including particle colliders such as the LHC and FCC, synchrotron light sources like ESRF and ALBA, and radio telescopes such as SARAO, illustrating the economic impacts of big science through a quantitative lens. In contrast to the more narrative and qualitative approach of Big Science, Innovation & Societal Contributions, The Economics of Big Science 2.0 distinguishes itself through a strong reliance on empirical data.

The post Two takes on the economics of big science appeared first on CERN Courier.

Ivan was born on 26 October 1933 into a family of literary scholars who played an active role in Bulgarian academic life. After graduating from the University of Sofia in 1956, he spent several years at JINR in Dubna and at IAS Princeton, before joining INRNE in Sofia. In 1974 he became a full member of the Bulgarian Academy of Sciences.

Ivan contributed substantially to the development of conformal quantum field theories in arbitrary dimensions. The classification and the complete description of the unitary representations of the conformal group have been collected in two well known and widely used monographs by him and his collaborators. Ivan’s research on constructive quantum field theories and the books devoted to the axiomatic approach have largely influenced modern developments in this area. His early scientific results related to the analytic properties of higher loop Feynman diagrams have also found important applications in perturbative quantum field theory.

Ivan contributed substantially to the development of conformal quantum field theories in arbitrary dimensions

The scientifically highly successful international conferences and schools organised in Bulgaria during the Cold War period under the guidance of Ivan served as meeting grounds for leading Russian and East European theoretical physicists and their West European and American colleagues. They were crucial for the development of theoretical physics in Bulgaria.

Everybody who knew Ivan was impressed by his vast culture and acute intellectual curiosity. His profound and deep knowledge of modern mathematics allowed him to remain constantly in tune with new trends and ideas in theoretical physics. Ivan’s courteous and smiling way of discussing physics, always peppered with penetrating comments and suggestions, was inimitable. His passing is a great loss for theoretical physics, especially in Bulgaria, where he mentored a generation of researchers.

The post Ivan Todorov 1933–2025 appeared first on CERN Courier.

Jonathan L Rosner, a distinguished theoretical physicist and professor emeritus at the University of Chicago, passed away on 24 May 2025. He made profound contributions to particle physics, particularly in quark dynamics and the Standard Model.

Born in New York City, Rosner grew up in Yonkers, NY. He earned his Bachelor of Arts in Physics from Swarthmore College in 1962 and completed his PhD at Princeton University in 1965 with Sam Treiman as his thesis advisor. His early academic appointments included positions at the University of Washington and Tel Aviv University. In 1969 he joined the faculty at the University of Minnesota, where he served until 1982. That year, he became a professor at the University of Chicago, where he remained a central figure in the Enrico Fermi Institute and the Department of Physics until his retirement in 2011.

Rosner’s research spanned a broad spectrum of topics in particle physics, with a focus on the properties and interactions of quarks and leptons in the Standard Model and beyond.

In a highly influential paper in 1969, he pointed out that the duality between hadronic s-channel scattering and t-channel exchanges could be understood graphically, in terms of quark worldlines. Approximately three months before the “November revolution”, i.e. the experimental discovery of charm–anticharm particles, together with the late Mary K Gaillard and Benjamin W Lee, Jon published a seminal paper predicting the properties of hadronic states containing charm quarks.

He made significant contributions to the study of mesons and baryons, exploring their spectra and decay processes. His work on quarkonium systems, particularly the charmonium and bottomonium states, provided critical insights into the strong force that binds quarks together. He also made masterful use of algebraic methods in predicting and analysing CP-violating observables.

In more recent years, Jon focused on exotic combinations of quarks and antiquarks, tetra­quarks and pentaquarks. In 2017 he co-authored a Physical Review Letters paper that provided the first robust prediction of a bbud tetraquark that would be stable under the strong interaction (CERN Courier November/December 2024 p33).

What truly set Jon apart was his rare ability to seamlessly integrate theoretical acumen with practical experimental engagement. While primarily a theoretician, he held a deep appreciation for experimental data and actively participated in the experimental endeavour. A prime example of this was his long-standing involvement with the CLEO collaboration at Cornell University.

He also collaborated on studies related to the detection of cosmic-ray air showers and contributed to the development of prototype systems for detecting radio pulses associated with these high-energy events. His interdisciplinary approach bridged theoretical predictions with experimental observations, enhancing the coherence between theory and practice in high-energy physics.

Unusually for a theorist, Jon was a high-level expert in electronics, rooted through his deep life-long interest in amateur short-wave radio. As with everything else, he did it very thoroughly, from physics analysis to travelling to solar eclipses to take advantage of the increased propagation range of the electromagnetic waves caused by changes in the ionosphere. Rosner was also deeply committed to public service within the scientific community. He served as chair of the Division of Particles and Fields of the American Physical Society in 2013, during which he played a central role in organising the “Snowmass on the Mississippi” conference. This event was an essential part of the long-term strategic planning for the US high-energy physics programme. His leadership and vision were widely recognised and appreciated by his peers.

Throughout his career, Rosner received numerous accolades. He was a fellow of the American Physical Society and was awarded fellowships from the Alfred P. Sloan Foundation and the John Simon Guggenheim Memorial Foundation. His publication record includes more than 500 theoretical papers, reflecting his prolific and highly impactful career in physics. He is survived by his wife, Joy, their two children, Hannah and Benjamin, and a granddaughter, Sadie.

The post Jonathan L Rosner 1941–2025 appeared first on CERN Courier.

César gained his PhD in 1981 from Universidad de Salamanca, where he became professor after working at Harvard, the Institute for Advanced Study and CERN. He held an invited professorship at the Université de Genève between 1987 and 1991, and in this same year, he moved to Consejo Superior de Investigaciones Científicas (CSIC) in Madrid, where he eventually became a founding member of the Instituto de Física Teórica (IFT) UAM–CSIC. He became emeritus in 2024.

Among the large number of topics he worked on during his scientific career, César was initially fascinated by the dynamics of gauge theories. He dedicated his postdoctoral years to problems concerning the structure of the quantum vacuum in QCD, making some crucial contributions.

Focusing in the 1990s on the physics of two-dimensional conformal field theories, he used his special gifts to squeeze physics out of formal structures, leaving his mark in works ranging from superstrings to integrable models, and co-authoring with Martí Ruiz-Altaba and Germán Sierra the book Quantum Groups in Two-Dimensional Physics (Cambridge University Press, 1996). With the new century and the rise of holography, César returned to the topics of his youth: the renormalisation group and gauge theories, now with a completely different perspective.

Far from settling down, in the last decade we discover a very daring César, plunging together with Gia Dvali and other collaborators into a radical approach to understand symmetry breaking in gauge theories, opening new avenues in the study of black holes and the emergence of spacetime in quantum gravity. The magic of von Neumann algebras inspired him to propose an elegant, deep and original understanding of inflationary universes and their quantum properties. This research programme led him to one of his most fertile and productive periods, sadly truncated by his unexpected passing at a time when he was bursting with ideas and projects.

César’s influence went beyond his papers. After his arrival at CSIC as an international leader in string theory, he acted as a pole of attraction. His impact was felt both through the training of graduate students, as well as by the many courses he imparted that left a lasting memory on the new generations.

Contrasting with his abstract scientific style, César also had a pragmatic side, full of vision, momentum and political talent. A major part of his legacy is the creation of the IFT, whose existence would be unthinkable without César among the small group of theoretical physicists from Universidad Autónoma de Madrid and CSIC who made a dream come true. For him, the IFT was more than his research institute, it was the home he helped to build.

Philosophy was a true second career for César, dating back to his PhD in Salamanca and strengthened at Harvard, where he started a lifelong friendship with Hilary Putnam. The philosophy of language was one of his favourite subjects for philosophical musings, and he dedicated to it an inspiring book in Spanish in 2003.

Cesar’s impressive and eclectic knowledge of physics always transformed blackboard discussions into a delightful and fascinating experience, while his extraordinary ability to establish connections between apparently remote notions was extremely motivating at the early stages of a project. A regular presence at seminars and journal clubs, and always conspicuous by his many penetrating and inspiring questions, he was a beloved character among graduate students, who felt the excitement of knowing that he could turn every seminar into a unique event.

César was an excellent scientist with a remarkable personality. He was a wonderful conversationalist on any possible topic, encouraging open discussions free of prejudice, and building bridges with all conversational partners. He cherished his wife Carmen and daughters Ana and Pepa, who survive him.

Farewell, dear friend. May you rest in peace, and may your memory be our blessing.

The post César Gómez 1954–2025 appeared first on CERN Courier.

A classical bit can exist in one of two states: |0> or |1>. A quantum bit, or qubit, exists in a superposition α|0> + β|1>, where α and β are complex amplitudes with real and imaginary parts. This superposition is the core feature that distinguishes quantum bits and classical bits. While a classical bit is either |0> or |1>, a quantum bit can be a blend of both at once. This is what gives quantum computers their immense parallelism – and also their fragility.

The difference becomes profound with scale. Two classical bits have four possible states, and are always in just one of them at a time. Two qubits simultaneously encode a complex-valued superposition of all four states.

Resources scale exponentially. N classical bits encode N boolean values, but N qubits encode 2^N complex amplitudes. Simulating 50 qubits with double-precision real numbers for each part of the complex amplitudes would require more than a petabyte of memory, beyond the reach of even the largest supercomputers.

Direct mimicry

Feynman proposed a different approach to quantum simulation. If a classical computer struggles, why not use one quantum system to emulate the behaviour of another? This was the conceptual birth of the quantum simulator: a device that harnesses quantum mechanics to solve quantum problems. For decades, this visionary idea remained in the realm of theory, awaiting the technological breakthroughs that are now rapidly bringing it to life. Today, progress in quantum hardware is driving two main approaches: analog and digital quantum simulation, in direct analogy to the history of classical computing.

In analog quantum simulators, the physical parameters of the simulator directly correspond to the parameters of the quantum system being studied. Think of it like a wind tunnel for aeroplanes: you are not calculating air resistance on a computer but directly observing how air flows over a model.

A striking example of an analog quantum simulator traps excited Rydberg atoms in precise configurations using highly focused laser beams known as “optical tweezers”. Rydberg atoms have one electron excited to an energy level far from the nucleus, giving them an exaggerated electric dipole moment that leads to tunable long-range dipole–dipole interactions – an ideal setup for simulating particle interactions in quantum field theories (see “Optical tweezers” figure).

The positions of the Rydberg atoms discretise the space inhabited by the quantum fields being modelled. At each point in the lattice, the local quantum degrees of freedom of the simulated fields are embodied by the internal states of the atoms. Dipole–dipole interactions simulate the dynamics of the quantum fields. This technique has been used to observe phenomena such as string breaking, where the force between particles pulls so strongly that the vacuum spontaneously creates new particle–antiparticle pairs. Such quantum simulations model processes that are notoriously difficult to calculate from first principles using classical computers (see “A philosophical dimension” panel).

Universal quantum computation

Digital quantum simulators operate much like classical digital computers, though using quantum rather than classical logic gates. While classical logic manipulates classical bits, quantum logic manipulates qubits. Because quantum logic gates obey the Schrödinger equation, they preserve information and are reversible, whereas most classical gates, such as “AND” and “OR”, are irreversible. Many quantum gates have no classical equivalent, because they manipulate phase, superposition or entanglement – a uniquely quantum phenomenon in which two or more qubits share a combined state. In an entangled system, the state of each qubit cannot be described independently of the others, even if they are far apart: the global description of the quantum state is more than the combination of the local information at every site.

By applying sequences of quantum logic gates, a digital quantum computer can model the time evolution of any target quantum system. This makes them flexible and scalable in pursuit of universal quantum computation – logic able to run any algorithm allowed by the laws of quantum mechanics, given enough qubits and sufficient time. Universal quantum computing requires only a small subset of the many quantum logic gates that can be conceived, for example Hadamard, T and CNOT. The Hadamard gate creates a superposition: |0> → (|0> + |1>) / √2. The T gate applies a 45° phase rotation: |1> → e^iπ/4|1>. And the CNOT gate entangles qubits by flipping a target qubit if a control qubit is |1>. These three suffice to prepare any quantum state from a trivial reference state: |ψ> = U₁ U₂ U₃ … U_N |0000…000>.

To bring frontier physics problems within the scope of current quantum computing resources, the distinction between analog and digital quantum simulations is often blurred. The complexity of simulations can be reduced by combining digital gate sequences with analog quantum hardware that aligns with the interaction patterns relevant to the target problem. This is feasible as quantum logic gates usually rely on native interactions similar to those used in analog simulations. Rydberg atoms are a common choice. Alongside them, two other technologies are becoming increasingly dominant in digital quantum simulation: trapped ions and superconducting qubit arrays.

Trapped ions offer the greatest control. Individual charged ions can be suspended in free space using electromagnetic fields. Lasers manipulate their quantum states, inducing interactions between them. Trapped-ion systems are renowned for their high fidelity (meaning operations are accurate) and long coherence times (meaning they maintain their quantum properties for longer), making them excellent candidates for quantum simulation (see “Trapped ions” figure).

Superconducting qubit arrays promise the greatest scalability. These tiny superconducting circuit materials act as qubits when cooled to extremely low temperatures and manipulated with microwave pulses. This technology is at the forefront of efforts to build quantum simulators and digital quantum computers for universal quantum computation (see “Superconducting qubits” figure).

The noisy intermediate-scale quantum era

Despite rapid progress, these technologies are at an early stage of development and face three main limitations.

The first problem is that qubits are fragile. Interactions with their environment quickly compromise their superposition and entanglement, making computations unreliable. Preventing “decoherence” is one of the main engineering challenges in quantum technology today.

The second challenge is that quantum logic gates have low fidelity. Over a long sequence of operations, errors accumulate, corrupting the result.

Finally, quantum simulators currently have a very limited number of qubits – typically only a few hundred. This is far fewer than what is needed for high-energy physics (HEP) problems.

This situation is known as the noisy “intermediate-scale” quantum era: we are no longer doing proof-of-principle experiments with a few tens of qubits, but neither can we control thousands of them. These limitations mean that current digital simulations are often restricted to “toy” models, such as QED simplified to have just one spatial and one time dimension. Even with these constraints, small-scale devices have successfully reproduced non-perturbative aspects of the theories in real time and have verified the preservation of fundamental physical principles such as gauge invariance, the symmetry that underpins the fundamental forces of the Standard Model.

Quantum simulators may chart a similar path to classical lattice QCD, but with even greater reach. Lattice QCD struggles with real-time evolution and finite-density physics due to the infamous “sign problem”, wherein quantum interference between classically computed amplitudes causes exponentially worsening signal-to-noise ratios. This renders some of the most interesting problems unsolvable on classical machines.

Quantum simulators do not suffer from the sign problem because they evolve naturally in real-time, just like the physical systems they emulate. This promises to open new frontiers such as the simulation of early-universe dynamics, black-hole evaporation and the dense interiors of neutron stars.

Quantum simulators will powerfully augment traditional theoretical and computational methods, offering profound insights when Feynman diagrams become intractable, when dealing with real-time dynamics and when the sign problem renders classical simulations exponentially difficult. Just as the lattice revolution required decades of concerted community effort to reach its full potential, so will the quantum revolution, but the fruits will again transform the field. As the aphorism attributed to Mark Twain goes: history never repeats itself, but it often rhymes.

Quantum information

One of the most exciting and productive developments in recent years is the unexpected, yet profound, convergence between HEP and quantum information science (QIS). For a long time these fields evolved independently. HEP explored the universe’s smallest constituents and grandest structures, while QIS focused on harnessing quantum mechanics for computation and communication. One of the pioneers in studying the interface between these fields was John Bell, a theoretical physicist at CERN.

Just as the lattice revolution needed decades of concerted community effort to reach its full potential, so will the quantum revolution

HEP and QIS are now deeply intertwined. As quantum simulators advance, there is a growing demand for theoretical tools that combine the rigour of quantum field theory with the concepts of QIS. For example, tensor networks were developed in condensed-matter physics to represent highly entangled quantum states, and have now found surprising applications in lattice gauge theories and “holographic dualities” between quantum gravity and quantum field theory. Another example is quantum error correction – a vital QIS technique to protect fragile quantum information from noise, and now a major focus for quantum simulation in HEP.

This cross-disciplinary synthesis is not just conceptual; it is becoming institutional. Initiatives like the US Department of Energy’s Quantum Information Science Enabled Discovery (QuantISED) programme, CERN’s Quantum Technology Initiative (QTI) and Europe’s Quantum Flagship are making substantial investments in collaborative research. Quantum algorithms will become indispensable for theoretical problems just as quantum sensors are becoming indispensable to experimental observation (see “Sensing at quantum limits”).

The result is the emergence of a new breed of scientist: one equally fluent in the fundamental equations of particle physics and the practicalities of quantum hardware. These “hybrid” scientists are building the theoretical and computational scaffolding for a future where quantum simulation is a standard, indispensable tool in HEP. 

The post Quantum simulators in high-energy physics appeared first on CERN Courier.

The latest recognition of the fertility of studying the interpretation of quantum mechanics was the award of the 2022 Nobel Prize in Physics to Alain Aspect, John Clauser and Anton Zeilinger. The motivation for the prize pointed out that the bubbling field of quantum information, with its numerous current and potential technological applications, largely stems from the work of John Bell at CERN the 1960s and 1970s, which in turn was motivated by the debate on the interpretation of quantum mechanics.

The majority of scientists use a textbook formulation of the theory that distinguishes the quantum system being studied from “the rest of the world” – including the measuring apparatus and the experimenter, all described in classical terms. Used in this orthodox manner, quantum theory describes how quantum systems react when probed by the rest of the world. It works flawlessly.

Sense and sensibility

The problem is that the rest of the world is quantum mechanical as well. There are of course regimes in which the behaviour of a quantum system is well approximated by classical mechanics. One may even be tempted to think that this suffices to solve the difficulty. But this leaves us in the awkward position of having a general theory of the world that only makes sense under special approximate conditions. Can we make sense of the theory in general?

Today, variants of four main ideas stand at the forefront of efforts to make quantum mechanics more conceptually robust. They are known as physical collapse, hidden variables, many worlds and relational quantum mechanics. Each appears to me to be viable a priori, but each comes with a conceptual price to pay. The latter two may be of particular interest to the high-energy community as the first two do not appear to fit well with relativity.

The idea of the physical collapse is simple: we are missing a piece of the dynamics. There may exist a yet-undiscovered physical interaction that causes the wavefunction to “collapse” when the quantum system interacts with the classical world in a measurement. The idea is empirically testable. So far, all laboratory attempts to find violations of the textbook Schrödinger equation have failed (see “Probing physical collapse” figure), and some models for these hypothetical new dynamics have been ruled out by measurements.

The second possibility, hidden variables, follows on from Einstein’s belief that quantum mechanics is incomplete. It posits that its predictions are exactly correct, but that there are additional variables describing what is going on, besides those in the usual formulation of the theory: the reason why quantum predictions are probabilistic is our ignorance of these other variables.

The work of John Bell shows that the dynamics of any such theory will have some degree of non-locality (see “Non-locality” image). In the non-relativistic domain, there is a good example of a theory of this sort, that goes under the name of de Broglie–Bohm, or pilot-wave theory. This theory has non-local but deterministic dynamics capable of reproducing the predictions of non-relativistic quantum-particle dynamics. As far as I am aware, all existing theories of this kind break Lorentz invariance, and the extension of hidden variable theories to quantum-field theoretical domains appears cumbersome.

Relativistic interpretations

Let me now come to the two ideas that are naturally closer to relativistic physics. The first is the many-worlds interpretation – a way of making sense of quantum theory without either changing its dynamics or adding extra variables. It is described in detail in this edition of CERN Courier by one of its leading contemporary proponents (see “The minimalism of many worlds“), but the main idea is the following: being a genuine quantum system, the apparatus that makes a quantum measurement does not collapse the superposition of possible measurement outcomes – it becomes a quantum superposition of the possibilities, as does any human observer.

If we observe a singular outcome, says the many-worlds interpretation, it is not because one of the probabilistic alternatives has actualised in a mysterious “quantum measurement”. Rather, it is because we have split into a quantum superposition of ourselves, and we just happen to be in one of the resulting copies. The world we see around us is thus only one of the branches of a forest of parallel worlds in the overall quantum state of everything. The price to pay to make sense of quantum theory in this manner is to accept the idea that the reality we see is just a branch in a vast collection of possible worlds that include innumerable copies of ourselves.

Relational interpretations are the most recent of the four kinds mentioned. They similarly avoid physical collapse or hidden variables, but do so without multiplying worlds. They stay closer to the orthodox textbook interpretation, but with no privileged status for observers. The idea is to think of quantum theory in a manner closer to the way it was initially conceived by Born, Jordan, Heisenberg and Dirac: namely in terms of transition amplitudes between observations rather than quantum states evolving continuously in time, as emphasised by Schrödinger’s wave mechanics (see “A matter of taste” image).

Observer relativity

The alternative to taking the quantum state as the fundamental entity of the theory is to focus on the information that an arbitrary system can have about another arbitrary system. This information is embodied in the physics of the apparatus: the position of its pointer variable, the trace in a bubble chamber, a person’s memory or a scientist’s logbook. After a measurement, these physical quantities “have information” about the measured system as their value is correlated with a property of the observed systems.

Quantum theory can be interpreted as describing the relative information that systems can have about one another. The quantum state is interpreted as a way of coding the information about a system available to another system. What looks like a multiplicity of worlds in the many-worlds interpretation becomes nothing more than a mathematical accounting of possibilities and probabilities.

The relational interpretation reduces the content of the physical theory to be about how systems affect other systems. This is like the orthodox textbook interpretation, but made democratic. Instead of a preferred classical world, any system can play a role that is a generalisation of the Copenhagen observer. Relativity teaches us that velocity is a relative concept: an object has no velocity by itself, but only relative to another object. Similarly, quantum mechanics, interpreted in this manner, teaches us that all physical variables are relative. They are not properties of a single object, but ways in which an object affects another object.

The QBism version of the interpretation restricts its attention to observing systems that are rational agents: they can use observations and make probabilistic predictions about the future. Probability is interpreted subjectively, as the expectation of a rational agent. The relational interpretation proper does not accept this restriction: it considers the information that any system can have about any other system. Here, “information” is understood in the simple physical sense of correlation described above.

Like many worlds – to which it is not unrelated – the relational interpretation does not add new dynamics or new variables. Unlike many worlds, it does not ask us to think about parallel worlds either. The conceptual price to pay is a radical weakening of a strong form of realism: the theory does not give us a picture of a unique objective sequence of facts, but only perspectives on the reality of physical systems, and how these perspectives interact with one another. Only quantum states of a system relative to another system play a role in this interpretation. The many-worlds interpretation is very close to this. It supplements the relational interpretation with an overall quantum state, interpreted realistically, achieving a stronger version of realism at the price of multiplying worlds. In this sense, the many worlds and relational interpretations can be seen as two sides of the same coin.

Every theoretical physicist who is any good knows six or seven different theoretical representations for exactly the same physics

I have only sketched here the most discussed alternatives, and have tried to be as neutral as possible in a field of lively debates in which I have my own strong bias (towards the fourth solution). Empirical testing, as I have mentioned, can only test the physical collapse hypothesis.

There is nothing wrong, in science, in using different pictures for the same phenomenon. Conceptual flexibility is itself a resource. Specific interpretations often turn out to be well adapted to specific problems. In quantum optics it is sometimes convenient to think that there is a wave undergoing interference, as well as a particle that follows a single trajectory guided by the wave, as in the pilot-wave hidden-variable theory. In quantum computing, it is convenient to think that different calculations are being performed in parallel in different worlds. My own field of loop quantum gravity treats spacetime regions as quantum processes: here, the relational interpretation merges very naturally with general relativity, because spacetime regions themselves become quantum processes, affecting each other.

Richard Feynman famously wrote that “every theoretical physicist who is any good knows six or seven different theoretical representations for exactly the same physics. He knows that they are all equivalent, and that nobody is ever going to be able to decide which one is right at that level, but he keeps them in his head, hoping that they will give him different ideas for guessing.” I think that this is where we are, in trying to make sense of our best physical theory. We have various ways to make sense of it. We do not yet know which of these will turn out to be the most fruitful in the future.

The post Four ways to interpret quantum mechanics appeared first on CERN Courier.

Over the past two decades, quantum sensors have taken on leading roles in the search for ultra-light dark matter and in precision tests of fundamental symmetries. Examples include the use of atomic clocks to probe whether Earth is sweeping through oscillating or topologically structured dark-matter fields, and cryogenic detectors to search for electric dipole moments – subtle signatures that could reveal new sources of CP violation. These areas have seen rapid progress, as challenges related to detector size, noise, sensitivity and complexity have been steadily overcome, opening new phase space in which to search for physics beyond the Standard Model. Could high-energy particle physics benefit next?

Low-energy particle physics

Most of the current applications of quantum sensors are at low energies, where their intrinsic sensitivity and characteristic energy scales align naturally with the phenomena being probed. For example, within the Project 8 experiment at the University of Washington, superconducting sensors are being developed to tackle a longstanding challenge: to distinguish the tiny mass of the neutrino from zero (see “Quantum-noise limited” image). Inward-looking phased arrays of quantum-noise-limited microwave receivers allow spectroscopy of cyclotron radiation from beta-decay electrons as they spiral in a magnetic field. The shape of the endpoint of the spectrum is sensitive to the mass of the neutrino and such sensors have the potential to be sensitive to neutrino masses as low as 40 meV.

Beyond the Standard Model, superconducting sensors play a central role in the search for dark matter. At the lowest mass scales (peV to meV), experiments search for ultralight bosonic dark-matter candidates such as axions and axion-like particles (ALPs) through excitations of the vacuum field inside high–quality–factor microwave and millimetre-wave cavities (see “Quantum sensitivity” image). In the meV range, light-shining-through-wall experiments aim to reveal brief oscillations into weakly coupled hidden-sector particles such as dark photons or ALPs, and may employ quantum sensors for detecting reappearing photons, depending on the detection strategy. In the MeV to sub-GeV mass range, superconducting sensors are used to detect individual photons and phonons in cryogenic scintillators, enabling sensitivity to dark-matter interactions via electron recoils. At higher masses, reaching into the GeV regime, superfluid helium detectors target nuclear recoils from heavier dark matter particles such as WIMPs.

These technologies also find broad application beyond fundamental physics. For example, in superconducting and other cryogenic sensors, the ability to detect single quanta with high efficiency and ultra-low noise is essential. The same capabilities are the technological foundation of quantum communication.

Raising the temperature

While many superconducting quantum sensors require ultra-low temperatures of a few mK, some spin-based quantum sensors can function at or near room temperature. Spin-based sensors, such as nitrogen-vacancy (NV) centres in diamonds and polarised rubidium atoms, are excellent examples.

NV centres are defects in the diamond lattice where a missing carbon atom – the vacancy – is adjacent to a lattice site where a carbon atom has been replaced by a nitrogen atom. The electronic spin states in NV centres have unique energy levels that can be probed by laser excitation and detection of spin-dependent fluorescence.

Researchers are increasingly exploring how quantum-control techniques can be integrated into high-energy-physics detectors

Rubidium is promising for spin-based sensors because it has unpaired electrons. In the presence of an external magnetic field, its atomic energy levels are split by the Zeeman effect. When optically pumped with laser light, spin-polarised “dark” sublevels – those not excited by the light – become increasingly populated. These aligned spins precess in magnetic fields, forming the basis of atomic magnetometers and other quantum sensors.

Being exquisite magnetometers, both devices make promising detectors for ultralight bosonic dark-matter candidates such as axions. Fermion spins may interact with spatial or temporal gradients of the axion field, leading to tiny oscillating energy shifts. The coupling of axions to gluons could also show up as an oscillating nuclear electric dipole moment. These interactions could manifest as oscillating energy-level shifts in NV centres, or as time-varying NMR-like spin precession signals in the atomic magnetometers.

Large-scale detectors

The situation is completely different in high-energy physics detectors, which require numerous interactions between a particle and a detector. Charged particles cause many ionisation events, and when a neutral particle interacts it produces charged particles that result in similarly numerous ionisations. Even if quantum control were possible within individual units of a massive detector, the number of individual quantum sub-processes to be monitored would exceed the possibilities of any realistic device.

Increasingly, however, researchers are exploring how quantum-control techniques – such as manipulating individual atoms or spins using lasers or microwaves – can be integrated into high-energy-physics detectors. These methods could enhance detector sensitivity, tune detector response or enable entirely new ways of measuring particle properties. While these quantum-enhanced or hybrid detection approaches are still in their early stages, they hold significant promise.

Quantum dots

Quantum dots are nanoscale semiconductor crystals – typically a few nanometres in diameter – that confine charge carriers (electrons and holes) in all three spatial dimensions. This quantum confinement leads to discrete, atom-like energy levels and results in optical and electronic properties that are highly tunable with size, shape and composition. Originally studied for their potential in optoelectronics and biomedical imaging, quantum dots have more recently attracted interest in high-energy physics due to their fast scintillation response, narrow-band emission and tunability. Their emission wavelength can be precisely controlled through nanostructuring, making them promising candidates for engineered detectors with tailored response characteristics.

While their radiation hardness is still under debate and needs to be resolved, engineering their composition, geometry, surface and size can yield very narrow-band (20 nm) emitters across the optical spectrum and into the infrared. Quantum dots such as these could enable the design of a “chromatic calorimeter”: a stack of quantum-dot layers, each tuned to emit at a distinct wavelength; for example red in the first layer, orange in the second and progressing through the visible spectrum to violet. Each layer would absorb higher energy photons quite broadly but emit light in a narrow spectral band. The intensity of each colour would then correspond to the energy absorbed in that layer, while the emission wavelength would encode the position of energy deposition, revealing the shower shape (see “Chromatic calorimetry” figure). Because each layer is optically distinct, hermetic isolation would be unnecessary, reducing the overall material budget.

Rather than improving the energy resolution of existing calorimeters, quantum dots could provide additional information on the shape and development of particle showers if embedded in existing scintillators. Initial simulations and beam tests by CERN’s Quantum Technology Initiative (QTI) support the hypothesis that the spectral intensity of quantum-dot emission can carry information about the energy and species of incident particles. Ongoing work aims to explore their capabilities and limitations.

Beyond calorimetry, quantum dots could be formed within solid semiconductor matrices, such as gallium arsenide, to form a novel class of “photonic trackers”. Scintillation light from electronically tunable quantum dots could be collected by photodetectors integrated directly on top of the same thin semiconductor structure, such as in the DoTPiX concept. Thanks to a highly compact, radiation-tolerant scintillating pixel tracking system with intrinsic signal amplification and minimal material budget, photonic trackers could provide a scintillation-light-based alternative to traditional charge-based particle trackers.

Living on the edge

Low temperatures also offer opportunities at scale – and cryogenic operation is a well-established technique in both high-energy and astroparticle physics, with liquid argon (boiling point 87 K) widely used in time projection chambers and some calorimeters, and some dark-matter experiments using liquid helium (boiling point 4.2 K) to reach even lower temperatures. A range of solid-state detectors, including superconducting sensors, operate effectively at these temperatures and below, and offer significant advantages in sensitivity and energy resolution.

Magnetic microcalorimeters (MMCs) and transition-edge sensors (TESs) operate in the narrow temperature range where a superconducting material undergoes a rapid transition from zero resistance to finite values. When a particle deposits energy in an MMC or TES, it slightly raises the temperature, causing a measurable increase in resistance. Because the transition is extremely steep, even a tiny temperature change leads to a detectable resistance change, allowing precise calorimetry.

Functioning at millikelvin temperatures, TESs provide much higher energy resolution than solid-state detectors made from high-purity germanium crystals, which work by collecting electron–hole pairs created when ionising radiation interacts with the crystal lattice. TESs are increasingly used in high-resolution X-ray spectroscopy of pionic, muonic or antiprotonic atoms, and in photon detection for observational astronomy, despite the technical challenges associated with maintaining ultra-low operating temperatures.

By contrast, superconducting nanowire and microwire single-photon detectors (SNSPDs and SMSPDs) register only a change in state – from superconducting to normal conducting – allowing them to operate at higher temperatures than traditional low-temperature sensors. When made from high–critical-temperature (T_c) superconductors, operation at temperatures as high as 10 K is feasible, while maintaining excellent sensitivity to energy deposited by charged particles and ultrafast switching times on the order of a few picoseconds. Recent advances include the development of large-area devices with up to 400,000 micron-scale pixels (see “Single-photon phase transitions” figure), fabrication of high-T_c SNSPDs and successful beam tests of SMSPDs. These technologies are promising candidates for detecting milli-charged particles – hypothetical particles arising in “hidden sector” extensions of the Standard Model – or for high-rate beam monitoring at future colliders.

Rugged, reliable and reproducible

Quantum sensor-based experiments have vastly expanded the phase space that has been searched for new physics. This is just the beginning of the journey, as larger-scale efforts build on the initial gold rush and new quantum devices are developed, perfected and brought to bear on the many open questions of particle physics.

Partnering with neighbouring fields such as quantum computing, quantum communication and manufacturing is of paramount importance

To fully profit from their potential, a vigorous R&D programme is needed to scale up quantum sensors for future detectors. Ruggedness, reliability and reproducibility are key – as well as establishing “proof of principle” for the numerous imaginative concepts that have already been conceived. Challenges range from access to test infrastructures, to standardised test protocols for fair comparisons. In many cases, the largest challenge is to foster an open exchange of ideas given the numerous local developments that are happening worldwide. Finding a common language to discuss developments in different fields that at first glance may have little in common, builds on a willingness to listen, learn and exchange.

The European Committee for Future Accelerators (ECFA) detector R&D roadmap provides a welcome framework for addressing these challenges collaboratively through the Detector R&D (DRD) collaborations established in 2023 and now coordinated at CERN. Quantum sensors and emerging technologies are covered within the DRD5 collaboration, which ties together 112 institutes worldwide, many of them leaders in their particular field. Only a third stem from the traditional high-energy physics community.

These efforts build on the widespread expertise and enthusiastic efforts at numerous institutes and tie in with the quantum programmes being spearheaded at high-energy-physics research centres, among them CERN’s QTI. Partnering with neighbouring fields such as quantum computing, quantum communication and manufacturing is of paramount importance. The best approach may prove to be “targeted blue-sky research”: a willingness to explore completely novel concepts while keeping their ultimate usefulness for particle physics firmly in mind.

The post Sensing at quantum limits appeared first on CERN Courier.

The ATLAS and ALICE collaborations have announced the first results of a new way to measure the “radial flow” of quark–gluon plasma (QGP). The two analyses offer a fresh perspective into the fluid-like behaviour of QCD matter under extreme conditions, such as those that prevailed after the Big Bang. The measurements are highly complementary, with ALICE drawing on their detector’s particle-identification capabilities and ATLAS leveraging the experiment’s large rapidity coverage.

At the Large Hadron Collider, lead–ion collisions produce matter at temperatures and densities so high that quarks and gluons momentarily escape their confinement within hadrons. The resulting QGP is believed to have filled the universe during its first few microseconds, before cooling and fragmenting into mesons and baryons. In the laboratory, these streams of particles allow researchers to reconstruct the dynamical evolution of the QGP, which has long been known to transform anisotropies of the initial collision geometry into anisotropic momentum distributions of the final-state particles.

Compelling evidence

Differential measurements of the azimuthal distributions of produced particles over the last decades have provided compelling evidence that the outgoing momentum distribution reflects a collective response driven by initial pressure gradients. The isotropic expansion component, typically referred to as radial flow, has instead been inferred from the slope of particle spectra (see figure 1). Despite its fundamental role in driving the QGP fireball, radial flow lacked a differential probe comparable to those of its anisotropic counterparts.

That situation has now changed. The ALICE and ATLAS collaborations recently employed the novel observable v₀(p_T) to investigate radial flow directly. Their independent results demonstrate, for the first time, that the isotropic expansion of the QGP in heavy-ion collisions exhibits clear signatures of collective behaviour. The isotropic expansion of the QGP and its azimuthal modulations ultimately depend on the hydrodynamic properties of the QGP, such as shear or bulk viscosity, and can thus be measured to constrain them.

Traditionally, radial flow has been inferred from the slope of p_T-spectra, with the p_T-integrated radial-flow extracted via fits to “blast wave” models. The newly introduced differential observable v₀(p_T) captures fluctuations in spectral shape across p_T bins. v₀(p_T) retains differential sensitivity, since it is defined as the correlation (technically the normalised covariance) between the fraction of particles in a given p_T-interval and the mean transverse momentum of the collision products within a single event, [p_T]. Roughly speaking, a fluctuation raising [p_T] produces a positive v₀(p_T) at high p_T due to the fractional yield increasing; conversely, the fractional yield decreasing at low p_T causes a negative v₀(p_T). A pseudorapidity gap between the measurement of mean p_T and the particle yields is used to suppress short-range correlations and isolate the long-range, collective signal. Previous studies observed event-by-event fluctuations in [p_T], related to radial flow over a wide p_T range and quantified by the coefficient v₀^ref, but they could not establish whether these fluctuations were correlated across different p_T intervals – a crucial signature of collective behaviour.

Origins

The ATLAS collaboration performed a measurement of v₀(p_T) in the 0.5 to 10 GeV range, identifying three signatures of the collective origin of radial flow (see figure 2). First, correlations between the particle yield at fixed p_T and the event-wise mean [p_T] in a reference interval show that the two-particle radial flow factorises into single-particle coefficients as v₀(p_T) × v₀^ref for p_T < 4 GeV, independent of the reference choice (left panel). Second, the data display no dependence on the rapidity gap between correlated particles, suggesting a long-range effect intrinsic to the entire system (middle panel). Finally, the centrality dependence of the ratio v₀(p_T)/v₀^ref followed a consistent trend from head-on to peripheral collisions, effectively cancelling initial geometry effects and supporting the interpretation of a collective QGP response (right panel). At higher p_T, a decrease in v₀(p_T) and a splitting with respect to centrality suggest the onset of non-thermal effects such as jet quenching. This may reveal fluctuations in jet energy loss – an area warranting further investigation.

Using more than 80 million collisions at a centre-of-mass energy of 5.02 TeV, ALICE extracted v₀(p_T) for identified pions, kaons and protons across a broad range of centralities. ALICE observes v₀(p_T) to be negative at low p_T, reflecting the influence of mean-p_T fluctuations on the spectral shape (see figure 3). The data display a clear mass ordering at low p_T, from protons to kaons to pions, consistent with expectations from collective radial expansion. This mass ordering reflects the greater “push” heavier particles experience in the rapidly expanding medium. The picture changes above 3 GeV, where protons have larger v₀(p_T) values than pions and kaons, perhaps indicating the contribution of recombination processes in hadron production.

The results demonstrate that the isotropic expansion of the QGP in heavy-ion collisions exhibits clear signatures of collective behaviour

The two collaborations’ measurements of the new v₀(p_T) observable highlight its sensitivity to the bulk-transport properties of the QGP medium. Comparisons with hydrodynamic calculations show that v₀(p_T) varies with bulk viscosity and the speed of sound, but that it has a weaker dependence on shear viscosity. Hydrodynamic predictions reproduce the data well up to about 2 GeV, but diverge at higher momenta. The deviation of non-collective models like HIJING from the data underscores the dominance of final-state, hydrodynamic-like effects in shaping radial flow.

These results advance our understanding of one of the most extreme regimes of QCD matter, strengthening the case for the formation of a strongly interacting, radially expanding QGP medium in heavy-ion collisions. Differential measurements of radial flow offer a new tool to probe this fluid-like expansion in detail, establishing its collective origin and complementing decades of studies of anisotropic flow.

The post A new probe of radial flow appeared first on CERN Courier.

Perhaps the most intriguing open question surrounding neutron stars is what is actually inside them. Clearly they are primarily composed of neutrons, but many theories suggest that other forms of matter should appear in the highest density regions near the centre of the star, including free quarks, hyperons and kaon or pion condensates. Diverse data can constrain these hypotheses, including astronomical inferences of the masses and radii of neutron stars, observations of the mergers of neutron stars by LIGO, and baryon production patterns and correlations in heavy-ion collisions at the LHC. Theoretical consistency is critical here. Several talks highlighted the importance of low-energy nuclear data to understand the behaviour of nuclear matter at low densities, though also emphasising that at very high densities and energies any description should fall within the realm of QCD – a theory that beautifully describes the dynamics of quarks and gluons at the LHC.

Another key question for neutron stars is how fast they cool. This depends critically on their composition. Quarks, hyperons, nuclear resonances, pions or muons would each lead to different channels to cool the neutron star. Measurements of the temperatures and ages of neutron stars might thereby be used to learn about their composition.

Research into neutron stars has progressed so rapidly in recent years that it allows key tests of fundamental physics

The workshop revealed that research into neutron stars has progressed so rapidly in recent years that it allows key tests of fundamental physics including tests of particles beyond the Standard Model, including the axion: a very light and weakly coupled dark-matter candidate that was initially postulated to explain the “strong CP problem” of why strong interactions are identical for particles and antiparticles. The workshop allowed particle theorists to appreciate the various possible uncertainties in their theoretical predictions and propagate them into new channels that may allow sharper tests of axions and other weakly interacting particles. An intriguing question that the workshop left open is whether the canonical QCD axion could condense inside neutron stars.

While many uncertainties remain, the workshop revealed that the field is open and exciting, and that upcoming observations of neutron stars, including neutron-star mergers or the next galactic supernova, hold unique opportunities to understand fundamental questions from the nature of dark matter to the strong CP problem.

The post Neutron stars as fundamental physics labs appeared first on CERN Courier.

Rogues’ gallery

The story told by the book combines lucid explanations of a rogues’ gallery of modern cosmological theories, some astonishingly successful, others less so, interspersed with anecdotes culled from Halper’s numerous interviews with key players in the game. These stories of the real people behind the theories add humanistic depth to the science, and the balance between Halper’s engaging storytelling and Afshordi’s steady-handed illumination of often esoteric scientific ideas is mostly a winning combination; the book is readable, without sacrificing too much scientific depth. In this respect, Battle of the Big Bang is reminiscent of Dennis Overbye’s 1991 Lonely Hearts of the Cosmos. As with Overbye’s account of the famous conference-banquet fist fight between Rocky Kolb and Gary Steigman, there is no shortage here of renowned scientists behaving like children, and the “mean girls of cosmology” angle makes for an entertaining read. The story of University of North Carolina professor Paul Frampton getting catfished by cocaine smugglers posing as model Denise Milani and ending up in an Argentine prison, for example, is not one you see coming.

A central conflict propelling the narrative is the longstanding feud between Andrei Linde and Alan Guth, both originators of the theory of cosmological inflation, and Paul Steinhardt, also an originator of the theory who later transformed into an apostate and bitter critic of the theory he helped establish.

Inflation – a hypothesised period of exponential cosmic expansion by more than 26 orders of magnitude that set the initial conditions for the hot Big Bang – is the gorilla in the room, a hugely successful theory that over the past several decades has racked up win after win when confronted by modern precision cosmology. Inflation is rightly considered by most cosmologists to be a central part of the “standard” cosmology, and its status as a leading theory inevitably makes it a target of critics like Steinhardt, who argue that inflation’s inherent flexibility means that it is not a scientific theory at all. Inflation is introduced early in the book, and for the remainder, Afshordi and Halper ably lead the reader through a wild mosaic of alternative theories to inflation: multiverses, bouncing universes, new universes birthed from within black holes, extra dimensions, varying light speed and “mirror” universes with reversed time all make appearances, a dizzying inventory of our most recent collective obsessions and schizophrenias.

In the later chapters, Afshordi describes some of his own efforts to formulate an alternative to inflation, and it is here that the book is at its strongest; the voice of a master of the craft confronting his own unconscious assumptions and biases makes for compelling reading. I have known Niayesh as a friend and colleague for more than 20 years. He is a fearlessly creative theorist with deep technical skill, but he has the heart of a rebel and a poet, and I found myself wishing that the book gave his unique voice more room to shine, instead of burying it beneath too many mundane pop-science tropes; the book could have used more of the science and less of the “science communication”. At times the pop-culture references come so thick that the reader feels as if he is having to shake them off his leg.

Compelling arguments

Anyone who reads science blogs or follows science on social media is aware of the voices, some of them from within mainstream science and many from further out on the fringe, arguing that modern theoretical physics suffers from a rigid orthodoxy that serves to crowd out worthy alternative ideas to understand problems such as dark matter, dark energy and the unification of gravity with quantum mechanics. This has been the subject of several books such as Lee Smolin’s The Trouble with Physics and Peter Woit’s Not Even Wrong. A real value in Battle of the Big Bang is to provide a compelling counterargument to that pessimistic narrative. In reality, ambitious scientists like nothing better than overturning a standard paradigm, and theorists have put the standard model of cosmology in the cross hairs with the gusto of assassins gunning for John Wick. Despite – or perhaps because of – its focus on conflict, this book ultimately paints a picture of a vital and healthy scientific process, a kind of controlled chaos, ripe with wild ideas, full of the clash of egos and littered with the ashes of failed shots at glory.

What the book is not is a reliable scholarly work on the history of science. Not only was the manuscript rather haphazardly copy-edited (the renowned Mount Palomar telescope, for example, is not “two hundred foot”, but in fact 200 inches), but the historical details are sometimes smoothed over to fit a coherent narrative rather than presented in their actual messy accuracy. While I do not doubt the anecdote of David Spergel saying “we’re dead”, referring to cosmic strings when data from the COBE satellite was first released, it was not COBE that killed cosmic strings. The blurry vision of COBE could accommodate either strings or inflation as the source of fluctuations in the cosmic microwave background (CMB), and it took a clearer view to make the distinction. The final nail in the coffin came from BOOMERanG nearly a decade later, with the observation of the second acoustic peak in the CMB. And it was not, as claimed here, BOOMERanG that provided the first evidence for a flat geometry to the cosmos; that happened a few years earlier, with the Saskatoon and CAT experiments.

Afshordi and Halper ably lead the reader through a wild mosaic of alternative theories to inflation

The book makes a point of the premature death of Dave Wilkinson, when in fact he died at age 67, not (as is implied in the text) in his 50s. Wilkinson – who was my freshman physics professor – was a great scientist and a gifted teacher, and it is appropriate to memorialise him, but he had a long and productive career.

Besides these points of detail, there are some more significant omissions. The book relates the story of how the Ukrainian physicist Alex Vilenkin, blacklisted from physics and working as a zookeeper in Kharkiv, escaped the Soviet Union. Vilenkin moved to SUNY Buffalo, where I am currently a professor, because he had mistaken Mendel Sachs, a condensed matter theorist, for Ray Sachs, who originally predicted fluctuations in the CMB. It’s a funny story, and although the authors note that Vilenkin was blacklisted for refusing to be an informant for the KGB, they omit the central context that he was Jewish, one of many Jews banished from academic life by Soviet authorities who escaped the stifling anti-Semitism of the Soviet Union for scientific freedom in the West. This history resonates today in light of efforts by some scientists to boycott Israeli institutes and even blacklist Israeli colleagues. Unlike the minutiae of CMB physics, this matters, and Battle of the Big Bang should have been more careful to tell the whole story.

The post The battle of the Big Bang appeared first on CERN Courier.

Alfred D Stone (Yale University) called upon participants to challenge the folklore surrounding quantum theory’s birth. Philosopher Elise Crull (City College of New York) drew overdue attention to Grete Hermann, who hinted at entanglement before it had a name and anticipated Bell in identifying a flaw in von Neumann’s no-go theorem, which had been taken as proof that hidden-variable theories are impossible. Science writer Philip Ball questioned Heisenberg’s epiphany itself: he didn’t invent matrix mechanics in a flash, claims Ball, nor immediately grasp its relevance, and it took months, and others, to see his contribution for what it was (see “Lend me your ears” image).

Building on a strong base

A clear takeaway from Helgoland 2025 was that the foundations of quantum mechanics, though strongly built on Helgoland 100 years ago, nevertheless remain open to interpretation, and any future progress will depend on excavating them directly (see “Four ways to interpret quantum mechanics“).

Does the quantum wavefunction represent an objective element of reality or merely an observer’s state of knowledge? On this question, Helgoland 2025 could scarcely have been more diverse. Christopher Fuchs (UMass Boston) passionately defended quantum Bayesianism, which recasts the Born probability rule as a consistency condition for rational agents updating their beliefs. Wojciech Zurek (Los Alamos National Laboratory) presented the Darwinist perspective, for which classical objectivity emerges from redundant quantum information encoded across the environment. Although Zurek himself maintains a more agnostic stance, his decoherence-based framework is now widely embraced by proponents of many-worlds quantum mechanics (see “The minimalism of many worlds“).

The foundations of quantum mechanics remain open to interpretation, and any future progress will depend on excavating them directly

Markus Aspelmeyer (University of Vienna) made the case that a signature of gravity’s long-speculated quantum nature may soon be within experimental reach. Building on the “gravitational Schrödinger’s cat” thought experiment proposed by Feynman in the 1950s, he described how placing a massive object in a spatial superposition could entangle a nearby test mass through their gravitational interaction. Such a scenario would produce correlations that are inexplicable by classical general relativity alone, offering direct empirical evidence that gravity must be described quantum-mechanically. Realising this type of experiment requires ultra-low pressures and cryogenic temperatures to suppress decoherence, alongside extremely low-noise measurements of gravitational effects at short distances. Recent advances in optical and opto­mechanical techniques for levitating and controlling nanoparticles suggest a path forward – one that could bring evidence for quantum gravity not from black holes or the early universe, but from laboratories on Earth.

Information insights

Quantum information was never far from the conversation. Isaac Chuang (MIT) offered a reconstruction of how Heisenberg might have arrived at the principles of quantum information, had his inspiration come from Shannon’s Mathematical Theory of Communication. He recast his original insights into three broad principles: observations act on systems; local and global perspectives are in tension; and the order of measurements matters. Starting from these ingredients, one could in principle recover the structure of the qubit and the foundations of quantum computation. Taking the analogy one step further, he suggested that similar tensions between memorisation and generalisation – or robustness and adaptability – may one day give rise to a quantum theory of learning.

Helgoland 2025 illustrated just how much quantum mechanics has diversified since its early days. No longer just a framework for explaining atomic spectra, the photoelectric effect and black-body radiation, it is at once a formalism describing high-energy particle scattering, a handbook for controlling the most exotic states of matter, the foundation for information technologies now driving national investment plans, and a source of philosophical conundrums that, after decades at the margins, has once again taken centre stage in theoretical physics.

The post Quantum theory returns to Helgoland appeared first on CERN Courier.

The spectral energy distribution of blazars generally has two broad peaks. The low-energy peak from radio to X-rays is well explained by synchrotron radiation from relativistic electrons spiraling in magnetic fields, but the origin of the higher-energy peak from X-rays to γ-rays is a longstanding point of contention, with two classes of models, dubbed hadronic and leptonic, vying to explain it. Polarisation measurements offer a key diagnostic tool, as the two models predict distinct polarisation signatures.

Model signatures

In hadronic models, high-energy emission is produced by protons, either through synchrotron radiation or via photo-hadronic interactions that generate secondary particles. Hadronic models predict that X-ray polarisation should be as high as that in the optical and millimetre bands, even in complex jet structures.

Leptonic models are powered by inverse Compton scattering, wherein relativistic electrons “upscatter” low-energy photons, boosting them to higher energies with low polarisation. Leptonic models can be further subdivided by the source of the inverse-Compton-scattered photons. If initially generated by synchrotron radiation in the AGN (synchrotron self-Compton, SSC), modest polarisation (~50%) is expected due to the inherent polarisation of synchrotron photons, with further reductions if the emission comes from inhomogeneous or multiple emitting regions. If initially generated by external sources (external Compton, EC), isotropic photon fields from the surrounding structures are expected to average out their polarisation.

IXPE launched on 9 December 2021, seeking to resolve such questions. It is designed to have 100-fold better sensitivity to the polarisation of X-rays in astrophysical sources than the last major X-ray polarimeter, which was launched half a century ago (CERN Courier July/August 2022 p10). In November 2023, it participated in a coordinated multiwavelength campaign spanning radio, millimetre and optical, and X-ray bands targeted the blazar BL Lacertae, whose X-ray emission arises mostly from the high-energy component, with its low-energy synchrotron component mainly at infrared energies. The campaign captured an exceptional flare, providing a rare opportunity to test competing emission models.

Optical telescopes recorded a peak optical polarisation of 47.5 ± 0.4%, the highest ever measured in a blazar. The short-mm (1.3 mm) polarisation also rose to about 10%, with both bands showing similar trends in polarisation angle. IXPE measured no significant polarisation in the 2 to 8 keV X-ray band, placing a 3σ upper limit of 7.4%.

The striking contrast between the high polarisation in optical and mm bands, and a strict upper limit in X-rays, effectively rules out all single-zone and multi-region hadronic models. Had these processes dominated, the X-ray polarisation would have been comparable to the optical. Instead, the observations strongly support a leptonic origin, specifically the SSC model with a stratified or multi-zone jet structure that naturally explains the low X-ray polarisation.

A key feature of the flare was the rapid rise and fall of optical polarisation

A key feature of the flare was the rapid rise and fall of optical polarisation. Initially, it was low, of order 5%, and aligned with the jet direction, suggesting the dominance of poloidal or turbulent fields. A sharp increase to nearly 50%, while retaining alignment, indicates the sudden injection of a compact, toroidally dominated magnetic structure.

The authors of the analysis propose a “magnetic spring” model wherein a tightly wound toroidal field structure is injected into the jet, temporarily ordering the magnetic field and raising the optical polarisation. As the structure travels outward, it relaxes, likely through kink instabilities, causing the polarisation to decline over about two weeks. This resembles an elastic system, briefly stretched and then returning to equilibrium.

A magnetic spring would also explain the multiwavelength flaring. The injection boosted the total magnetic field strength, triggering an unprecedented mm-band flare powered by low-energy electrons with long cooling times. The modest rise in mm-wavelength polarisation (green points) suggests emission from a large, turbulent region. Meanwhile, optical flaring (black points) was suppressed due to the rapid synchrotron cooling of high-energy electrons, consistent with the observed softening of the optical spectrum. No significant γ-ray enhancement was observed, as these photons originate from the same rapidly cooling electron population.

Turning point

These findings mark a turning point in high-energy astrophysics. The data definitively favour leptonic emission mechanisms in BL Lacertae during this flare, ruling out efficient proton acceleration and thus any associated high-energy neutrino or cosmic-ray production. The ability of the jet to sustain nearly 50% polarisation across parsec scales implies a highly ordered, possibly helical magnetic field extending far from the supermassive black hole.

The results cement polarimetry as a definitive tool in identifying the origin of blazar emission. The dedicated Compton Spectrometer and Imager (COSI) γ-ray polarimeter is soon set to complement IXPE at even higher energies when launched by NASA in 2027. Coordinated campaigns will be crucial for probing jet composition and plasma processes in AGNs, helping us understand the most extreme environments in the universe.

The post Exceptional flare tests blazar emission models appeared first on CERN Courier.

The muon g-2 anomaly dates back to the late 1990s and early 2000s, when measurements at Brookhaven National Laboratory (BNL) uncovered a possible discrepancy by comparison to theoretical predictions of the so-called muon anomaly, a_μ = (g-2)/2. a_μ expresses the magnitude of quantum loop corrections to the leading-order prediction of the Dirac equation, which multiplies the classical gyromagnetic ratio of fundamental fermions by a “g-factor” of precisely two. Loop corrections of a_μ ~ 0.1% quantify the extent to which virtual particles emitted by the muon further increase the strength of its interaction with magnetic fields. Were measurements to be shown to deviate from SM predictions, this would indicate the influence of virtual fields beyond the SM.

Move on up

In 2013, the BNL experiment’s magnetic storage ring was transported from Long Island, New York, to Fermilab in Batavia, Illinois. After years of upgrades and improvements, the new experiment began in 2017. It now reports a final precision of 127 parts per billion (ppb), bettering the experiment’s design precision of 140 ppb, and a factor of four more sensitive than the BNL result.

“First and foremost, an increase in the number of stored muons allowed us to reduce our statistical uncertainty to 98 ppb compared to 460 ppb for BNL,” explains co-spokesperson Peter Winter of Argonne National Laboratory, “but a lot of technical improvements to our calorimetry, tracking, detector calibration and magnetic-field mapping were also needed to improve on the systematic uncertainties from 280 ppb at BNL to 78 ppb at Fermilab.”

This formidable experimental precision throws down the gauntlet to the theory community

The final Fermilab measurement is (116592070.5 ± 11.4 (stat.) ± 9.1(syst.) ± 2.1 (ext.)) × 10^–11, fully consistent with the previous BNL measurement. This formidable precision throws down the gauntlet to the Muon g-2 Theory Initiative (TI), which was founded to achieve an international consensus on the theoretical prediction.

The calculation is difficult, featuring contributions from all sectors of the SM (CERN Courier March/April 2025 p21). The TI published its first whitepaper in 2020, reporting a_μ = (116591810 ± 43) × 10^–11, based exclusively on a data-driven analysis of cross-section measurements at electron–positron colliders (WP20). In May, the TI updated its prediction, publishing a value a_μ = (116592033 ± 62) × 10^–11, statistically incompatible with the previous prediction at the level of three standard deviations, and with an increased uncertainty of 530 ppb (WP25). The new prediction is based exclusively on numerical SM calculations. This was made possible by rapid progress in the use of lattice QCD to control the dominant source of uncertainty, which arises due to the contribution of so-called hadronic vacuum polarisation (HVP). In HVP, the photon representing the magnetic field interacts with the muon during a brief moment when a virtual photon erupts into a difficult-to-model cloud of quarks and gluons.

Significant shift

“The switch from using the data-driven method for HVP in WP20 to lattice QCD in WP25 results in a significant shift in the SM prediction,” confirms Aida El-Khadra of the University of Illinois, chair of the TI, who believes that it is not unreasonable to expect significant error reductions in the next couple of years. “There still are puzzles to resolve, particularly around the experimental measurements that are used in the data-driven method for HVP, which prevent us, at this point in time, from obtaining a new prediction for HVP in the data-driven method. This means that we also don’t yet know if the data-driven HVP evaluation will agree or disagree with lattice–QCD calculations. However, given the ongoing dedicated efforts to resolve the puzzles, we are confident we will soon know what the data-driven method has to say about HVP. Regardless of the outcome of the comparison with lattice QCD, this will yield profound insights.”

We are making plans to improve experimental precision beyond the Fermilab experiment

On the experimental side, attention now turns to the Muon g-2/EDM experiment at J-PARC in Tokai, Japan. While the Fermilab experiment used the “magic gamma” method first employed at CERN in the 1970s to cancel the effect of electric fields on spin precession in a magnetic field (CERN Courier September/October 2024 p53), the J-PARC experiment seeks to control systematic uncertainties by exercising particularly tight control of its muon beam. In the Japanese experiment, antimatter muons will be captured by atomic electrons to form muonium, ionised using a laser, and reaccelerated for a traditional precession measurement with sensitivity to both the muon’s magnetic moment and its electric dipole moment (CERN Courier July/August 2024 p8).