From 18 to 22 May, the 14th Large Hadron Collider Physics conference (LHCP 2026) brought 424 physicists to Sorbonne University’s Pierre and Marie Curie campus, in Paris. Over five days of plenary talks, parallel sessions and poster presentations, the ALICE, ATLAS, CMS and LHCb collaborations, and the theory community, presented a flurry of new results, alongside updates on detector upgrades for the High-Luminosity LHC (HL-LHC) phase. The picture that emerged was of a Standard Model (SM) holding up under ever-sharper scrutiny, with experimental uncertainties increasingly rivalling those from theory.
Direct implications
When ATLAS and CMS discovered the Higgs boson, in 2012, they did so with only about 10 fb–1 of data. Today, the field they opened is entering the era of precision measurements. The Higgs production rate measured across all channels after Run 2 of the LHC agrees with the SM prediction within 5%, and theory is now the dominant source of uncertainty. A preliminary combination of the four most precise ATLAS and CMS measurements using Run 1 and Run 2 data yields mH = 125.09 ± 0.07 GeV, a precision of 0.6‰ that places the Higgs mass among the best-known parameters of the SM, with direct implications for the stability of the electroweak vacuum.
Precision measurements of fundamental SM parameters continue to advance on multiple fronts. CMS and LHCb measured the W- and Z-boson masses to be 80360.2 ± 9.9 MeV and 91184.2 ± 9.3 MeV, respectively. Together with the ATLAS + CMS combined top-quark mass of 172.52 ± 0.33 GeV and the strong coupling αs (see How strong is the strong interaction?), determined by a wide range of experiments and methods, these results provide essential inputs to global electroweak fits. Vector-boson scattering, first observed during Run 2, has now also been established in Run-3 data in the WW, WZ and ZZ channels, and ATLAS and CMS presented first evidence for ZZγ production, the smallest cross-section yet measured at the LHC.
The picture that emerged was of a Standard Model holding up under ever-sharper scrutiny
The Higgs-boson mass tells us how sharply the Higgs potential curves at its minimum, but not its full shape. The self-coupling λ, which provides a first look beyond the minimum, bears on whether the electroweak vacuum is truly stable, and therefore on the ultimate fate of our universe (CERN Courier January/February 2026 p31). Directly accessing λ requires measuring Higgs pair production, a process roughly a thousand times rarer than single Higgs production and one of the most ambitious goals of the LHC and HL-LHC programme. Combining their Run-2 searches, ATLAS and CMS constrained κλ, the measured self-coupling divided by its SM value, to between –0.71 and 6.1 at 95% confidence level (CL). Newly presented HH analyses using Run-3 data in the HH → bbbb, HH → bbγγ and HH → bbWW channels significantly improved on the sensitivity of the Run-2 measurements, even with smaller datasets, thanks to refined analysis techniques. ATLAS also presented the first search for ttHH production, excluding cross-sections above 20 times the SM prediction at 95% CL.
Sharpening up
In the flavour sector, LHCb and CMS presented new precision results on the Cabibbo–Kobayashi–Maskawa matrix angles γ and β, with CMS measuring time-dependent CP violation in neutral B decays using the full Run-3 statistics. These measurements provide increasingly stringent tests of the unitarity triangle and sharpen the sensitivity to potential new-physics contributions.
Quantum chromodynamics (QCD) governs both the dominant and some of the least understood processes at the LHC. New results on the dead-cone effect, the suppression of gluon radiation close to the flight direction of a massive quark, confirm simulation predictions and reveal large differences between b and light quarks. The catalogue of bound states keeps growing as well, with new exotic ones from LHCb bringing the number of hadrons discovered at the LHC to 82 and toponium candidates coming from ATLAS and CMS (CERN Courier May/June 2026 p8). The first oxygen–oxygen and neon–neon collisions at the LHC, delivered in 2025, offered several different views of the quark–gluon plasma, from collective flow to nuclear geometry (CERN Courier November/December 2025 p8). Among them, the rates at which J/ψ and Υ mesons melt in the medium and re-form from it differ distinctly from those seen in lead–lead.
The most interesting years may still lie ahead
Some 90 theorists attended, with many young researchers presenting new ideas and precision calculations reaching next-to-next-to- and next-to-next-to-next-to-leading order accuracy for key benchmark processes. The SM effective-field-theory framework has become a central tool for interpreting precision data and probing new physics indirectly, while lattice QCD has matured into an indispensable tool for controlling flavour observables. Achieving consistent precision across perturbative calculations, parton distribution functions, parton showers and hadronisation remains a major challenge, and reliable uncertainty estimates are now as important as the higher perturbative orders themselves.
An extensive beyond-the-SM programme covered supersymmetry, dark matter, exotic Higgs decays and rare processes, with much emphasis on searches for long-lived particles. No discovery has emerged, but exclusion limits now reach multi-TeV scales, driven by new triggers, dedicated reconstruction and dramatic machine-learning improvements deployed at every stage of the analysis chain.
Bright lights
The LHC has delivered luminosity at an extraordinary rate. With more than 1 ab–1 of collisions provided to ATLAS and CMS, half of which in the last two and a half years, the field has firmly entered the attobarn era. Improved analysis techniques are now providing sensitivity gains that rival integrated luminosity itself, through better efficiencies and signal-to-background discrimination. The most interesting years may still lie ahead, with progress being driven by the ingenuity and creativity of the young community.
The 21st International Conference on B-Physics at Frontier Machines, Beauty 2026, took place from 1 to 5 June in the charming setting of St. Jan’s Church in Maastricht, the Netherlands. Maastricht University and Nikhef, the Dutch National Institute for Subatomic Physics, welcomed around 100 participants from all over the world for a programme ranging from CP violation and rare decays to spectroscopy, the production of heavy-flavoured hadrons and the prospects for quark-flavour studies at the High-Luminosity LHC (HL-LHC), Belle II, the Super Tau Charm Facility (STCF) and the Future Circular Collider (FCC).
Powerful probes
Heavy-flavour decays, in particular those of hadrons containing b quarks, offer powerful probes for testing the Standard Model (SM) and searching for new physics at the high-precision frontier. Reviewing the latest results in this field and discussing its future directions has been the central goal of the Beauty conference series since its first edition, held in Prague in 1993. This year, the LHC experiments showed an impressively broad range of new results.
LHCb reported a new study of the highly suppressed flavour-changing neutral-current decay B+→ π+μ+μ–, based on data from Runs 1 and 2 of the LHC, as well as early results from Run 3. Notably, the other LHC experiments are also contributing a growing breadth of flavour results. For example, CMS presented its first study of CP violation in the “golden mode” B0 → J/ψKS, which is sensitive to the unitarity-triangle angle β. To control hadronic uncertainties in the interpretation of this result, the analysis also determined CP-violating observables in the sister mode B0s → J/ψKS to record precision.
The collaborations also reported on their achievements in hadron spectroscopy, including through the observation by ATLAS of a low-lying excited Bc meson (see Hyperfine splitting in the Bc system). The conference hosted the exciting announcement of LHCb’s discovery of the doubly charmed baryon Ω+cc (CERN Courier July/August 2026 p12), a result that showcases the improved capabilities of the upgraded Run-3 detector. This particle completes the family of doubly charmed ground-state baryons, extensively studied by theorists since the 1970s. Interesting in themselves, these and other multi-charmed baryons may also serve as important probes of the quark–gluon plasma at ALICE and other experiments in the HL-LHC era. Beyond the LHC, participants heard about the impressive studies of lower-mass hadrons possible at BESIII in Beijing, including the X(2370), a tantalising candidate for the elusive glueball (see A case for pure glue).
The LHC experiments showed an impressively broad range of new results
The event featured more talks from Belle II than any previous edition, reflecting the steadily improving performance of the SuperKEKB accelerator, which has delivered a dataset at the Υ(4S) resonance that now exceeds that collected by the earlier Belle experiment. One of the most striking results was a measurement of the time-dependent CP-violating asymmetry in B0 → π0π0 decays, exploiting a novel technique that circumvents the need to reconstruct the decay vertex of the signal B meson and exploits the quantum-entanglement of the two beauty mesons produced in the Υ(4S) decay.
First observed in 1947, kaons were fundamental in establishing flavour physics as a discipline, and remain of high interest today. A report was given of the ongoing measurement programme of the super-rare decay K+ → π+νν at CERN’s NA62 (CERN Courier May/June 2026 p9), and on the prospects for observing the even more suppressed neutral-kaon analogue KL → π0νν at the KOTO experiment in Japan, predicted to occur only three times every 100 billion decays.
A flavour of the future
The final day of the conference turned to the future, laying out how the upgrades of the LHC experiments for Run 4 and beyond will enable substantial increases in sensitivity. Complementary studies, targeting decays with neutrals and missing energy, will take place at Belle II, while the STCF in China, if realised, would extend them to lower energies. On a longer timescale, the FCC, proposed to begin operation at CERN in the 2040s, would open up rich flavour opportunities, in particular with a vast sample of b hadrons produced at the Z resonance.
A key aspect of the event was the close interaction between theorists and experimentalists, fostered by 14 invited theory talks ranging from the global flavour-physics landscape to lattice QCD and models with Z′ bosons and invisible particles. Furthermore, the next generation took central stage through new sessions dedicated to early-career researchers, including lightning talks and posters presented by Maastricht University BSc students. The week left no doubt that flavour physics remains a vibrant field and a key player in the search for physics beyond the SM.
The 16th workshop of the Long-Lived Particle Community (LLP2026) gathered more than 70 theorists, phenomenologists and experimentalists at Cambridge’s Ray Dolby Centre, the new home of the Cavendish Laboratory, from 29 June to 3 July. Discussions centred on particles that live long enough to travel a measurable distance before decaying, and on the searches, algorithms and detectors designed to catch them.
Long-lived particles (LLPs) arise in a wide range of theories beyond the Standard Model (SM), from supersymmetry and extended Higgs sectors to models of neutrino masses or dark matter. Due to their long lifetimes, LLPs could decay far from the collision point and leave signatures that are exotic and hard to identify.
On the hunt
The CMS collaboration presented eight new searches in which displaced muons and dedicated LLP triggers played a central role. Anna Mascellani (ETH Zurich) presented a search for heavy neutral leptons (HNLs). Among the simplest and best-motivated LLP candidates, they could explain the origin of neutrino masses through the so-called “seesaw” mechanism. The new channel targets HNL decays beyond the tracker using standalone displaced muons, reconstructed exclusively in the muon system. The gain in sensitivity at longer lifetimes yields the best expected limits on HNL mixing with muon neutrinos for masses between 2.5 and 4.2 GeV. Celia Fernandez Madrazo (Boston University) presented a search for light, narrow resonances decaying to two displaced muons, recorded with a dedicated scouting trigger stream, in which events are kept at a high rate by recording only a reduced amount of trigger-level information. The saving in bandwidth allows muons with transverse momenta down to 3 GeV to be collected, and relaxed requirements on the number of hits give Run-3 scouting access – for the first time – to muons displaced by more than 11 cm. Together, the two extensions yield the most stringent limits to date on dark-shower models, which predict light, long-lived dark pions decaying after 10 cm or more.
The workshop showed how new triggers and analysis strategies are testing a wide range of theoretical models
ATLAS presented several results with 13 and 13.6 TeV data. Paul Swallow (University of Cambridge) reported a search for displaced vertices and missing transverse momentum built on a novel fuzzy-vertexing algorithm, optimised for displaced decays to heavy quarks. Since b quarks themselves travel a sizeable distance before decaying, an LLP decaying to them need not produce particles pointing back to a single vertex. Rather than forcing one, the algorithm reconstructs several nearby seed vertices and then merges them into a single one, improving efficiency for the high-mass, high-multiplicity displaced vertices expected from such decays.
Models predicting quirks, hypothetical particles charged both under the SM and a new confining force analogous to the strong force, drew particular interest. Produced in pairs and bound to each other by a confining string, quirks would cross a detector slowly, as charged long-lived particles. Arash Jofrehei (University of Geneva) presented the first direct LHC search for quirks with masses above the weak scale, performed with FASER, and Alejandro Novo Cal (IGFAE, University of Santiago de Compostela) proposed a search at LHCb, complementary in reach to those of heavy stable charged particles or monojets at ATLAS and CMS. Thanks to the forward acceptance and lack of magnetic field of the LHCb Vertex Locator (VELO), alongside a flexible trigger design, the approach could probe values of the new confining scale between 0.1 and 10 keV. The stakes reach into cosmology, as Graham Kribs (CERN and University of Oregon) stressed. If macroscopic dark-colour strings were observed, the reheating temperature at which the hot Big Bang began after inflation would have to lie below about 100 GeV, since a hotter start would have left long-lived bound states of dark gluons in conflict with early-universe observations.
Martin Hirsch (IFIC, CSIC and University of Valencia) showed that axion-like particles (ALPs) can couple HNLs to gluons, giving the LHC and proposed LLP detectors high sensitivity to long-lived HNLs for GeV-scale ALP masses. As Oleg Brandt (University of Cambridge) observed, the two candidates are usually treated separately, and their combination offers interesting new benchmarks.
Future facilities
On the facilities side, John Anders (University of Liverpool) reviewed design and detector studies for the proposed Forward Physics Facility, including a full simulation of its cavern. Updates followed from SHiP/NA67, CODEX-b, MATHUSLA and ANUBIS, with sensitivity studies for benchmark models ranging from HNLs and Higgs-portal scalars to dark photons and ALPs. Looking further ahead, Rhitaja Sengupta (BCTP, University of Bonn) presented dedicated LLP detector concepts for FCC-ee and FCC-hh, including DELIGHT, FOREHUNT and DELIGHT-SHIELD, stressing how a future collider’s design phase offers the best opportunity to integrate dedicated LLP detectors within the main detector designs.
Discussions extended beyond CERN. Zeren Simon Wang (Hefei University of Technology) presented prospects for searching for very long-lived HNLs through baryon-number-violating decays of charmed baryons at the Super Tau Charm Facility proposed in Hefei, China. The same symmetry violation motivated Patrick Adolf (TU Dortmund) to propose a new class of nucleon-decay observables involving LLPs, in which a vector mediator produced in the decay travels a macroscopic distance before turning into an electron–positron pair inside a detector such as Super-Kamiokande, leaving temporally correlated “echo” vertices.
LLP2026 opened just as the LHC entered Long Shutdown 3, and the results presented drew on only part of the Run-3 dataset. Even so, the workshop showed how new triggers and analysis strategies are testing a wide range of theoretical models. The detectors and facilities now taking shape promise to go further, so that no new physics remains hidden at the long-lifetime frontier.
From 8 to 12 June, FCC Week brought 656 scientists, engineers, industry representatives and policymakers from 38 countries to the University of Helsinki. With its primary focus on FCC-ee, the meeting examined how the FCC study can progress from demonstrating feasibility to a coherent reference design, treating physics goals, accelerator parameters, detector concepts, theory, software and the machine–detector interface as parts of a single system.
Paula Eerola, president of the Research Council of Finland, opened the week by reflecting on CERN as a place where scientific responsibility is carried across borders and generations. CERN Director-General Mark Thomson described the coming years as a critical period, in which CERN must deliver the High-Luminosity LHC while advancing the FCC-ee as a possible next flagship project (CERN Courier July/August 2026 p7). Both efforts will rely on CERN’s close cooperation with its Member States, and Costas Fountas, president of the CERN Council, underlined their growing alignment with the laboratory and the particle-physics community. For European Commissioner Valdis Dombrovskis, the same model of international cooperation underpins Europe’s competitiveness, its technological capacity and its ability to invest over the long term.
Broad programme
The scientific case for the FCC-ee is often summarised in terms of the Higgs boson, but the programme discussed in Helsinki was considerably broader. Runs at the Z resonance, the W-pair threshold, the Higgs-production maximum and the top-quark threshold would yield around six trillion Z bosons and hundreds of millions of W pairs, improving statistical precision by three orders of magnitude and probing the Standard Model as an interconnected whole. The top-threshold run, for example, would determine the top mass, width, Yukawa coupling and strong coupling in a uniquely controlled environment, and a precise top mass is needed in turn to exploit the projected W-mass precision. The FCC-ee would also be a major flavour facility, with its clean initial state, vertex reconstruction and near-complete angular coverage, enabling studies of beauty, charm and tau decays, many of which are statistically limited today. As a precision-QCD machine, it could determine the strong coupling at the per-mille level (see How strong is the strong interaction?).
Discovery potential would not depend solely on producing new particles. Heavy states can alter measured quantities through quantum effects, leaving correlated deviations across electroweak, Higgs, flavour and top observables that global analyses could trace well beyond the machine’s direct reach. Direct searches would complement this programme, with the enormous Z and Higgs samples providing sensitivity to long-lived particles, heavy neutral leptons, axion-like particles and other hidden-sector states. The projected precision also creates a theoretical challenge. For the W-boson mass, a possible experimental uncertainty of about 0.24 MeV stands against a current theoretical uncertainty of several MeV, and closing such gaps will require higher-order calculations, better event generators and improved control of hadronisation, in a coordinated effort lasting many years.
On the accelerator side, the main news concerned the radiofrequency system. Studies indicate that it could be built entirely from 800 MHz superconducting cavities, instead of the planned mix of 400 and 800 MHz, and still reach the target collision rate with the new beam optics, although beam lifetime and the mutual disturbance of the colliding beams remain to be assessed. That collision rate calls for beams that are exceptionally flat, their vertical spread up to 2000 times smaller than the horizontal, demanding tight control of alignment, vibration and focusing. The rings would also need continuous refilling to hold the luminosity steady, a top-up injection that simulations now show can be performedwith efficiencies above 90%.
Advancing hardware
Hardware is advancing too. Drawings for the 400 MHz superconducting cavities are nearly complete, and a first integrated cryomodule demonstrator is due by 2031. The cavities will only be as efficient as the klystrons that power them, and these are gaining ground, with a single-beam device in China reaching 78% and a CERN tristron targeting above 90%. Magnet studies now feed manufacturing imperfections into predictions of field quality, and alignment on the supporting girders aims at 10 to 20 micrometres. The synchrotron radiation emitted by the circulating electrons, an intense X-ray glare, would deposit its power along the vacuum chamber, with absorbers every 5 to 6 m to intercept it. At the Z pole, however, where the circulating current would be highest, the budget for the chamber’s electromagnetic response to the beam remains tight, and the effects of stray-electron clouds in the pipe are not yet fully resolved.
Six detector concepts are under study, providing redundancy and comparative optimisation
The interaction region is where accelerator and detector choices meet most directly. A longitudinal detector-opening scenario is now the baseline, and a full-scale mock-up of the cooled central beam pipe and vertex detector has validated assembly and cooling. Moving the first final-focus element to 2.4 m from the interaction point has reduced the radiation load on the superconducting coils. Synchrotron-radiation backgrounds, however, have emerged as one of the most serious potential problems. Current estimates indicate that unless the radiation can be reduced by orders of magnitude, it would leave unacceptably high hit rates in the trackers. Masks, revised beam-pipe geometry, shielding and optics changes all show promise, but a final solution is still to be defined, making the interaction region a defining test of the integrated design.
Detailed simulation
Six detector concepts are under study, providing redundancy and comparative optimisation. All concepts place silicon pixel sensors closest to the beam, in chips that combine sensor and readout. Ideas diverge further out. Calorimeter concepts range from silicon-tungsten and crystal systems to noble-liquid and dual-readout approaches, while trackers include full-silicon, drift-chamber and time-projection-chamber solutions. Comparing the concepts, and finding their weaknesses early, takes detailed simulation, and simulation in turn calls for common tools. The shared software platform Key4hep has matured, and a distributed computing model is already operating ahead of formal collaborations.
The Industry and Technology Day, opened by Finland’s minister of economic affairs Sakari Puisto, asked how companies can become involved early enough to shape the design, from superconducting RF and cryogenics to civil construction. Many technologies will need to be adapted, scaled up or made more efficient, while others will have to be co-developed.
The FCC-ee tunnel could later host a much higher-energy proton collider, the FCC-hh, which Helsinki discussed as a possible second stage rather than as the justification for the whole programme. The case for the FCC-ee rests on its own precision measurements, flavour physics and direct searches, while the FCC-hh would extend direct searches to masses of tens of TeV and improve the measurement of the Higgs self-coupling.
FCC Week 2026 showed substantial progress in all project domains, and equally how much remains to be done. Synchrotron-radiation backgrounds must fall dramatically, theory precision must catch up with experimental ambition, and industrial capacity and long-term expertise need sustained investment. The central outcome was the growing integration of the many strands of a credible international project.
Memories from Paris to Stanford is a deeply personal account of the fascinating and varied life of Gregory Loew. A renowned accelerator physicist, Loew made seminal contributions to the accelerators at the Stanford Linear Accelerator Center (SLAC) that underpinned its Nobel Prize-winning scientific programme. Among them were the klystron drive and phasing systems for the 20 GeV linac, the S-band radiofrequency (RF) particle separators for the initial linac-based experiments, and the electron–positron injection system for the B-factory. He also directed the three-year effort that cured the beam breakup plaguing early operation, and through the 1970s, as head of the Accelerator Physics Department, led all the linac improvement projects. Later, Loew went on to chair two large international committees, whose comparisons of the competing linear-collider proposals paved the way for the 2004 selection of the superconducting technology of the DESY-led TESLA collaboration.
Core values
The memoir is divided into three parts: his early life and education, his career at SLAC, and his later engagement in politics. The diverse cultural environments and upheavals of his pre-college years recur throughout, intertwined with his science and activism, and matched by the value he places on family, friendship and collegiality.
The early chapters follow a childhood scattered by war. Loew was born in Vienna in 1930 to Jewish parents, with roots in Prague and Hamburg. In 1932, the family moved from Bucharest to Paris and switched their home tongue from German to French. In 1939, with antisemitism rife and war declared, they were forced out again, reaching Buenos Aires on 1 January 1940 – two weeks after the German battleship Admiral Graf Spee was scuttled off Montevideo. Somewhat screened in Argentina from the ravages of the war, the family nonetheless followed its perils, finding immense relief in the liberation of Paris in August 1944. A three-month visit to New York at the end of the same year marked his first encounter with the United States of America. There, operas at the Met and concerts conducted by Toscanini and Stravinsky sparked a lifelong love of the performing arts.
Loew knew from an early age that he wanted to pursue a career in physics, a desire cemented by excellent maths and science teachers at his high school in Buenos Aires. He returned to Paris in 1948 for undergraduate studies at the Sorbonne, and his account of those four highly eventful years is captivating. After his licence-ès-sciences in mathematics, physics, chemistry and radioactivity, he chose Caltech for his PhD studies, where he enjoyed easy access to figures such as Richard Feynman, Robert Millikan, Fred Hoyle and James Watson. Gregarious by nature, Loew never quite took to campus life at Caltech and, on completing his master’s in electrical engineering, accepted a generous scholarship from Stanford University to do his PhD. So began an illustrious 54-year career at Stanford and SLAC, including about five years as deputy director from 2001 to 2005.
The heart of the book, and its strongest claim on a physicist’s attention, is Loew’s chronicle of the internationally competitive development of linear electron accelerators from its Stanford origins. It began with William Hansen in the 1930s and advanced when the Varian brothers, Sigurd and Russell, arrived in 1937. That year, they built the first working klystron, a technology that still powers RF cavities today (CERN Courier May/June 2025 p30). After the war, Hansen resumed the work with Ed Ginzton and Richard Neal. Hansen died in 1949, and in 1951 Wolfgang “Pief” Panofsky arrived from Berkeley and carried the programme forward. By the mid-1950s, Stanford had a machine capable of delivering electron beams up to 1 GeV, which Robert Hofstadter used for his Nobel Prize-winning study of the structure of protons and neutrons. Loew himself arrived in 1954 as an electrical-engineering graduate student. His thesis, fortuitous given his later role, involved a microwave amplifier and oscillator system much like those used in the 1 GeV machine.
The Monster
Panofsky had larger ambitions. In 1957, he led a Stanford group that audaciously proposed Project M, the Monster, a two-mile accelerator for electron beams of up to 20 GeV. Loew completed his PhD in 1958 and joined the project, taking on the critical task of designing copper microwave structures to accelerate the electrons. Project M was funded in 1961, with Panofsky as SLAC’s first director, and construction began in 1962.
SLAC’s scientific breakthroughs resulted from decade after decade of far-reaching technical invention driven by a group of brilliant, ambitious and colourful scientists. And who better to describe that history than Loew? From the start until his retirement in 2008, he contributed to the technical development, the unfolding science and the laboratory’s management. His astute people skills and benevolent character enliven his account and lend it veracity.
One further thread gives these chapters their texture: the laboratory’s role, largely driven by Panofsky, in building relationships and intergovernmental agreements with high-energy physics laboratories in Russia, Japan and China. Those friendships outlasted the science and at times served as back channels during political tension, and Loew’s insider account of those brave endeavours opens a window onto their workings.
Ambassador extraordinaire
As SLAC’s “ambassador extraordinaire”, Loew was often called on to host its most prominent visitors. He describes two, the visits of French president Georges Pompidou in 1970 and of the emperor and empress of Japan in 1994. He also devotes a chapter to his wife of 30 years, Gilda, who passed away in 2001. She was a PhD quantum chemist, with an adjunct professorship at Stanford, and worked with Nobel laureate Joshua Lederberg.
A fascinating blend of Stanford and SLAC’s groundbreaking accelerator-based science
The third part of the book steps outside SLAC. Loew had been alert to politics and the causes of war since boyhood, and a call from the Stanford administration prompted him, in 1970, to devise a one-quarter course on the origins of war. It proved popular enough to run for more than a decade. Its range, from international relations and history to economics, anthropology and biology, later fed his first book, The Human Condition: Reality, Science and History (CERN Courier March/April 2020 p68). Since 1995, he has been active in democratic politics, serving on local and national committees and as a delegate to the 2012 convention. He closes with chapters on the threats to democracy and the existential threat of global warming.
Memories from Paris to Stanford is a fascinating blend of Stanford and SLAC’s groundbreaking accelerator-based science from the early 1950s to the present and the socio-political currents that shaped that post-WWII period. The scientific reader will likely be captivated by the discovery potential unleashed by innovative technology in the hands of brilliant practitioners and by how world events played into that adventure. The non-scientific reader can equally appreciate the transformative nature of the technology, but for them the primary fascination may well be how dramatic world events shaped the multifaceted career of a brilliant scientist and humanitarian.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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).
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.
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.”
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 expectedto 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.
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 1015 solar 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.
To provide the best experiences, we use technologies like cookies to store and/or access device information. Consenting to these technologies will allow us to process data such as browsing behavior or unique IDs on this site. Not consenting or withdrawing consent, may adversely affect certain features and functions.
Functional
Always active
The technical storage or access is strictly necessary for the legitimate purpose of enabling the use of a specific service explicitly requested by the subscriber or user, or for the sole purpose of carrying out the transmission of a communication over an electronic communications network.
Preferences
The technical storage or access is necessary for the legitimate purpose of storing preferences that are not requested by the subscriber or user.
Statistics
The technical storage or access that is used exclusively for statistical purposes.The technical storage or access that is used exclusively for anonymous statistical purposes. Without a subpoena, voluntary compliance on the part of your Internet Service Provider, or additional records from a third party, information stored or retrieved for this purpose alone cannot usually be used to identify you.
Marketing
The technical storage or access is required to create user profiles to send advertising, or to track the user on a website or across several websites for similar marketing purposes.
