Jobs – CERN Courier

When particles went global

**Balance of power** A prototype storage cavity for LEP’s copper radio-frequency system, photographed in 1980. The largest electron–positron collider ever built, LEP came of age at a time when particle physics had grown to a planetary scale. Credit: CERN

Once in a while, the particle-physics community takes stock of its own history. The latest such occasion was the 4th International Symposium on the History of Particle Physics, held at CERN from 10 to 13 November 2025.

The series began at Fermilab, with meetings in 1980 and 1985 that traced the field from its origins to the particle zoo of the 1950s. A third, at SLAC in 1992, took the story through the 1960s and 1970s, when much of the Standard Model (SM) fell into place. The 2025 edition brought together about 200 physicists and historians, with many more following online, to pick up where its predecessor left off, turning to the period from 1980 to 2000. With some 40 contributions across four days, the programme was far richer than any single report can convey. The symposium’s Indico page (indico.cern.ch/event/1480892) carries it in full, along with recordings of most of the talks.

Strong foundations

By the end of the 1970s, the foundations were laid. Quarks had been accepted as physical constituents of matter, neutral currents had validated the electroweak theory in its SU(2) × U(1) form – ruling out an alternative that contained no Z – and the proof that gauge theories yield finite predictions had given the SM its mathematical backbone. The talks painted a shiny and coherent image of what came next: fundamental discoveries of new particles, the stabilisation of the SM, outstanding – and sometimes revolutionary – progress in the construction of accelerators and detectors. These achievements were spread across laboratories worldwide, and made possible by two seemingly contradictory forces: fierce international, national and “intramural” competition, and close cooperation among thousands of dedicated scientists and engineers, on a scale unheard of in civilian industry. A unique and precious ecosystem, that of the high-energy physics community, had established this “coopetition” as its own modus operandi.

Starting from the early 1980s, discovery truly became a global endeavour, against a steady shift in experimental activity from the US to Europe. The W and Z bosons, massive carriers of the weak force, were found at CERN’s SPS (Super Proton Synchrotron) collider by the pioneering UA1 and UA2 experiments between 1982 and 1983 (see “The opening act” and “Sharing the ring” images). The top quark eluded a long search before being found at Fermilab in 1995, at the mass scale of the weak interaction. Its existence, required by anomaly cancellation, completed the third generation of quarks (see “Top, at last” image). Three years later, Super-Kamiokande caught neutrinos oscillating between flavours, building on a tradition of neutrino physics to which Soviet groups had made decisive contributions.

**The opening act** The final stages of the construction of CERN’s Antiproton Accumulator in 1980. The machine accumulated and stochastically cooled the antiprotons that fed the SPS collider for the discovery of the W and Z bosons. Credit: CERN

At both LEP (Large Electron–Positron collider) and the SLC (Stanford Linear Collider) at SLAC, the number of light neutrino species coupling to the Z was measured to be three, implying no more were left to find. Cornell’s CESR/CLEO explored the upsilon family and the physics of the b-quark, while ARGUS, at DESY’s DORIS storage ring, made the first observation of B-meson mixing in 1987. The B-factory programmes at KEK and SLAC then opened the door to the observation of charge–parity (CP) violation in B-meson decays. Not all searches bore fruit: no leptoquarks appeared at HERA and no supersymmetric particles at LEP. These null results, too, shaped the direction of the field.

Still, participants could revel in how one prediction of the SM after another found experimental confirmation, be it the nature of the particles discovered or the detailed structure of hadronic jets, the most direct way quarks and gluons can manifest themselves as constituents of hadrons in a detector far from the interaction region. The same holds for the deep-inelastic structure functions measured at HERA, which mapped the proton’s internal structure with unprecedented precision.

On the theoretical side, several talks described the laborious process of assembling the many pieces of the SM jigsaw, a striking example of a concise and predictive compendium of human knowledge. The Brout–Englert–Higgs mechanism received particular attention, along with the emergence of the concrete Higgs-hunting strategies that would ultimately lead to the 2012 discovery at the LHC (Large Hadron Collider). The solar neutrino problem, a persistent and puzzling shortfall in the detected flux of electron neutrinos from the Sun, found its resolution in the conjecture that neutrinos are massive and can oscillate between flavours, later confirmed experimentally. More broadly, the talks exposed the characteristic dissatisfaction of theorists with any present state of understanding, paired with a restless curiosity for finding in it clues to the next frontier.

The hunt for new physics

The Peccei–Quinn solution to the strong CP problem, proposed in 1977, was soon shown to imply a new pseudoscalar boson: the axion (CERN Courier January/February 2026 p21). By the 1980s, the idea was mature enough to start searching for it. Grand unified theories (GUTs) sought to merge the electroweak and strong forces at energies not far below the Planck scale, corresponding to the shortest distance our current physics allows us to contemplate, and at which quantum gravitational effects become unavoidable, but the early predictions for proton decay were soon contradicted at the Irvine–Michigan–Brookhaven (IMB) detector and Kamiokande (see “The proton stays” image). Supersymmetry offered a potential resolution of the naturalness problem, the puzzle of why the Higgs-boson mass is so much lighter than the Planck scale, and predicted a host of new particles. Its presence would soothe the nagging concern that a long energy desert might separate the electroweak scale from any new physics. Spoiler alert: at the time this article goes to press, no sign of supersymmetry has yet emerged.

**Sharing the ring** The multipurpose magnetic detector UA1 (left), integrating tracking, calorimetry and muon chambers for the widest possible coverage of the collision, and the calorimetric experiment UA2 (right), with high-granularity calorimetry optimised to detect electrons and hadronic jets. Together, they discovered the W and Z bosons at the SPS collider. Credit: CERN

The 1980s also witnessed the resuscitation of string theory, following a rather technical proof that specific string theories could be made free of quantum anomalies – a problem most theoretical physicists were unaware of anyway. The mid 1990s brought matrix models, the non-local objects called D-branes and the unifying vision of M-theory, drawing much of the theoretical community into the programme. Unexpected connections between gauge theory and gravitational physics followed, culminating in the AdS/CFT correspondence at the decade’s end. String theory, when combined with supersymmetry, provided a first candidate for a finite quantum theory of gravity, turning questions such as the number of flavour generations, their mass structure and even the number of spacetime dimensions from speculations suitable only for after-dinner talks into scientific ones. Finally, the period saw astrophysicists, cosmologists and particle physicists joining forces, giving rise to the new field of particle cosmology. The absence of the abundant monopoles predicted by GUTs was among the puzzles that led to inflation, the conjecture that the very early universe underwent a brief phase of exponential expansion.

**Top, at last** The audience in Fermilab’s Ramsey Auditorium on 2 March 1995, gathered to hear the joint CDF and D0 announcement of the discovery of the top quark. Credit: R Hahn, Fermilab

Technology was celebrated in its own right. Talks traced the development of the superconducting magnets and cooling methods that made the great colliders possible. One could hear of the Rutherford cables wound into the superconducting coils of the Tevatron, HERA, RHIC and the LHC, with precision collars and yoke structures containing the enormous magnetic forces. Impressive advances in detector technology, data management, computing and analysis were also reviewed. It was made clear that without tight international collaboration and knowledge flow extending from KEK and Novosibirsk through Europe to the USA and down to Antarctica, among many competing groups, such progress would have been extremely unlikely. The lesson for those now planning future colliders like the FCC (Future Circular Collider) was explicit: even more than for the LHC, pull together the centres of excellence well in advance and concretely prepare what is needed for the construction of both the accelerators and the detectors.

Successes and setbacks

The 1980s and 1990s were good years for opening champagne bottles in accelerator labs. Insightful talks stressed the pioneering role of the ISR (Intersecting Storage Rings) and described how the SPS and the Tevatron were built. The challenges and successes of HERA, the first and only electron–proton collider so far, were described, as were the birth pains and rewarding growth of the SLC, which demonstrated that linear colliders could do precision physics. The triumphs all involved facing and overcoming substantial scientific, engineering and financial challenges. Side by side with the exhilarating successes were the setbacks. Perhaps it was less pleasant to dwell on failures, but studying them while planning future projects is as important as basking in the description of successes.

**The proton stays** The Kamiokande detector during construction in spring 1983, a kilometre beneath Mount Ikeno in Japan. Designed to detect proton decay, it found none, ruling out the simplest grand unified theories. Its successor, Super-Kamiokande, observed neutrino oscillations in 1998. Credit: Institute for Cosmic Ray Research, University of Tokyo

A very telling story was that of ISABELLE, a high-energy proton collider planned at Brookhaven National Laboratory that ran into magnet trouble in the early 1980s. A proposal to double the energy fared no better, especially after the discoveries of the W and Z at CERN made the original physics case less compelling. Yet its rebirth as RHIC, the Relativistic Heavy Ion Collider, opened a whole new field of research for the community (see “Second life” image), allowing physicists to recreate and study the quark–gluon plasma that filled the universe microseconds after the Big Bang. A case of snatching victory from the jaws of defeat, and an important trajectory to keep in mind for the future.

A different story was that of the Superconducting Super Collider (SSC), cancelled by the US Congress in 1993 after construction began in Texas. Understanding all the causes of this failure and appreciating its full impact on both the US and the CERN programmes is still fertile ground for historians, and should be required reading for those planning new frontier colliders.

The 1980s and 1990s were good years for opening champagne bottles in accelerator labs

The dissolution of the Soviet Union terminated the UNK programme, which envisaged a 3 + 3 TeV proton collider at IHEP Protvino. The impact on high-energy research in Russia was profound, yet the community showed remarkable resilience, sustaining the heavy-ion programme at JINR in Dubna and continuing neutrino experiments in politically challenging times. A silver lining was the international call for scientific solidarity that crossed geopolitical lines, helping keep Soviet colleagues part of the high-energy physics community.

The protagonists of the 1980s and 1990s fulfilled, with honour, their duty – passed down from generation to generation – to lay the groundwork for their successors. Many of them were in the room, recounting their own experience. All endeavours were extremely challenging. Some overcame the obstacles and reaped success, some had to reconcile themselves with descoping, and some failed. All left lessons for the future.

Free energy

As Winston Churchill himself put it in the House of Commons, it is “much better by all parties to leave the past to history, especially as I propose to write that history myself.” Inevitably, those who lived through the events are not always best placed to write the conclusive story. Future editions would benefit from a larger contribution by historians of science, offering independent analysis of the processes, credits and timelines to which the physicists themselves may be too close. Still, many words of wisdom emerged from the combined experience of the speakers. Among them, a remark redefined “free energy” as the small reserve of discretionary funds that a visionary director-general can deploy to test major breakthrough ideas, free from the scrutiny of funding agencies.

**Second life** The construction of BNL’s Relativistic Heavy Ion Collider began in 1991, in the tunnel left by the cancellation of the ISABELLE proton collider eight years earlier. The machine would come into operation in 2000 and establish the quark–gluon plasma as a near-perfect liquid. Credit: Brookhaven National Laboratory

Various flavours of human behaviour in competition and in collaboration found expression throughout the talks: the intramural rivalry between UA1 and UA2 at CERN, the national competition between SLAC and Cornell, the international contests between LEP and the SLC, and between KEK and SLAC at the B-factories. All these stories contain lessons for the future. For me, none matters more than coopetition.

The symposium was dedicated to the memory of Herwig Schopper, who passed away in August 2025 at the age of 101 (CERN Courier November/December 2025 p32). He was CERN Director-General from 1981 to 1988, in the years of the SPS collider, of LEP’s approval and of the early conception of the LHC.

Interactive map A selection of discoveries, machines and ideas from 1980 to 2000, pinned to the places where they happened, with each pin opening a short account of its site. Progress relied on shared technologies, from the Rutherford cables wound into the superconducting coils of the Tevatron, HERA, RHIC and, later, the LHC, to the era’s advances in detectors and computing.

Ten years at the missing-energy frontier

Missing energy has long served as a clue to unseen physics. In December 1930, Wolfgang Pauli suggested a “desperate remedy” to explain the continuous spectrum of beta-decay electrons: an undetected, neutral state that accompanied the electron, carrying off the rest of the energy and saving its conservation (see “A desperate remedy” image). The particle, named the neutrino by Enrico Fermi, interacted so weakly that Frederick Reines and Clyde Cowan observed it only a quarter of a century later

Missing transverse energy and momentum were also essential to the 1983 discovery of the W boson at the SPS proton–antiproton collider (see “When particles went global”). At LEP, the same principle was applied to events whose only visible trace was a single photon, with a neutrino pair recoiling against it and escaping with the rest of the energy. The rate of such events agreed with the existence of three light neutrino species, a count fixed far more precisely by the shape of the Z resonance.

The same observables have driven searches for supersymmetric particles and invisible Higgs decays at the LHC. Their application to fixed-target experiments is more recent, and at its forefront is CERN’s NA64 search for feebly interacting, dark-sector particles.

**A desperate remedy** Wolfgang Pauli lecturing in Copenhagen in April 1929. Twenty months later, in an open letter to a meeting in Tübingen, he postulated the neutrino to account for the missing energy in beta decays, doubting it would ever be detected directly. Credit: CERN

The case for such states has been building for half a century. Several astrophysical and cosmological observations point to a “dark” form of matter, whose presence is inferred only through gravity. Together with a pervasive and no-less-mysterious dark energy, it accounts for about 95% of the energy content of the universe. Despite its extraordinary precision, then, the Standard Model (SM) of particle physics does not provide a complete description of matter.

For many years, the dominant dark-matter candidate has been the weakly interacting massive particle (WIMP), a thermal relic in equilibrium in the early universe, whose present-day abundance is set by its annihilation rate. Remarkably, models with masses and interaction strengths characteristic of the electroweak scale, around 100 GeV, naturally reproduce the observed dark-matter density – a coincidence often referred to as the WIMP miracle. This expectation motivated searches at high-energy colliders such as the LHC, as well as direct- and indirect-detection experiments.

The dark sector

With these efforts yielding no clear signal, the case for looking elsewhere has grown. In particular, it has become clear that the thermal mechanism behind the WIMP can be realised for much lighter particles, provided their interactions with ordinary matter are sufficiently weak. This is most naturally achieved if new physics resides in a “dark sector”, comprising additional particles and interactions beyond those of the SM, with only a tenuous coupling between the two. Much like ordinary matter, the dark sector could possess its own intricate, internal dynamics, beyond the reach of direct observation.

**Portal, ajar** A high-energy electron scatters off a nucleus in the active target and radiates a dark photon, which decays into two dark-matter particles. The dark photon’s decay products escape unseen, and the electron emerges with reduced energy. Credit: NA64 Collaboration

In addition to gravity, communication between the dark sector and ordinary matter may occur through a faint “portal” interaction, distinct from the known SM forces. The forms such a portal can take are tightly constrained, since the symmetries of the SM allow only a handful of ways for a hidden sector to couple to ordinary matter: a dark photon can mix with the ordinary one, a new scalar particle can couple to the Higgs boson and a hidden fermion can couple to neutrinos. In most scenarios, the dark sector lies at mass scales comparable to those of electrons and protons, with couplings tuned to set the relic abundance at the observed value. The portal coupling is typically so weak that dark-sector particles would be produced only rarely, so high-intensity beams are needed to reach detectable rates. Once made, they would traverse large volumes of ordinary matter almost unattenuated, making signatures hard to detect.

The success of NA64 has relied on the continuous support of the CERN accelerator and beams department

This feebleness limits the reach of traditional beam-dump experiments, such as CHARM at CERN and E137 at SLAC, which rely on producing a new particle in a target and observing its decay or interaction in a far, downstream detector. The signal rate is therefore suppressed twice by the small portal coupling: once in production and once in the subsequent decay or scattering. In the missing-energy approach, by contrast, a well-characterised particle is sent into a target, and the energy emerging from the interaction is measured with hermetic calorimetry (see “Portal, ajar” figure). Any deficit would point to a particle that escaped without interacting, with sensitivity suppressed only by the production probability. For a representative mixing parameter of order 10^–5 between the photon and its hypothetical dark counterpart, the number of beam particles required to reach a given sensitivity can be reduced by up to 10 orders of magnitude. This brings well-motivated regions of parameter space associated with light thermal dark matter within reach of existing accelerator facilities.

This perspective has motivated a new generation of high-intensity experiments, designed to probe rare processes associated with extremely weakly interacting particles. Among these, NA64 has played a pioneering role.

NA64 was proposed in 2013 to employ the missing-energy technique at the CERN SPS, using the H4 high-energy electron beam impinging on an instrumented target. Early test measurements carried out in 2014 demonstrated the feasibility of the approach and helped validate the detector concept. Building on these results, the experiment was approved in 2016 as the 64th experiment in the North Area, from which it takes its name. It is part of CERN’s broader Physics Beyond Colliders (PBC) initiative, which coordinates efforts to explore new physics using high-intensity and precision experiments. The success of NA64 has also relied on the continuous support of the CERN accelerator and beams department, in particular in optimising the beamline and preparing a dedicated experimental area following Long Shutdown 2.

Pure heart

At the heart of the experiment, an electromagnetic calorimeter serves as an active target (see “Hermetic chain” figure). Incoming electrons are individually tagged and their energy precisely measured before entering the calorimeter, where they initiate an electromagnetic shower. The tagging system, based on the detection of synchrotron radiation emitted by beam particles in the dipole magnets of the spectrometer, provides a highly pure electron beam. Additional tracking and identification detectors upstream of the target reject contaminating particles, further improving the purity of the selected events, while hadronic calorimeters downstream provide a veto against escaping particles and ensure a hermetic measurement of the event.

A candidate signal is an event in which the measured energy falls below a predefined threshold. SM processes can also give rise to missing energy, most notably through neutrinos produced in hadronic interactions in the target. These backgrounds are strongly suppressed and can be efficiently discriminated, as they are typically accompanied by significant energy deposition in either the electromagnetic or hadronic calorimeters. Detector effects such as energy leakage are likewise carefully controlled.

Over the past decade, NA64 has carried out a series of measurements using high-intensity electron beams, progressively improving its sensitivity to light, feebly coupled particles. The initial focus has been the search for dark photons that decay invisibly, a well-motivated benchmark scenario for dark-sector models. A dark photon was also considered a possible explanation for the muon g–2 anomaly (CERN Courier March/April 2025 p21), since it would contribute to the muon magnetic moment at one loop through kinetic mixing with the ordinary photon. In these searches, no significant excess of events with missing energy has been observed, allowing stringent constraints to be placed on the interaction strength between the dark sector and ordinary matter.

Long Shutdown 3 provides the opportunity to further extend the sensitivity of NA64 to increasingly rare processes

The NA64 programme has since expanded to a wider class of dark-sector mediators. The results are typically presented as exclusion regions in the plane defined by the mediator mass and its coupling to the SM. The collaboration has explored a broad range of masses in the MeV–GeV region, probing parameter space previously inaccessible to both collider searches and traditional beam-dump experiments.

Beyond electrons

Over time, the experimental programme has also expanded beyond the original electron-beam configuration. The use of positron beams enables the resonant production of dark photons through the annihilation of beam positrons with atomic electrons of the active target. In an electron beam, the same process is suppressed because it relies on secondary positrons produced in the electromagnetic shower. Complementary approaches using hadronic beams extend the reach to dark-sector particles coupled to quarks, while measurements with muon beams at the M2 beamline, based on the missing energy-momentum technique, probe portals that couple to second-generation leptons and reach higher mediator masses. Each beam targets a different region of the dark-sector model space, and the combined programme covers a far wider range than the original electron-beam design.

**A dark corner** Experimental bounds on the dark photon, plotted as the mixing parameter ε against the dark-photon mass m_A′ (left), and as the dimensionless variable y – which sets the relic abundance in the standard thermal scenario – against the dark-matter mass m_χ (right). On the left, NA64 sets the leading bounds at sub-GeV masses and excludes the region once favoured as an explanation of the muon g–2 anomaly. On the right, three black lines show where the scalar, Majorana and pseudo-Dirac fermion relics reproduce the observed dark-matter abundance. The parameter α_D denotes the dark fine-structure constant. Credit: NA64 Collaboration

These constraints have important implications for models of light dark matter. In a number of well-defined scenarios, the parameter-space region consistent with the observed relic abundance can be directly tested, and NA64 has already excluded significant portions of it (see “A dark corner” figure).

Thanks to its high-energy beam and compact detector, NA64 is also sensitive to particles with much shorter lifetimes than those targeted by traditional beam-dump experiments, which would typically decay inside the dump or shielding and so escape detection. By looking for visible decay products close to the production point, NA64 reaches a regime intermediate between prompt collider signatures and long-lived beam-dump ones. This has enabled, for example, searches for axions and axion-like particles decaying into two photons.

The exploration of dark sectors at high intensities is still in its early stages, and the recently started CERN Long Shutdown 3 (LS3) provides the opportunity to further extend the sensitivity of NA64 to increasingly rare processes (see “The LHC completes its third run”). Planned detector upgrades aim to improve hermeticity and background rejection, ensuring that the missing-energy signature remains robust even at higher beam intensities. The integrated statistics will then grow by up to two orders of magnitude beyond present datasets (see “Reaching the relic” figure).

**Reaching the relic** Projected NA64 reach for light dark matter, plotted as the dimensionless variable y against the dark-matter mass m_χ. The black line (relic curve) marks where a pseudo-Dirac fermion relic reproduces the observed dark-matter abundance. Dashed orange lines show NA64 projections in electron and muon modes with increasing data samples, while solid lines show other proposed experiments. With 10¹⁴ muons on target, NA64 reaches well below the relic curve across a wide range of masses. The grey region is already excluded by current experiments. Credit: NA64 Collaboration, adapted from ESPP input

The combined electron, positron, muon and hadronic beam programmes, together with the post-LS3 increase in statistics, are projected to probe a large portion of the parameter space of leading thermal-relic models of sub-GeV dark matter. Inelastic, semi-visible and muon-philic mediators will be tested in parallel, alongside searches for axion-like particles and other weakly coupled states. Together with complementary searches at colliders, in other fixed-target experiments and in direct detection, NA64 will help test a broad range of dark-sector scenarios.

The Swiss Army knife of beam simulation

Particle accelerators are complex engineering marvels, with thousands of components working in concert. Magnets bend, focus and shape particle beams, while radio-frequency cavities accelerate them and keep the bunches dense. Feedback systems stabilise their motion, collimators intercept stray particles to protect the machine and the detectors, and injection and extraction devices guide them into and out of the accelerator. In addition to these external fields, beams are shaped by their own collective effects, in which particles influence one another directly or through the electromagnetic fields they generate in the surrounding beam pipe and accelerator structures.

Faced with these intricate effects, physicists have long relied on computer simulations to design, study and operate particle accelerators. At CERN, the development of computer programs to calculate the orbit and optics of synchrotrons, transfer lines and experimental lines dates back to the late 1960s (see “Hello, world” image). The Methodical Accelerator Design program, better known as MAD, was developed in the 1980s as part of the design effort for the Large Electron–Positron Collider (LEP). It later evolved into MAD-X, which became the de facto standard for optics calculations and accelerator operation at CERN throughout the LHC era.

**Hello, world** CERN’s IBM 709 arrives at Geneva’s Cointrin airport in 1960. Successor to the vacuum-tube Ferranti Mercury of 1958, it was the first CERN machine that could be programmed in FORTRAN. Credit: CERN

With the growth of computing power, it became possible to follow individual particle trajectories over thousands, and later millions, of revolutions. This opened the door to more detailed studies of beam-quality preservation in the presence of non-linear forces and magnet imperfections. At CERN, much of this work was carried out with SixTrack, inspired by an earlier tool created for DESY’s HERA collider. Like MAD-X, SixTrack became one of the standard simulation tools of the LHC era. Further programs were tailored over the years to specific phenomena. For example, HEADTAIL and the later PyHEADTAIL became workhorses for studying transverse collective instabilities, in which the electromagnetic fields induced by a bunch in its surroundings act back on the bunch itself. Together, these programs distilled decades of accumulated expertise in accelerator physics, from which the community still benefits on a day-to-day basis.

As studies became more sophisticated, however, legacy tools began to show their limitations. Developed by separate teams to analyse different physical phenomena, they were not easy to combine when studying their interplay, often requiring slow, ad hoc interfaces. Many had not been designed with long-term extensibility in mind, and some of the experts familiar with the original technologies had since moved on. At the same time, the promise of workflows accelerated by GPUs was becoming increasingly attractive for accelerator physics, but difficult to retrofit into software architectures designed for a different computing era. The Xsuite framework was developed to pull all these threads together (see “Enter Xsuite” panel).

Enter Xsuite

Xsuite aims to bring together the functionality of MAD-X, SixTrack and PyHEADTAIL in a modern, extensible, Python-based toolkit, with GPU acceleration and collective effects included by design. Its architecture is modular, with clean, well-defined interfaces between components. The main building blocks include Xtrack, Xpart, Xfields, Xcoll and Xwakes, supported by the lower-level packages Xdeps and Xobjects (see “Division of labour” figure).

**Division of labour** The modular architecture of Xsuite, with each physics module handling a specific aspect of beam dynamics. All build on the Xobjects and Xdeps lower-level modules. Credit: CERN

This modular structure helps keep the individual components manageable as the framework grows, while making the code easier to understand, extend and contribute to. Development has been user-centric from the start, with features implemented in response to concrete needs and tested on real-use cases. A major investment in automated testing supports a fast release cycle, with new versions appearing several times a month. Comprehensive tests run every night on all supported platforms, helping to catch errors from developers and third-party dependencies early.

Much of Xsuite’s value lies in how readily it connects to other codes, allowing a single study to combine effects that would otherwise sit in separate programs. Integration with FLUKA and BDSIM-Geant4, for example, enables precise modelling of particle–matter interactions across the accelerator environment, while interfaces with RF-Track and BLonD extend simulations to include complex electromagnetic fields and RF feedback systems. An interface to MAD-NG adds methods to perform high-order non-linear optimisation.

Launched in 2021, within the Accelerators and Beam Physics group at CERN, Xsuite brings accelerator-specific tools for design and simulation into a modern, integrated computing environment, connected to the wider scientific Python ecosystem for data analysis, visualisation, optimisation and machine learning. This transition is particularly important for future accelerator projects. Machines such as the Future Circular Collider (FCC) push accelerator modelling to its limits: the underlying physics calls for detailed simulations, while their scale and complexity demand the full exploitation of modern computing hardware.

Xsuite now serves as a core tool for beam-dynamics simulations in all synchrotrons operating at CERN (see “Four crossings” figure), as well as in upgrade projects and future design studies. Beyond CERN, it is used by teams at GSI, the Heidelberg Ion–Beam Therapy Center (HIT), MedAustron and DESY in Europe, Fermilab and Brookhaven in the US, the Canadian Light Source, and J-PARC and KEK in Japan. A clear sign of Xsuite’s growing adoption is the steady rise in mentions in recent proceedings of the International Particle Accelerator Conference (IPAC), which now exceed those of MAD-X, for many years one of the most widely used accelerator codes.

Xsuite’s open-source model has been central to this success. Free availability invites scrutiny, builds trust by allowing users to inspect the source code, and lowers the barrier to contribution. The teams that use Xsuite increasingly help to shape it, returning improvements and new features to the shared codebase. Xsuite is used for student training in schools such as the CERN Accelerator School, the Joint Universities Accelerator School and the US Particle Accelerator School, where it gives students hands-on experience with tools directly applicable to the machines they will go on to build and operate.

Testing future limits

Proposed future colliders present daunting but exciting challenges for accelerator physicists, making simulation tools more important than ever. Following the success of the integrated LEP and LHC programme, the leading proposals for the next high-energy facility are once again electron–positron colliders, potentially followed by larger hadron machines. Among them are the FCC study at CERN (see “CERN Council updates the European Strategy”) and the CEPC study at IHEP in China (CERN Courier November/December 2025 p7).

The most luminous lepton collider currently operating is SuperKEKB at the KEK laboratory in Tsukuba, Japan. Its three-kilometre electron–positron rings hold the world record for the highest collider luminosity, and set the benchmark the next generation aims to pass (see “At the luminosity frontier” figure). As SuperKEKB works towards its design parameters, simulation has been central to understanding observations and optimising the machine configuration. This work has, in turn, driven the development of Xsuite and strengthened collaboration between CERN and KEK, supported by exchange schemes such as the European Union’s Europe–America–Japan Accelerator Development Exchange Programme (EAJADE).

**At the luminosity frontier** A cutaway of the Belle II detector at SuperKEKB. The colliding beams pass through the experiment solenoid, while accelerator magnets inside the detector steer and focus them close to the interaction point. Xsuite can model these nested magnetic fields, supporting studies to optimise SuperKEKB as it advances toward design parameters. Credit: Belle II/KEK

The clean way Xsuite’s components fit together enables integrated studies of several beam-dynamics effects at once – a capability that is increasingly important for the FCC-ee design. A faithful model must include thousands of magnetic and radio-frequency elements, together with synchrotron-radiation emission, the tapering of magnet strengths to follow the local beam energy, beam–beam interactions, beamstrahlung and scattering in collimators. Xsuite can treat these effects together, in a self-consistent and flexible way, making it possible to study how they influence one another in realistic machine configurations. The loss distribution around the FCC-ee ring, for example, was obtained from such integrated simulations (see “On the safe side” figure). These studies test whether the machine can operate safely while delivering its target luminosity, and help guide key design decisions early in the project’s life cycle.

Built for multi-tracking

Particle accelerators usually accelerate one kind of particle at a time. Nevertheless, secondary particle species are produced whenever beam particles hit the surface of beam pipes or collimators, and following them matters for the protection of sensitive components such as superconducting magnets and cavities, as well as detectors. Xsuite was therefore built to track more than one particle species at once. This feature is regularly used for collimation studies for the LHC complex and for the FCC design, and it is also proving valuable for advanced studies in hadron therapy, where carbon and helium ions are a case in point.

**On the safe side** Xsuite is used in the design of the proposed FCC-ee collider to predict beam losses and protect sensitive cold components. The resulting loss map shows the power lost per metre around the ring. The black peaks mark the collimators that catch the losses, so that little reaches the cold elements (blue), while the top labels name the eight access points. Credit: CERN/G Broggi

The charge-to-mass ratios of the two species are close enough that the two species can be accelerated together in one ring. At MedAustron, in Austria, researchers are studying mixed carbon–helium beams for “online range verification” during patient irradiation (see “Tag team” figure). In practice, while the carbon ions deposit their energy in the area to be treated, the helium ones pass through the patient and can be used to precisely localise the beam. Xsuite is used to simulate and optimise how the two ion species are injected, accelerated and extracted together in the synchrotron (CERN Courier November/December 2025 p45).

Shared expertise

Xsuite has by now become the working tool of hundreds of accelerator physicists. The advantages of having a large and diverse community working with the same toolkit are tangible, including synergies across projects, shared expertise, community support for new users, and collective validation of results. The current effort is to bring it into the control room. Connecting Xsuite to the CERN layout database and to the accelerator control system would let operators use the same simulation models to read, build and adjust machine configurations directly. Early control-room tests have been very positive – including a machine-development session where a full proof-of-concept LHC cycle was designed entirely with Xsuite and tested in the machine.

**Tag team** The ion-beam injectors (top) of the MedAustron synchrotron in Wiener Neustadt, Austria, where a mixed carbon-helium beam is produced. The two ions share almost the same charge-to-mass ratio, so they can be accelerated together to the same energy per nucleon. A schematic (bottom) shows the beam in a patient: the carbon stops in the target to deliver the dose, while the lighter helium, with about three times the range, passes through. The energy the helium still carries when leaving the body can be used for online monitoring of the treatment. Credit: MedAustron/T Kästenbauer (top); M Kausel *et al.* (bottom)

These steps towards operational readiness coincide with a broader shift in how accelerators are studied and controlled. Machine-learning methods and AI are becoming increasingly present in accelerator workflows, from beam tuning and injection optimisation to anomaly detection and correction of slow drifts across CERN’s complex (CERN Courier May/June 2025 p35). Their development benefits from simulation tools that can generate realistic scenarios, explore many configurations efficiently and connect naturally to modern data-analysis environments. By combining fast-tracking, GPU capabilities and Python integration, Xsuite is well placed to support this growing convergence of beam-dynamics modelling, machine learning and accelerator operation.

Built to bring established accelerator modelling capabilities into a modern, integrated framework, Xsuite now spans applications from future-collider design and accelerator operation to the synchrotrons used in cancer therapy. If Xsuite is relevant for your work, give it a try – the team is eager to hear your feedback!

CERN Council updates the European Strategy

On 22 May, at a dedicated session in Budapest, the CERN Council updated the European Strategy for Particle Physics. The update confirms the full exploitation of the scientific potential of the LHC, through the completion of its high-luminosity upgrade (HiLumi), as the highest medium-term priority for European particle physics. For the longer term, it recommends the electron–positron Future Circular Collider (FCC-ee) as the preferred option for the next flagship project at CERN, thereby maintaining Europe’s leadership in the field. A decision on the FCC-ee itself is targeted for 2028.

Critical importance

“The European Strategy reiterates the critical importance of the High-Luminosity LHC, which will use advanced accelerator and detector technologies to fully exploit the scientific potential of this incredible machine in the coming years,” said Mark Thomson, CERN Director-General. “Beyond HiLumi LHC, the FCC-ee would be a visionary global research infrastructure for the next decades that will deepen our knowledge of the fundamental building blocks of the universe through ultra-precise measurements of the Higgs boson and other elementary particles. CERN’s task now is to steer this unprecedented project towards a decision by the CERN Council.”

According to the Strategy, the FCC-ee would offer the broadest exploratory programme in fundamental physics. The feasibility study, published in March 2025 (CERN Courier May/June 2025 p9), describes a baseline machine in a 90.7-kilometre ring, running at four centre-of-mass energies: the Z pole, the WW threshold, the ZH production peak and the top-quark pair threshold. Over a 15-year programme, it would be expected to yield some 6 × 10¹² Z bosons, 2.4 × 10⁸ W-boson pairs, 2.7 × 10⁶ Higgs bosons and 2 × 10⁶ top-quark–antiquark pairs.

“The high-energy physics community and the CERN Council have been united for this critical update of the European Strategy for Particle Physics, and the FCC-ee has emerged as the preferred flagship project to maintain CERN’s world-leading role in collider physics and technology in the decades to come,” said Costas Fountas, Council president. “I wish the CERN management the greatest success in implementing the Council resolution between now and the 2028 target decision date.”

The FCC-ee would be a visionary global research infrastructure that will deepen our knowledge of the fundamental building blocks of the universe

The 2026 update follows more than two years of intense work by the European particle-physics community, under the auspices of the European Strategy Group. Initiated in March 2024, the process aimed to develop a concrete plan to advance fundamental physics by constructing a new flagship project at CERN, and drew on more than 260 written submissions. It builds on the 2020 update, which emphasised the importance of ensuring Europe’s continued scientific and technological leadership, and recommended an electron–positron “Higgs factory” as the highest-priority next facility after the LHC reaches the end of its operational lifetime in 2041.

In addition to updating the Strategy, the Council has invited CERN management to initiate discussions with the relevant authorities and entities in the Member and Associate Member States, as well as non-Member States and the European Union, with a view to developing a financially feasible funding plan for the possible FCC-ee project. In the next two years, CERN management will provide annual reports on the implementation of the Strategy update and the necessary information to support national decision-making processes so that the Council will be in a position, by 2028, to take a decision on the FCC-ee, taking into account elements such as the scientific, technical and financial feasibility of the project, as well as results from the public consultation exercises in CERN’s host states, France and Switzerland.

Strong engagement

“The Strategy process has seen a very strong engagement of the particle-physics community and has led to a very clear conclusion: the FCC-ee, if approved, would deliver the world’s broadest high-precision particle-physics programme, its technical feasibility has been demonstrated by the comprehensive FCC feasibility study, and its scope and cost are well defined,” said Karl Jakobs, Strategy secretary. “It would also pave the way for a possible future hadron collider reusing the tunnel and much of the infrastructure, providing direct discovery reach well beyond the 10 TeV parton energy scale.”

The LHC completes its third run

On 27 June 2026, proton beams circulated through the LHC for the final time before the machine entered its third Long Shutdown (LS3). After four years of operation, the world’s highest-energy particle accelerator now begins an ambitious programme of upgrades that will transform it into the High-Luminosity LHC (HL-LHC).

“The performance of the LHC during Run 3 surpassed all expectations,” says Oliver Brüning, director for accelerators and technology at CERN. “The LHC delivered nearly 550 fb^–1 of integrated luminosity since its first collisions – almost twice the original target of 300 fb^–1. This amounts to 50 times the total combined dataset produced by all hadron colliders operating before the start of the LHC.”

The LHC was approved by the CERN Council in December 1994 and built in the 27 km tunnel previously occupied by LEP, the electron–positron collider that operated at CERN until 2000 (When particles went global). Collisions began in 2009, reaching the 7 TeV centre-of-mass energy milestone in 2010. The years since have brought the discovery of the Higgs boson, a wealth of new hadrons, detailed studies of the quark-gluon plasma and measurements of a wide range of Standard Model (SM) processes with steadily improving precision.

Expanded searches

The recently completed third run exploited the accelerator and detector systems upgraded during the machine’s second Long Shutdown (LS2), from 2019 to 2022 (CERN Courier May/June 2022 p29). “The LHC’s Run 3 benefits from an increased proton–proton collision energy of 13.6 TeV and a data volume that is far greater than that of Run 2,” says Gautier Hamel de Monchenault, director for research and computing at CERN. “This enabled detailed studies of the Higgs-boson’s properties through high-precision measurements and explorations of its self-coupling and rarest decays. Searches for dark matter and new phenomena expanded considerably to include new topologies, such as exotic long-lived heavy particles, through improvements in trigger systems and advances in analysis techniques.”

Run 3 also brought advances in flavour physics, with more precise measurements of rare decays and CP violation across the beauty and charm sectors, and in the heavy- and light-ion programmes. In summer 2025, the LHC successfully delivered its first oxygen–oxygen and neon–neon collisions (CERN Courier November/December 2025 p8). “The LHC provided the experiments with extensive samples of lead–lead collisions, as well as samples of events involving lighter nuclei of oxygen and neon, in order to map the properties of the primordial quark–gluon plasma with greater precision,” remarks Hamel de Monchenault.

The upgrades to the experiments are the largest since the start of the LHC

During Run 3, the proton–proton collisions delivered to each of the general-purpose experiments ATLAS and CMS amounted to about 4 × 10¹⁶, while the lead–ion programme delivered about 2 × 10¹¹ collisions to the four experiments – ALICE being the one designed specifically for heavy-ion physics. LHCb, primarily a flavour experiment and the only one to also operate in fixed-target mode, collided the beams with several gas species injected through its SMOG2 system. Together, the experiments generated exabytes of data for processing, storage and analysis through the Worldwide LHC Computing Grid.

The final proton–proton collisions took place on 16 May, followed by the final lead–lead collisions on 14 June. The beams continued circulating for another two weeks for high-intensity machine tests before the accelerator was shut down. “The success of the LHC Run 3 is a testimony of the quality and ingenuity of the LHC design and hardware, and to the dedication and competence of the teams looking after the operation of the accelerator,” says Brüning.

Although collisions have now ceased, analysis of the collected data will continue well into the 2030s. Meanwhile, the LHC and its experiments enter LS3, a four-year period during which extensive upgrades throughout the accelerator complex and the detectors will turn the machine into the HL-LHC.

Unprecedented efficiency

“The HL-LHC aims at increasing the data set produced by the LHC by an order of magnitude,” says Brüning. “This implies not only an increase in the instantaneous performance but also an unprecedented availability and efficiency of the machine. To this end, new underground galleries and caverns were created next to the LHC tunnel, reducing radiation exposure to the electronics and thus the machine’s downtime, and facilitating access for technical experts to intervene in case of problems. All these new underground structures have been completed during the LS2 and the Run 3 period. The production of the remaining components is in full swing, with more than half of them already available for installation in the tunnel and the new underground areas.”

The upgrades to the experiments are the largest since the start of the LHC. Large sections of the detectors are being rebuilt from scratch, with new tracking systems, upgraded calorimeters and muon detectors, and next-generation trigger and data-acquisition systems built for the far higher event rates. These upgrades will let the experiments match and exceed their current performance under HL-LHC conditions, where around 200 proton–proton collisions are expected per bunch crossing, compared with about 60 during Run 3.

The central prize remains the Higgs boson itself

The HL-LHC’s target of more than 3 ab^–1, around six times the data collected so far, will open several fronts at once. In the Higgs sector, the experiments expect to observe rare decays such as H → μ⁺μ^– and H → Zγ, and to pin down its couplings to fermions and vector bosons at the per cent level. More ambitious still are Higgs pair production, expected to be observed with the combined HL-LHC dataset, and the charm Yukawa coupling. The programme should establish longitudinally polarised vector-boson scattering and bring rare processes such as four-top-quark production into precision reach, while the sensitivity of direct and indirect searches will push well past that of Run 3.

In flavour physics, precision measurements of rare decays and CP violation will test the SM and probe for deviations that would point to new physics. The study of quark–gluon plasma will enter a higher-precision phase of its own, with larger samples of lead–lead and proton–lead collisions mapping its properties in finer detail and tracing the collective behaviour of strongly interacting matter and the emergence of nuclear structure.

For John Ellis, theoretical physicist at King’s College London and CERN, the central prize remains the Higgs boson itself: “Is the Higgs boson really what it seems? Experiments at the HL-LHC will measure accurately how it interacts with other particles and itself, searching for evidence of new physics that it may be hiding, and probing its role in the history of the universe.”

arXiv’s one-strike rule on AI

**Steep climb** Monthly submissions to arXiv since its launch in 1991, climbing past 30,000 for the first time in early 2026. Credit: arXiv

Authors who submit a paper containing unchecked output from a large language model (LLM) will risk a year’s suspension from arXiv. The preprint server, long the main channel for circulating papers in physics, mathematics, computer science and other quantitative fields before peer review, has clarified its content policy in response to a rising tide of AI-generated submissions.

The threshold for suspension is “incontrovertible evidence of hallucinatory AI generation”, in the words of the arXiv scientific director Steinn Sigurðsson. Examples include hallucinated references, citations to non-existent papers, and meta-comments left by the model, such as an instruction to fill in the real numbers from an experiment. arXiv uses a detection algorithm to identify suspect papers, with readers also able to submit “Code of Conduct” complaints if papers have been released without being caught. “After the suspension period, the author can request reinstatement,” Sigurðsson says, “but in a number of cases, depending on the reason for the suspension, the author will be asked to submit work that has passed peer review with a reputable journal or conference. Typically, after three such submissions, there is no further restraint.”

The pressure behind the move is one of volume. Rejection rates have climbed, and the number of submissions held back for review, flagged by a quality-assurance tool or a moderator and often rejected in the end, has climbed faster still. “The increase in such cases puts a major strain on both the staff and the volunteer moderators,” says Sigurðsson.

arXiv typically holds all co-authors jointly responsible for a paper’s content. But that applies only when everyone listed has consented to it, a condition a fabricated paper may not satisfy. In such cases, arXiv relies on rules it already has. “Adding an author to a paper without their knowledge is a major academic misconduct,” Sigurðsson says, “and would generally lead to referral to the relevant academic institutions.”

For an offending submission from a large team of thousands, arXiv would not suspend the entire author list. It would defer instead to the collaboration, trusting it to identify the member who posted without clearance and to impose the primary sanction. That person would still be flagged for close inspection – and may be suspended by arXiv itself. “We have had several instances,” says Sigurðsson, “both where a member of a collaboration has submitted a manuscript to arXiv without clearing it through the internal process, or even where completely unaffiliated submitting authors have added a collaboration without notifying it at all.”

The one-strike rule on AI hallucinations is a matter of enforcement, rather than a new rule. arXiv has long held broad policies on content and scholarly standards, and recent internal discussions focused on how to apply them consistently to AI-generated content.

As LLMs develop, however, quality-control mechanisms are liable to date quickly. The difficulty, Sigurðsson says, is “not the lack of possible tools and counters, but the work involved in testing the tools and implementing them in a production system. This is due especially to their short useful lifetime, as AI tools evolve and users react to arXiv measures.”

Antihydrogen toes the line

Hydrogen’s ground state is split into two closely spaced energy levels, the tiny gap set by the magnetic tug between its proton and electron. Antihydrogen should split by exactly the same amount. The ALPHA experiment at CERN’s Antimatter Factory has now found that it does, a hundred times more precisely than before. The new result, with an uncertainty of four parts per million, delivers one of the most stringent comparisons between matter and antimatter, and a potential probe of the antiproton’s internal structure.

ALPHA is designed to measure the spectrum of antihydrogen, compare it with hydrogen, and test the limits of CPT symmetry. This principle holds that physics is unchanged when charge, parity and time are reversed together. Matter and antimatter must then have the same masses and spectra – hydrogen and antihydrogen included. “The current measurement represents the culmination of many years of effort,” says Jeffrey Hangst, spokesperson of the ALPHA Collaboration. “We have been pursuing the precise determination of the hyperfine splitting of antihydrogen since we demonstrated how to trap antimatter atoms in 2010.”

Early evidence

Precision measurements of atomic hyperfine structures in the late 1940s provided early evidence for the electron’s anomalous magnetic moment, and, together with the discovery of the Lamb shift, contributed to the development of quantum electrodynamics. Today, the ground-state hyperfine splitting of hydrogen is known to better than one part in a trillion.

Measuring antihydrogen, however, is much more challenging. To do it, the ALPHA collaboration uses microwaves to drive spin-flip transitions in trapped antihydrogen atoms. Within the magnetic field of the trap, the ground-state hyperfine structure resolves into four sublevels, separated in energy by the Zeeman effect. Antihydrogen atoms in the two high-field-seeking states are ejected from the trap and annihilate, while those in the two low-field-seeking ones stay confined. Researchers determine the two transition frequencies by scanning the microwave frequency and observing when trapped atoms flip, leave the trap and annihilate. The splitting then follows from the difference between the two, through the Breit–Rabi formula.

We have made strong progress in the control, stabilisation and characterisation of the magnetic fields in the antihydrogen trap

The first demonstration of this approach came in 2012, and in 2017 the collaboration made the first quantitative measurement of the ground-state hyperfine splitting with a precision of 400 parts per million, in agreement with hydrogen. The precision was limited by the amount of antihydrogen available. “We have made strong progress in the control, stabilisation and characterisation of the magnetic fields in the antihydrogen trap,” explains Hangst. “Our new accumulation technique has also been a real game changer.”

The technique uses laser-cooled beryllium ions to cool positrons before mixing them with antiprotons, raising the trapping rate eightfold. The data behind the 2017 result came from 22 runs of 11 to 14 trapped atoms. The new measurement comprised instead 16 runs, eight at each of two magnetic-field settings, each accumulating roughly 1500 antihydrogen atoms. This improved the precision on the ground-state hyperfine splitting by two orders of magnitude, finding it consistent with the hydrogen value.

ALPHA now plans to study the nuclear magnetic resonance transition between the two trapped levels. This measurement is far less prone to systematics from the magnetic trap, and is expected to improve on the present result by a further two orders of magnitude. In the same Antimatter Factory, the ASACUSA collaboration is developing a complementary approach to measure the hyperfine splitting with a beam of antihydrogen in a field-free region.

A hot start for cluster formation

**Ahead of schedule** The protocluster JADES-ID1, seen about one billion years after the Big Bang. The Chandra X-rays (blue), overlaid on a JWST image, trace hot gas that marks it as the earliest bound system of its kind yet found. The circles locate JADES-ID1’s known galaxies. Credit: X-ray, NASA/CXC/CfA/Á Bogdán; infrared, NASA/ESA/CSA/STScI; processing, NASA/CXC/SAO/P Edmonds and L Frattare

Galaxy clusters are the largest bound objects in the universe, and among the most spectacular astrophysical systems. Their gravity lenses the light of background sources and heats the intracluster gas to X-ray-emitting temperatures. Within them – or in their smaller counterparts, galaxy groups – resides a large fraction of today’s galaxies. Their dense environments also drive galaxy evolution, with the densest clusters hosting the most evolved systems in the local universe.

By studying clusters at different epochs, we can track how galaxies and their environments changed over time. They are also signposts of large-scale structure, and their number counts constrain cosmological models, including the nature and amount of dark matter, and the behaviour of dark energy. The South Pole Telescope, for example, searches for massive high-redshift clusters via the Sunyaev–Zel’dovich effect, in which hot electrons in the intracluster medium scatter cosmic microwave background (CMB) photons to higher energies, distorting their spectrum.

When deep optical and infrared imaging surveys began in the 1990s, it became clear that massive galaxy clusters are rare in the early universe – if they exist at all. In the first few billion years after the Big Bang, we find galaxies associated with one another spatially, but generally not in bound systems approaching the 10¹⁵solar masses of the largest present-day clusters. This reflects the growth of structure predicted by the cold dark matter (CDM) model, which reproduces the density fluctuations seen in the CMB and the clustering of galaxies on large scales, but is harder to test against the properties of galaxies themselves.

When and how?

And yet, a major question remains open: when and how did the first galaxy clusters and groups form? An answer could potentially come from the James Webb Space Telescope (JWST), which measures the three-dimensional positions of astrophysical objects back to when the universe was less than a billion years old. Galaxies in the same region of sky that share similar radial velocities, and thus distances, are likely physically associated, and may evolve over time into a massive present-day cluster.

Such a system was recently identified in the GOODS-S field, as part of the JWST Advanced Deep Extragalactic Survey (JADES), a guaranteed-time programme of two of JWST’s instrument teams. A recent analysis of galaxy positions and distances in this field by our group revealed significant overdensities as far back as when the universe was about a gigayear old. But proximity in the sky alone does not guarantee a bound structure: such systems could still be assembling, rather than already being mature and gravitationally bound. One way to tell is to look for X-ray emission from intracluster gas shocked to high temperatures as the system collapses. At these early times, however, such emission is expected to be extremely faint.

JADES–ID1 is an exceptionally rare but plausible example of early structure formation

Our team discovered an X-ray-emitting protocluster, JADES-ID1, at redshift z ≈ 5.68. Found just one billion years after the Big Bang, it is the earliest protocluster known to host a hot intracluster medium, a stage of cluster formation not previously seen before the universe was three billion years old. Measured in the deepest X-ray observations ever taken by the Chandra observatory, it has a total gravitational mass of about 20 trillion solar masses – far too large for a single galaxy. Instead, it must comprise many galaxies within a shared gravitational potential that has existed long enough to heat the intracluster gas, allowing it to emit X-rays.

This discovery may have important implications for both galaxy evolution and cosmology. The presence of an X-ray-emitting intracluster medium indicates that substantial gravitational collapse, shock heating and mass assembly have already occurred, whereas most cosmological models predict that protoclusters should have been at much earlier stages. JADES-ID1 appears as a mature, bound group significantly earlier than when models predict such systems should be abundant.

Early efficiency

The patch of sky in which JADES-ID1 was discovered is exceptional, with the deepest multiwavelength coverage available, X-rays included. Finding such an object in a small patch of sky suggests that it may not be a statistical fluke: standard predictions would expect the most massive bound systems in the surveyed volume to be roughly an order of magnitude lighter than JADES-ID1. If comparable systems are discovered in other regions, it may imply that current models of early universe structure and galaxy formation underestimate the efficiency of early structure growth. Caution is in order, however, as a single detection is not enough to demonstrate a failure of Lambda–CDM cosmology.

Still, JADES-ID1 adds to a growing set of JWST observations pointing to faster early structure formation (CERN Courier November/December 2025 p11). For now, the conservative reading is that JADES–ID1 is an exceptionally rare but plausible example of early structure formation. If upcoming JWST, X-ray and Sunyaev-Zel’dovich surveys reveal a substantial population of similarly massive systems, this may eventually warrant a revision of galaxy and cluster-formation models.

We have now been awarded JWST spectroscopy time to study the system in more detail, which may give us more clues to its origin as well as how an early structure overdensity drives the formation of its host galaxies.

Regrouping for success

As president of the CERN Council, I have followed the updating of the European Strategy for Particle Physics (ESPP) first hand. The proposals for CERN’s future flagship project – the central goal of the ESPP 2026 update – were set out in the open, and physicists fought for their ideas with passion, as they should when the stakes are this high. Such conviction is the sign of a healthy field, and I am grateful to all who took part. Eventually, the community reached an overwhelming consensus on the electron-positron Future Circular Collider (FCC-ee), distilled in the recommendation of the European Strategy Group (ESG). When the Council met in Budapest this May, I was delighted to see it vote unanimously to update the ESPP accordingly.

The road ahead

The path to that vote is well known. The Council launched this bottom-up process in 2024, instituting the ESG, and the particle-physics community took it up with enthusiasm. Hundreds gathered to air and debate the proposals at the open symposium in Venice in June 2025 (CERN Courier September/October 2025 p24), before the recommendations were drawn together at the December drafting session in Ascona (CERN Courier January/February 2026 p7).

The ESPP’s medium-term recommendation is to complete the high-luminosity LHC upgrade and fully exploit the scientific potential of the HL-LHC. Looking further ahead, it recommends the FCC-ee as the preferred choice for CERN’s next flagship collider, aiming to maintain Europe’s leading role in particle physics. Given the clear scientific and strategic superiority of the FCC-ee relative to all other potential options, a descoped FCC-ee is recommended as the preferred option if the full machine is not considered to be feasible.

The Council invited the CERN management to use the ESPP update as input for its medium-term plan and for the ongoing studies of the FCC-ee as the possible next flagship at CERN. As a sign of its strong support, the Council further asked management to initiate discussions with the relevant authorities and entities in the Member and Associate Member States, as well as non-Member States and the European Union. The goal is to obtain pledges of additional cash and/or in-kind contributions to the cost of constructing and operating the FCC-ee, and, on this basis, to develop a financially feasible funding plan.

Securing those funds is now central, and the current CERN management continues an effort begun under its predecessor. The draft Multiannual Financial Framework (MFF) released by the European Commission in July 2025 foresees €3 billion towards the FCC, and pledges amounting to €860 million have already been secured by private donors in 2025 (CERN Courier January/February 2026 p9).

Certain Member States requested specific information to approve the FCC project through their finance agency system, and management has been instructed to provide it. It then promised to do an exercise on paper, which will result in an option analysis to assist these Member States in their approval procedures for the FCC. A large part of this information exists already in the ESPP documents. However, the Council did not support the development of alternative options in parallel with the FCC reference design.

In June, the Council approved the management’s plans and the milestones leading to a reference design for the FCC. The strategic question is now answered. From here, the task is one of carrying the project to approval, and the Council stands fully behind that effort.

The debate leading here was substantial, but it was conducted on the merits of the scientific case, and that is the common ground from which the community can now move forward. The time has come for us to unite behind the choice that has been made, see it through to success in 2028, support the management and contribute to CERN’s standing as the leading laboratory in particle physics.

**Costas Fountas** is president of the CERN Council. Credit: M Brice, CERN

A light on partons at the nuclear edge

ATLAS figure 1 — **Fig. 1.** The ratio of peripheral to inclusive photonuclear jet production cross-sections as a function of x₊, a proxy for the Bjorken-x. Theoretical predictions corresponding to two models in which PDF modifications are limited to inclusive collisions (red, blue) are shown, as well as the case of identical PDF modifications in both classes of collisions (green). Credit: ATLAS Collab 2026

When nucleons are bound within nuclei, their parton distribution functions (PDFs) differ from when they are free. The origin of these modifications, first discovered by the European Muon Collaboration in 1982 (CERN Courier November 1982 p362), remains one of the central open questions in nuclear physics, since the MeV-scale energies that bind nucleons together should have little bearing on the GeV-scale processes that probe their quarks and gluons. Results presented by the ATLAS collaboration at the Deep Inelastic Scattering (DIS) conference now provide the first observation that nucleons near the edge of a nucleus exhibit different PDFs from those near its centre.

The measurement relies on ultra-peripheral collisions (UPCs) of lead ions at the LHC, where – unlike a typical head-on collision – the ions glance past each other without their nucleons overlapping. They still interact, however, through the intense electromagnetic fields that surround each ion. Photonuclear UPCs occur when one nucleus emits a high-energy photon that strikes the other. One possible outcome is the production of jets through hard scattering between the photon and the nucleus, whose cross-section is sensitive to the parton distribution.

Such hard scatterings usually break the nucleus up, producing forward neutrons that are detected in the ATLAS zero-degree calorimeters located 140 metres from the interaction point. The key insight of this measurement is that, in about 4% of photonuclear scatterings, the photon strikes a single nucleon and leaves the rest of the nucleus whole. These events can be tagged by the absence of neutrons in the direction of the struck nucleus. Since nucleon densities are lower near the edge of the nucleus, collisions that leave it intact are also typically the more peripheral ones.

To study these collisions, the collaboration analysed lead–lead collision data collected in 2018 during Run 2 of the LHC, corresponding to an integrated luminosity of 1.72 nb^–1. Events were selected by requiring at least two jets in the calorimeter with large rapidity gaps – regions of the detector devoid of particle production, characteristic of these photon-initiated processes. These were then categorised as “inclusive” or “peripheral” according to the presence or absence of forward neutrons, respectively.

To interpret the results, proxy variables for the parton kinematics were defined using the properties of the measured jets. One such variable is x₊, which provides an estimate for the Bjorken-x of the struck parton, the fraction of the nucleon’s momentum that it carries. The measured ratio of the x₊ distributions between inclusive and peripheral collisions has a clear slope (see figure 1), indicating a significant difference in the nuclear modification of the parton distributions between the two classes. The size of this effect is quantified through a statistical test of the difference between them, which reaches a significance of 6σ.

These results provide the first observation that nucleons near the edge of a nucleus have different PDFs from those near the centre, and mark a significant step forward in understanding the structure of nuclei. The analysis also has important implications for the interpretation of relativistic heavy-ion collisions, which are sensitive to the PDFs of the colliding ions, and demonstrates a novel methodology that could be applied at the future Electron-Ion Collider (EIC). While this analysis uses Run 2 data, the larger Run 3 lead–lead dataset and the HL-LHC programme will offer substantial opportunities to improve both the precision and scope of these measurements.

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