Roger Barlow passed away suddenly on 1 February 2026 at his home in Wales. Roger had an illustrious career in particle physics and, latterly, also in accelerator physics. He was well known internationally for his work in statistics, in particular for his widely used textbook, Statistics: A Guide to the Use of Statistical Methods in the Physical Sciences, published in 1989.
Roger was born on 14 April 1951 in Canterbury. After attending Edinburgh Academy, he obtained a place to study for his first degree at Oxford. He then went to Cambridge, where he completed his PhD in 1977 on proton–deuteron interactions at CERN’s 2 metre bubble chamber. Roger then took up a research post at Oxford, working on the TASSO experiment, and contributed to the discovery of the gluon in 1979.
In 1980, Roger was appointed to a lectureship at the University of Manchester and joined the JADE Collaboration, where his work on event reconstruction and Monte Carlo simulation led to one of the early measurements of the B-meson lifetime. During his early years at Manchester, Roger moved on to the OPAL experiment, becoming leader of the Manchester team in 1991. On OPAL, he helped design, build, commission and operate the muon chambers, which were crucial for many Standard Model physics studies, including precision measurements of the Z boson.
Roger became the overall leader of the Manchester particle-physics group in 2005, after the retirement of Robin Marshall. The group was then also involved in ATLAS, D0, several neutrino experiments and BaBar, which Roger had joined. Under his leadership, the particle-physics group grew to more than 100 members. As a collaborator on BaBar, he helped design the electromagnetic endcap, and he supervised the construction of half of the detector in Manchester. His data analyses included setting new limits on the existence of second-class weak currents in tau–lepton decays. As BaBar wound down, he took his group into LHCb.
In the early 2000s, Roger began researching accelerator science, forming an accelerators group in Manchester and becoming a founding member of the Cockcroft Institute of Accelerator Science and Technology. He was principal investigator for the CONFORM project that led to the successful operation of EMMA, the world’s first non-scaling FFAG accelerator. This provided a proof of principle for a new type of accelerator with many potential applications. In 2011, he left Manchester for a post at the University of Huddersfield, where he formed another accelerator-science group.
In addition to his textbook, Roger produced several influential works on statistics, including a description of extended maximum likelihood, a highly cited paper on fitting using finite Monte Carlo samples, and a detailed paper on the treatment of systematic uncertainties.
Roger was a dedicated and skilled teacher, who cared deeply about educating the next generations. Among his many contributions to the public understanding of science, he introduced the Particle Physics Masterclasses for high-school students, which quickly expanded across the UK, before becoming truly international. In recognition of this, he was awarded the Institute of Physics’ Lise Meitner Medal and Prize in 2022.
Roger retired from Huddersfield in 2017, but continued to work on BaBar and LHCb, and to publish papers and lecture on statistics, right up until his passing.
Outside of physics, Roger was active in UK national politics as a member of the Liberal Democrats. He was selected three times to stand as a candidate for the UK parliament. He will be greatly missed by his wife, Ann, his children Edward and Eleanor, his extended family, and his many friends and colleagues across the world.
Every new instrument needs its mysteries, and no discovery of the James Webb Space Telescope (JWST) has been more surprising than the “little red dots” it discovered in the early universe. Four years after their discovery, their nature is still an open question, with new papers purporting to solve the mystery on an almost daily basis.
These unexpected objects came into view in JWST’s first data release in 2022 thanks to its sharp images and sensitivity in the near infrared. By summer of 2023, a number of discovery papers had been written about them, identifying three traits in common: they were compact in size, had unusual “V-shaped” spectra and they showed emission from high-velocity hydrogen gas. Due to their compact size and red colour in the rest frame, they were dubbed little red dots. A few appeared in every pointing of the JWST imaging camera NIRCam, accounting for a few percent of all known galaxies in the first billion years of cosmic time. The race was on to determine their nature.
Two options initially appeared possible, but both were extraordinary and required a very precise tuning of parameters to fit the observations: too-dense galaxies or too-massive supermassive black holes. In either case, the objects had to be enshrouded in a cocoon of dust.
Galaxies or black holes?
The first paper assumed they were very massive galaxies, with their stars all assembled less than a billion years after the Big Bang. In favour of the galactic hypothesis were the V-shaped spectra, which are difficult to model without invoking massive stars. The vertex of the V-shape resembles a “Balmer break”, which is produced by the absorption of hydrogen atoms in the n = 2 level. Longward of the break, the optical continuum rises steeply toward the red, which this model attributed to the reddening of these stars by dust, with the UV being produced by starlight that was scattered out of the dust screen. However, the very high masses and early-universe star formation rates required for these models were difficult to reconcile with our understanding of the rate at which galaxies and their dark-matter halos assemble.
The first paper assumed they were very massive galaxies, with their stars all assembled less than a billion years after the Big Bang
The black-hole hypothesis was supported by evidence for very dense gas clouds moving at thousands of kilometres per second in the potential of a massive black hole. In this picture, surrounding dust would preferentially absorb ultraviolet light and re-emit it at longer wavelengths, producing the observed red colour. Though this explanation promised to alleviate the tension arising from the implied galaxy masses, it quickly became clear that these objects were not typical growing black holes. They were not detected in X-rays, nor did they show the characteristic 1000 K dust signature that is ubiquitous in actively accreting black holes. However, the most concerning piece of the black-hole interpretation was the implied black-hole masses. Applying local calibrations to the observed motion of gas in the little red dots implied black-hole masses of ten million to a billion suns, compared with galaxy masses of the same order – a stark contrast with local black holes, which have masses roughly a thousandth of their host galaxies. These overly massive black holes are hard to grow so far in advance of the galaxies, and also overproduce the total amount of black-hole mass created at such an early time.
Explaining their redness
Two major breakthroughs occurred in 2024 that clarified the nature of the little red dots. All the aforementioned models invoked heavy amounts of dust to suppress ultraviolet emission and produce the observed red colours. The conservation of energy implies that all the absorbed radiation should be re-emitted by the dust. However, multiple studies of populations and of luminous individual sources turned up non-detections of dust emission. These stringent limits on the far-infrared energy output were enough to conclusively rule out these entire classes of models, invoking reddening by dust to explain the observed red colours.
At the same time, campaigns to observe the broad population of little red dots discovered a remarkable class of sources with very little ultraviolet emission and extreme Balmer breaks. These breaks could not be produced by anything resembling a stellar population we have observed before, and served as conclusive evidence that normal stars cannot be responsible for producing the optical emission in little red dots; the photoabsorption by hydrogen in the n = 2 energy state must nevertheless be a crucial physical aspect of the little red dots, even if it wasn’t happening in the atmospheres of massive stars.
Plausible scenarios
The challenge is therefore to explain the characteristic red colour of the little red dots without dust obscuration. Any successful model would also need a substantial reservoir of hydrogen around to cause the hydrogen absorption that looked like starlight, but wasn’t. One plausible scenario that could satisfy these requirements is very dense gas arranged quasi-spherically around the black hole. In this scenario, the black holes powering the little red dots could be significantly less massive than we had originally thought, when we had assumed that dust was obscuring most of the light from the growing black hole.
The task is to explain the characteristic red colour of the little red dots without dust obscuration
In this new picture, the little red dots are powered by black holes that are accreting at much higher rates than are typically seen at later times. A higher accretion rate implies greater luminosity for a given black-hole mass, and therefore we infer much lower black-hole masses, perhaps closer to a million suns, and much more aligned with the measured galaxy masses. As a side benefit, lower black-hole masses are much more natural for objects that are so prevalent, because the number of low-mass dark-matter halos and low-mass galaxies is much higher than the number of high-mass systems.
Astronomers are still arguing about how this dense gas is configured and accretes onto the black hole, and everyone has their favourite model. We do not know if the geometry of the system is completely spherical, or if we are seeing a mixed-phase medium where the viewing angle is an important parameter. These details matter, because if we can pin down the characteristic size and density of these gas envelopes, we may be able to infer more robust black-hole masses for the population. There has been some recent speculation that the little red dots may be marking the end stages of black-hole seed growth, in which case they could be a critical missing link in our understanding of the formation of the first black holes. However, without more concrete constraints on black-hole mass, we cannot know for sure. At the same time, we need a much better theoretical understanding of what makes little red dots so distinct from the more typical growing black holes we have studied for decades, and why that mode of growth becomes so much less common as the universe ages.
One thing we do know for sure: the more we learn about the little red dots, the more complex and unexpected they become. We are excited to see what new wrinkles arise as we enter our fifth year of JWST operations.
Applying the Standard Model (SM) to early cosmological times leads to an uninhabitable universe, with tiny and equal amounts of matter and antimatter. Yet the universe is habitable and the local universe strongly matter-dominated. Observations of the diffuse gamma-ray background and cosmic microwave background show no evidence for the presence of antimatter on large scales and rule out a matter–antimatter symmetric universe.
From 19 to 22 January, 80 particle physicists, astronomers and cosmologists gathered at CERN for the first “All that Antimatters in the Universe” workshop to explore the frontier between the laboratory and astrophysical perspectives on the matter–antimatter asymmetry of the universe.
Broad panorama
Julia Harz (Mainz University) reviewed a broad panorama of baryogenesis models in which physics beyond the SM produces a homogeneous matter excess within the first seconds after the Big Bang, before light elements are synthesised. She highlighted their features and potential tests and constraints, including searches at colliders like the LHC and indirectly with experiments such as those looking for neutrinoless double-beta decays.
Questioning our assumptions about antimatter was a central thread of the workshop, with several presentations highlighting non-standard baryogenesis models that allow domains of antimatter to survive the Big Bang, as well as others in which antimatter is hidden in compact nuggets that could also constitute dark matter. A lively discussion explored how to hunt for these scenarios using astrophysical and cosmological observables. For example, spectral distortions of the cosmic microwave background could indicate energy injections from matter–antimatter annihilation in the early universe. Observations at 21 cm-wavelengths offer another probe: these signals trace neutral hydrogen during the cosmic-dawn epoch, when the first stars and galaxies formed, and could reveal anomalous heating or ionisation patterns characteristic of antimatter annihilation.
Questioning assumptions about antimatter was a central thread of the workshop
The discrete symmetries of charge conjugation (C), parity (P) and time reversal (T) have been central to particle physics since the discovery that nature violates them individually, yet their combined action (CPT) appears to be preserved in all standard interactions. In a particularly sharp presentation, Gabriela Barenboim (University of Valencia) stressed that while much attention is devoted to the search for differences in the interactions between particles and antiparticles through CP-symmetry violation, the more fundamental possibility of CPT violation remains largely unexplored. Unlike CP violation, which can occur within the Standard Model, any breakdown of CPT symmetry would signal new physics and could manifest as differences in the intrinsic properties of particles and antiparticles, including their masses and lifetimes.
Leading stress-tests of CPT symmetry are now carried out at CERN’s Antimatter Factory (AF), whose experiments presented an array of impressive results at the workshop. Eric Hunter (CERN) highlighted the potential of boosting the yield of antihydrogen formation at the AF experiments, showing how this could improve our knowledge of antimatter physics enormously. Improved yields of antimatter replicas of naturally occurring matter-based atoms would enable higher precision tests of key electromagnetic transitions and gravitational interactions of antimatter.
Much attention went to antimatter in cosmic rays. Primary cosmic rays are particles accelerated at astrophysical sources such as supernova remnants and injected into the galaxy, whereas secondary cosmic rays are produced when those primaries collide with gas and dust in the interstellar medium. In standard galactic cosmic-ray models, antimatter is purely a secondary product of the interactions of primary cosmic rays with the interstellar medium. However, the AMS-02 experiment operating on the International Space Station has firmly established a positron excess requiring a primary source, possibly pulsars. AMS-02 antiproton data also show some anomalies, but uncertainties in the propagation models and interaction cross-sections remain large.
Mind the GAPS
Complementary searches for cosmic-ray antimatter are also carried out by balloon-borne experiments. Principal investigator Chuck Hailey (Columbia University) described how the GAPS balloon experiment, uniquely suited to probe low-energy antiprotons, antideuterons and antihelium, reported its first data from a 25-day flight completed in early 2026. The specificity of GAPS is the exploitation of the characteristic X-ray emission produced by short-lived bound states between antimatter nuclei and ordinary atoms, which results in excellent particle-identification and background-rejection capabilities.
The atmosphere at the workshop was excellent, with participants curious to learn from other communities and expand their horizons everywhere that antimatter matters in the universe, from the cosmos to the lab, via astrophysical systems. While antimatter still holds many mysteries, All that Antimatters in the Universe brought us one step closer to answering them.
One hundred researchers gathered in Santiago de Compostela from 21 to 23 January for Iberian Strings, the annual meeting of the vibrant Spanish and Portuguese string theory community. From the idea that black holes may test quantum gravity to the new, string-inspired ways of organising quantum field theories using symmetries and defects, the programme offered a broad overview of where string theory and holography currently sit. What stood out was the extent to which very different problems are now being tackled with a shared set of theoretical tools.
Black holes remain a clean laboratory for probing ideas about quantum gravity. Decades of work have shown they behave much like ordinary thermodynamic systems, with quantities such as temperature and entropy. A central question is how this simple large-scale behaviour arises from an underlying quantum description. Vijay Balasubramanian (University of Pennsylvania) emphasised that the challenge is not only reproducing the familiar area law – which links entropy to the area of the event horizon – but also understanding what different semiclassical calculations are really describing.
Calculations under control
One way to address this problem is to count the quantum states that give rise to a black hole’s entropy. To make progress, researchers often focus on settings where calculations are under better control. Gabriel Cardoso (IST Lisbon) discussed BPS black holes, highly symmetric solutions that allow precise calculations using holography. Stefano Trezzi (University of Barcelona) showed that near-extremal black holes, systems close to a zero-temperature limit, exhibit a universal near-horizon behaviour that provides a clean setting to study how quantum effects modify the semiclassical picture.
So much for static black holes; what about their evolution in time? Marija Tomašević (CERN) suggested that quantum effects can form a horizon where classical gravity would predict a naked singularity. Pablo A Cano (University of Murcia) and Marina David (KU Leuven) explored instead how black holes react when they are perturbed, emitting gravitational waves as they settle back to equilibrium through a process known as ringdown. Across these contributions, the focus was on separating what can be understood within controlled semiclassical calculations from what requires genuinely microscopic, quantum-gravitational input.
Some particle theories may have been gravity all along. And vice versa. These seemingly disparate worlds, with particle beams and colour confinement in one (particle physics) and curved spacetime in the other (gravity), may simply be two languages for the same physics. To translate between them, the particle side must live in one fewer dimension. Just as a hologram stores a 3D image on a 2D plate, a gravitational theory in D dimensions may be exactly equivalent to a non-gravitational quantum field theory in D–1 dimensions. This holographic correspondence is central to modern approaches to quantum gravity. The focus at the workshop was on its more applied uses, as a controlled way to learn about dynamics at strong coupling.
Elias Kiritsis (University of Crete) provided a concrete example. Using familiar spacetime physics, he studied how strongly interacting quantum systems respond to gentle deformations at low temperature, a standard probe of transport. In this setting, quantum effects can modify quantities such as the ratio of viscosity to entropy density beyond the semiclassical value.
To round the picture, Francesco Nitti (APC Paris), explored holographic models in which varying the curvature of spacetime can affect confinement, while Shota Komatsu (CERN) presented an overview of matrix-model methods in holography, emphasising how they can provide tractable descriptions of strong-coupling dynamics in specific regimes, such as large-N limits. Following ’t Hooft, theorists often treat the number of colours in an SU(N) gauge theory as a tunable parameter, providing a controlled simplification of strongly coupled dynamics.
Black holes remain a clean laboratory for probing ideas about quantum gravity
Working in simplified settings can be an effective way to make progress. In holography, a quantum field theory in two dimensions can map to a three-dimensional spacetime with a negative cosmological constant. Symmetries then constrain the gravity side, allowing us to pose – and sometimes answer – questions that would be far harder to tackle in higher dimensions or less symmetric settings. In this spirit, Stéphane Detournay (Université Libre de Bruxelles) showed how near-extremal black holes themselves can behave like two-dimensional systems, where effects due to thermodynamics, symmetry and quantum corrections can often be disentangled cleanly.
Rapid progress in understanding generalised symmetries and defects was a hot topic. Guillermo Arias-Tamargo (Imperial College London) described how recent work on non-invertible symmetries in non-linear sigma models pushes beyond the traditional picture of symmetries as simple group actions on local fields. In this modern framework, symmetries are realised through extended objects, such as defects or interfaces. Tracking how observables transform across these structures provides concrete constraints on the dynamics and phases of the theory.
A particularly sharp application came from José Calderón Infante (Caltech), who used defect-based arguments to rule out global shift symmetries in quantum gravity. Interfaces also featured prominently as physically meaningful probes, naturally connecting abstract symmetry ideas to concrete quantities such as boundary degrees of freedom and entropy-like measures – as discussed by Carlos Hoyos (Universidad de Oviedo).
The meeting covered a wide range of active topics, but controlled semiclassical arguments, low-dimensional holographic models and defect-based symmetry arguments resurfaced throughout the programme. In that sense, Iberian Strings provided an overview not only of open questions but also of modern methods.
Supersymmetry has so far eluded discovery at the LHC, yet it retains strong theoretical appeal as an extension of the Standard Model (SM), and potential hiding places remain. In two recent analyses, the ATLAS collaboration sets new bounds on compressed higgsino models, where the proposed particles lie very close in mass. The collaboration used machine-learning techniques to target some of the most elusive signatures at the LHC: low-momentum decay products.
Without extreme fine tuning, quantum corrections would drive the Higgs-boson mass far above the electroweak scale. Supersymmetry prevents this by introducing fermion partners for the SM bosons (and vice versa) so that their quantum contributions naturally cancel. The result is a partner for every SM particle – including higgsinos, the fermionic counterparts of the Higgs field. Higgsinos mix with the partners of the electroweak gauge bosons to form electrically neutral and charged states known as neutralinos (χ̃0) and charginos (χ̃±). The lightest neutralino (χ̃01) is stable in a wide class of models and may naturally account for the observed dark-matter abundance.
In compressed scenarios, the tiny mass-splitting between these new particles poses a distinct experimental challenge. When a heavier state decays to χ̃01, the small mass difference leaves little energy for the accompanying SM particles. The visible decay products therefore carry very low momentum and may fall below reconstruction and identification thresholds. The new analyses focus precisely on this regime using the full Run 2 dataset collected at √s = 13 TeV, with two complementary strategies optimised for different values of the mass splitting.
Firstly, a “displaced track” search targets scenarios with a mass difference between the lightest chargino χ̃±1 and χ̃01 of 0.3 to 1 GeV, in which the χ̃±1 has a non-negligible lifetime and can travel a few millimetres before decaying into an invisible χ̃01 and a low-momentum charged pion. The resulting event signature is a pion track with a large transverse impact parameter and high missing transverse momentum from the neutralinos. Significant improvement in signal sensitivity is achieved by the use of two dedicated neural networks (NNs), where one exploits the global event kinematics and the other focuses on the displaced track characteristics.
A “one-lepton-one-track (1ℓ1T)” search instead targets scenarios with a larger mass splitting of 1 to 3 GeV, in which the heavier neutralino χ̃02 promptly decays into the χ̃01 and two low-momentum leptons. Since these could elude the existing ATLAS identification techniques, dedicated low-momentum electron and muon identification algorithms have been developed using NNs that exploit track and calorimeter information. The new algorithms are applied to leptons with momentum as low as 0.5 GeV for electrons and 1 GeV for muons, below the standard reconstruction thresholds, resulting in a signature consisting of one lepton and one lepton-like track. An additional NN enhances sensitivity for event classification, exploiting kinematic features that depend strongly on the mass splitting.
The observed data are consistent with the SM predictions, with no signs of new physics emerging in the targeted phase-space. Based on this result, lower limits on the higgsino masses are set at 95% confidence level (CL) (see figure 1). The 1ℓ1T search excludes a mass-splitting region between 0.8 and 2.0 GeV, extending previous limits from the LEP experiments up to a maximum χ̃±1 mass of 132 GeV for a 1.8 GeV mass splitting. The displaced track search extends the exclusion limits previously set by the ATLAS experiment by about 30 GeV, reaching a χ̃±1 mass of 199 GeV for a 0.6 GeV mass splitting. Together, the two searches exclude χ̃±1 masses below 126 GeV at 95% CL over the targeted mass splitting range. Limits set by the ATLAS collaboration now supersede those from the LEP experiments in all mass-splitting ranges.
With this result, ATLAS is now able to set limits over the full range of higgsino mass splittings that are interesting for naturalness, marking a significant milestone in the search for supersymmetry. The new Run 3 dataset, along with advanced analysis techniques, will push these searches even further – perhaps towards the discovery of physics beyond the SM.
When atomic nuclei collide at the LHC, they produce tiny droplets of quark–gluon plasma (QGP) and energetic partons plough through it, slowing down in the process. In a new analysis, the CMS collaboration compared high transverse momentum (pT) particle yields in oxygen–oxygen, neon–neon, xenon–xenon and lead–lead collisions, with the nucleon numbers of the colliding particles increasing in the sequence 16 < 20 < 129 < 208. The results suggest a steady growth of parton energy loss with the size of the colliding system.
High-pT particles come from the fragmentation of quarks and gluons produced in the earliest hard scatterings of a collision. As these partons cross the QGP, they interact with the medium and radiate, losing energy in the process. This is one of the clearest signatures of QGP formation. How much energy partons lose depends on how far they travel inside the medium, which in turn grows with the size of the colliding nuclei. Although firmly established in xenon–xenon and lead–lead collisions, the precise way this quenching depends on the path length is not yet fully understood.
Light-ion collisions provide a controlled way to vary the system size and isolate this path-length dependence. In July 2025, the LHC delivered its first ever oxygen–oxygen and neon–neon collisions (CERN Courier November/December 2025 p8). The CMS collaboration analysed the data from this dedicated one-week run to perform a systematic study of high-pT charged-particle suppression across multiple collision systems.
The analysis combines existing measurements in oxygen–oxygen, xenon–xenon and lead–lead collisions with the first measurement of the charged-particle nuclear modification factor, RAA, in neon–neon collisions at a centre-of-mass energy of 5.36 TeV per nucleon pair. The observable RAA quantifies how particle yields deviate from expectations based on proton–proton collisions. The four systems were analysed using identical pT-intervals, enabling a consistent comparison across systems.
The results should help inform the choice of ion species
For smaller nuclei, such as oxygen and neon, many experimental uncertainties shared with the proton–proton reference largely cancel, for example, those related to tracking. This leads to particularly precise measurements of RAA across a wide pT range, which is difficult to achieve in larger systems. Combined with the wide span of nuclear sizes, this precision enables a more direct assessment of how parton energy loss depends on in-medium path length.
For a fixed transverse momentum interval, the suppression increases smoothly with system size, from light to heavy ion collisions (see figure 1). Conversely, for a given nuclear system, the suppression is stronger at lower transverse momenta and progressively weakens as it increases. Expressed in terms of the cube root of the nucleon number, which is proportional to the nuclear radius, the results follow a simple ordering with the size of the system, offering a natural framework to test the evolution of energy loss with system size.
The data indicate that nuclear suppression develops gradually as the nuclear system grows, consistent with a picture in which partons interact with QGP droplets whose extent and density evolve smoothly across collision systems. Calculations that omit energy loss show little variation with system size and do not describe the observed suppression, whereas models that include it qualitatively reproduce the observed trend within uncertainties. The data, presented this way, offer a guide for further improvements on their A-dependence.
This study places new quantitative constraints on parton-energy-loss mechanisms and on the emergence of QGP-like behaviour in small nuclear systems. The results should help guide future theoretical developments and inform the choice of ion species in upcoming heavy-ion studies at the LHC.
The 18th International Workshop on Top Quark Physics (TOP2025) brought the top-quark community to Seoul, South Korea, from 21 to 26 September 2025. Hosted at Hanyang University, the event offered 135 experimentalists and theorists a chance to exchange results, discuss open questions and explore the future of top-quark physics.
2025 marked the 30th anniversary of the top quark’s discovery by the DØ and CDF experiments at Fermilab. Three decades on, and despite ever-increasing experimental precision, the top quark’s properties remain only partially understood. While its mass is now known at the sub-GeV level and its production cross sections agree well with Standard Model predictions, questions persist about its electroweak couplings, its interactions with the Higgs boson, and the detailed structure of top–antitop production at high energies. Because of its large mass and correspondingly strong coupling to the electroweak sector, many in the community continue to view the top quark as a sensitive probe of physics beyond the Standard Model.
The conference opened with an inspiring keynote address by Juan Antonio Aguilar Saavedra (IFT Madrid), who explored the connections between top-quark physics and quantum science and technology. A notable example is the recent observation of quantum entanglement in top-quark pair production by the ATLAS and CMS experiments, which has opened a promising new line of research linking collider physics with concepts more familiar to quantum information researchers. Entanglement can be measured in the top–antitop system because top quarks decay before hadronisation takes place, allowing direct access to their spin correlations.
Top physics is currently enjoying a golden era. Last year, the CMS collaboration reported an excess near the top–antitop threshold (CERN Courier May/June 2025 p7), later confirmed by ATLAS with a significance of 7.7σ above the background predicted by perturbative quantum chromodynamics (CERN Courier September/October 2025 p9). This excess is consistent with expectations from non-relativistic quantum chromodynamics, an effective theory that describes the dynamics of heavy quark pairs near threshold and with simplified models involving a pseudoscalar “quasi-bound-state”, called toponium.
During a mini-workshop dedicated to toponium, Benjamin Fuks (LPTHE) presented an intriguing scenario in which the excess could be explained by two contributions: one from a top–antitop bound state and another from a beyond-the-Standard-Model signature, although the data are also compatible with Standard-Model-only components.
The next edition of the TOP conference will take place in Antalya, Turkey, from 5 to 9 October 2026.
Proton–proton collisions at the LHC fling quarks and gluons out at massive energies. As they radiate and split into ever more partons, the strong force confines them into sprays of hadrons called jets. The total momentum of a jet, split among its components, approximates that of the initial quark or gluon, which cannot be accessed directly. By tracking how much of a jet’s momentum each hadron carries, the LHCb collaboration has now compared how charm, beauty and light quarks hadronise.
While the production and radiation of individual quarks and gluons can be treated perturbatively, their conversion into hadrons occurs in the non-perturbative regime and cannot be calculated from first principles. Instead, the transition is described using phenomenological probability distributions, called fragmentation functions, which encode how a quark of a given flavour produces specific hadrons. Measuring the content and structure of jets, as well as their kinematic properties, can help constrain these functions.
Previously, the LHCb collaboration measured observables sensitive to fragmentation functions in samples dominated by light-quark-initiated jets. The same measurements were recently carried out for charm- and beauty-quark-initiated jets, allowing a direct comparison of hadronisation across three different jet flavour categories at a single experiment. The light-quark sample was obtained by selecting jets produced nearly back-to-back with a Z boson. In such events, the single parton initiating the jet is typically a gluon or a light quark. In the forward kinematic region accessible to the LHCb detector, where one incoming parton often carries a large fraction of the proton momentum, the proportion of light-quark jets gets further enhanced. Samples of predominantly charm- and beauty-quark-initiated jets were instead obtained using a dedicated flavour-tagging algorithm, which makes use of LHCb’s excellent performance at heavy flavour identification and reconstruction.
The new measurements allow a direct comparison of hadronisation across three different jet flavour categories
A key observable for constraining fragmentation functions is the longitudinal momentum fraction z, defined as the share of jet momentum carried by a hadron along its axis. With respect to their light-quark analogues, heavy-quark-initiated jets appear suppressed at high z, consistent with the leading heavy-flavour hadron carrying most of the jet momentum (see figure 1).
Previous measurements of the hadronisation of a heavy quark into a single heavy-flavour hadron showed that this hadron carries most of the parent quark’s momentum. The new LHCb analysis extends this picture to the full multi-hadron structure of heavy-quark-initiated jets and is consistent with single-hadron measurements: relatively few charged hadrons possess a large fraction of the jet momentum – a result compatible with the heavy-flavour hadron carrying most of it. This result demonstrates the complementarity of single- and multi-hadron measurements, which are both necessary to fully understand high-energy hadronisation.
The analysis also measured the transverse momentum of the hadron with respect to the jet axis, which is sensitive to transverse-momentum-dependent fragmentation functions. Experimental constraints on these functions remain limited, yet they are crucial in reconstructing a three-dimensional description of hadronisation.
Partons produced in heavy-ion collisions at the LHC must push their way through a hot, dense quark–gluon plasma (QGP). In doing so, they experience medium-induced energy loss that depends on the parton’s mass. In a recent analysis, the ALICE collaboration compared the yields of charged particles associated with electrons from heavy-flavour hadron decays with those of the light hadrons. Both show a suppression of high-momentum particles emitted opposite to the tagged particle, with no significant difference between the two.
After a hard scattering, high-energy partons fragment into collimated sprays of hadrons known as jets. These are well described in proton–proton (pp) collisions, where their substructures provide stringent tests of perturbative QCD. In heavy-ion collisions, instead, they propagate through the QGP and emerge modified – a phenomenon known as jet quenching. Previous measurements (CERN Courier March/April 2025 p13) suggest that jets initiated by charm and beauty quarks lose less energy than those from light quarks and gluons, owing to their larger mass. This difference is commonly attributed to the dead-cone effect, which suppresses gluon emission by heavy quarks at small angles. Jet-quenching effects can be further characterised by measuring the transverse-momentum distribution of particles within jets, providing insight into the redistribution of the quenched energy.
To study this, the ALICE collaboration employs azimuthal-correlation measurements. This technique measures the angular correlation between a heavy-flavour hadron or its decay daughter (“trigger” particle) and other associated charged particles in the same event. The resulting distribution features two correlation peaks: a near-side peak from particles produced alongside the trigger and an away-side one from the recoiling jet, particularly sensitive to jet-medium interactions. Jet quenching is then quantified by the per-trigger nuclear modification factor IAA, which is the ratio of away-side charged-particle yield in heavy-ion collisions to pp collisions. Values of IAA deviating from unity indicate QGP-induced modifications of the jet.
The ALICE collaboration now reports the measurements of jet-like structures in the heavy-flavour sector of lead-lead collisions at a centre-of-mass energy of 5.02 TeV per nucleon pair. The analysis, based on LHC Run 2 data, uses electrons from semi-leptonic decays of charm and beauty hadrons as trigger particles. Electron identification relies on a combination of energy-loss measurements in the time-projection chamber, energy-momentum matching in the calorimeter and selection of shower shapes. Invariant-mass tagging techniques allowed for the subtraction of the large backgrounds from photon conversions and light-meson decays to electron-positron pairs.
The measurement is challenging due to the high multiplicity of lead-lead collisions and the need to extract jet-like correlations from large combinatorial and collective-motion backgrounds. A corresponding analysis of pp collisions at the same energy provides the reference needed to compare jet evolution in the presence of the QGP.
The away-side shows a suppression for associated particles with transverse momenta between 4 and 7 GeV/c (see figure 1), indicating relevant jet quenching with a 2.5σ significance. Conversely, a hint of an enhancement is observed below 2 GeV/c, possibly signalling the redistribution of lost energy into the medium and the subsequent formation of additional low-momentum particles.
These results are consistent with corresponding measurements using light-flavour triggers across all measured intervals. While this suggests that the QGP modifies jets consistently regardless of the initiating parton’s mass, important caveats remain. Variations in parton-to-hadron momentum scaling, as well as the fact that the heavy flavour is tagged via decay electrons, could introduce kinematic differences that complicate a direct comparison. Whether QCD predicts a deviation remains an open question for future modelling of mass-dependent parton-medium interactions.
LHC Run 3 will provide an order of magnitude more heavy-ion events. This increased luminosity will enable higher-precision analyses, offering a deeper understanding of how QGP modifies heavy- and light-flavour jets.
The Higgs boson is uniquely simple – the only Standard Model particle with no spin. Paradoxically, this allows its behaviour to be uniquely complex, notably due to the “scalar potential” built from the strength of its own field. Shaped like a Mexican hat, the Higgs potential has a local maximum of potential energy at zero field, and a ring of minima surrounding it.
In the past, the Higgs field settled into this ring, where it still dwells today. Since then, the field has been permanently “switched on” – a directionless field with a nonzero “vacuum expectation value” that is ubiquitous throughout the universe. Its interactions with a number of other fundamental particles give them mass. What remains unclear is how the Higgs field behaves once pushed from this familiar minimum. Where will it go next, how did it get there in the first place and might new physics modify this picture?
The LHC alone has shed experimental light on this physics. Further progress on this compelling frontier of fundamental science requires upgrades and new colliders. The next step along this path is the High-Luminosity LHC (HL-LHC), which is scheduled to begin operations in 2030. The HL-LHC is set to outperform the LHC by far, with a total dataset of 380 million Higgs bosons created inside the ATLAS and CMS experiments – a sample more than 10 times larger than any studied so far (see “A leap in technology” panel). We still need to unlock the full reach of the HL-LHC, but three scientific questions may serve to illustrate what can be studied with 380 million Higgs bosons.
What is the fate of the universe?
The stability of our universe hangs in a delicate balance. Quantum corrections could make the Higgs potential bend downward again at high values of the Higgs field, creating a lower-energy state beneath our own (see “The Higgs potential” panel). Through quantum tunnelling, tiny regions of space could spontaneously make the transition, releasing energy as the Higgs field settles into a new minimum of the Higgs potential. Bubbles of the new vacuum would expand at the speed of light, changing the vacuum state of the regions they encounter.
Details matter. The Higgs potential is modified by the effect of virtual loops from all particles interacting with the Higgs field. Bosons push the Higgs potential upwards at high field values, and fermions pull it downwards. If the Standard Model remains valid up to high field values, perhaps as high as the Planck scale where quantum gravity is expected to become relevant, these corrections may determine the ultimate fate of the vacuum. As the most massive Standard Model particle yet discovered, the top quark makes a dominant negative contribution at high energies and field strengths. Together with a smaller effect from the mass of the Higgs boson itself, the top-quark mass defines three possible regimes.
In the stable case, the Higgs potential remains above the current minimum up to high field values, and no deeper minimum is present.
If a second, lower minimum forms at high field values, but is shielded by a large energy barrier, the vacuum can be “metastable”. In that case, quantum tunnelling could in principle occur, but on timescales exceeding the age of the universe.
In the unstable regime, the barrier is low enough for decay to have already occurred.
Current observations place our universe safely within the metastable zone, far from any immediate change (see “A second minimum?” figure). Yet the precision of the latest LHC measurements, based on independent determinations of the top-quark mass (purple ellipses), leaves unresolved whether the universe is stable or metastable. Other uncertainties, such as that on the strength of nature’s strong coupling, also affect the distinction between the two regimes, shifting the boundary between stability and metastability (orange band).
The HL-LHC will be well placed to help resolve the question of the stability of the vacuum thanks to improvements in the measurements of the top quark and Higgs-boson masses (red ellipse). This will rely on combining the HL-LHC’s large dataset, the ingenuity of expected analysis improvements and theoretical progress in the fundamental interpretation of these measurements.
The Higgs potential
The Higgs boson is the only Standard Model particle with no spin – a quantum number that behaves as if fundamental particles were spinning, but which cannot correspond to a physical rotation without violating relativity theory.
This allows the Higgs field to experience a scalar potential – energy penalties that depend on the strength of the Higgs field itself. This is forbidden for fermions
(spin ½) and massless bosons (spin 1) by Lorentz symmetry and gauge invariance.
In the Standard Model, the Higgs field is subject to the Higgs potential, shaped like a Mexican hat, with a maximum of potential energy at zero field, and a minimum at a ring in the complex plane of values of the Higgs field. Its polynomial form is restricted by gauge symmetry. Experimentally, it can be inferred by measuring properties of the Higgs boson such as its self-coupling λ3.
Two effects then modify the Mexican-hat shape in ways that are difficult to predict but have important consequences for particle physics and cosmology. These are due to the interactions of the Higgs field with virtual particles and real thermal excitations. Quantum fluctuations modify the energy penalty of exciting the Higgs field due to virtual loops from all Standard Model particles. Changes in the temperature of the universe also generate changes in the shape of the Higgs potential due to the interaction of the Higgs field with real thermal excitations in the hot early universe. Properties such as λ3 are also affected by these effects.
The Higgs potential wasn’t always a Mexican hat. If the early universe got hot enough, interactions between the Higgs field and a hot plasma of particles shaped the Higgs potential into a steep bowl with a minimum at zero field, yielding no vacuum expectation value. As the universe cooled, this potential drooped into its familiar Mexican-hat shape, with a central peak surrounded by a ring of minima, where the Higgs field sits today. But did the Higgs field pass through an intermediate stage, with a “bump” separating the inner minimum from the ring?
The answer depends on the strength of the Higgs self-coupling, λ3, which governs the trilinear coupling where three Higgs-boson lines meet at a single vertex in a Feynman diagram. But λ3 is not yet measured. The most recent joint ATLAS and CMS analysis excludes values outside of –0.71 to 6.1 times its expected value in the Standard Model with 95% confidence.
In the Standard Model, the vacuum smoothly rolled from zero Higgs field to its new minimum in the outer ring. But if λ3 were at least 50% stronger than in the Standard Model, this smooth “crossover” phase transition may have been prevented by an intermediate bump. The vacuum would then have experienced a strong first-order phase transition (FOPT), like ice melting or water boiling at everyday pressures. As the universe cooled, regions of space would have tunnelled into the new vacuum, forming bubbles that expanded and merged. These bubble-wall collisions, combined with additional processes beyond the Standard Model that violate the conservation of both charge and parity together, could have contributed to the observed excess of matter over antimatter – one of the deepest mysteries of modern physics, wherein there appears to have been an excess of baryons over antibaryons in the early universe of roughly one part in a billion, resulting in the surplus we observe today after the annihilation of the others into photons.
The most direct probe of λ3 comes from Higgs-boson pair production (HH). HH production happens most often by the fusion of gluons from the colliding protons to create a top-quark loop that emits either two Higgs bosons or one Higgs boson splitting into two, yielding sensitivity to λ3.
HH production happens only once for every thousand Higgs bosons produced in the LHC. Searches for this process are already underway, with analyses of the Run 2 dataset by the ATLAS and CMS collaborations showing that a signal 2.5 times larger than the Standard Model expectation is already excluded. This progress far exceeds early expectations, suggesting that the HL-LHC may finally bring λ3 within experimental reach, clarifying the shape of the Higgs potential near its current minimum (see “Constraining the Higgs potential” figure).
Measuring λ3 at the HL-LHC would shed light on whether the Higgs potential follows the Standard Model prediction (black line) or alternative shapes (dashed lines), which may arise from physics beyond the Standard Model (BSM). The corresponding sensitivity can be illustrated through two complementary approaches: one based on HH production, assuming no effects beyond λ3 and providing a largely model-independent view near the potential’s minimum (red bands); and an approach that incorporates higher-order effects, which extend the reach over a broader range of the Higgs field (blue bands).
Since the previous update of the European Strategy for Particle Physics, the projected sensitivity has vastly improved. The combined ATLAS and CMS results are now expected to yield a discovery significance exceeding 7σ, should HH production occur at the Standard Model rate. By the end of the HL-LHC programme, the two experiments are expected to determine λ3 with a 1σ uncertainty of about 30% – enough to exclude the considered BSM potentials at the 95% confidence level if the self-coupling matches the Standard Model prediction.
What lurks beyond the Standard Model?
Puzzles such as the origin of dark matter and the nature of neutrino masses suggest that new physics must lie beyond the Standard Model. With greatly expanded data sets at the HL-LHC, new phenomena may become detectable as resonant peaks from undiscovered particles or deviations in precision observables.
As an example, consider a BSM scenario that includes an additional scalar boson “S” that mixes with the Higgs boson but remains blind to other Standard Model fields (see “Spotting a new scalar” figure). S could induce observable differences in λ3 (horizontal axis) and the coupling of the Higgs boson to the Z boson, gHZZ (vertical axis). Both couplings are plotted as a factor of their expected Standard Model values. The figure explores scenarios where the coupling deviates from its Standard Model value by as little as a tenth of a permille, and where the trilinear self-coupling may be between 0.5 and 2.5 times the value. Such models could prove to be the underlying cause of deviations from the Standard Model such as contributing to the matter–antimatter asymmetry in the universe. Combinations of model parameters that could allow for a strong FOPT in the early universe are plotted as black dots.
This example analysis serves to illustrate the complementarity of precision measurements and direct searches at the HL-LHC. The parameter space can be narrowed by measuring the axis variables λ3 and gHZZ (blue and orange bands). Direct searches for S → HH and S → ZZ will be able to probe or exclude many of the remaining models (red and purple regions), leaving room for scenarios in which new physics is almost entirely decoupled from the Standard Model.
What’s next?
What once might have seemed like science fiction has become a milestone in our understanding of nature. When Ursula von der Leyen, president of the European Commission, last visited CERN, she reflected on recent progress in the field.
“When you designed a 27 km underground tunnel where particles would clash at almost the speed of light, many thought you were daydreaming. And when you started looking for the Higgs boson, the chances of success seemed incredibly low, but you always proved the sceptics wrong. Your story is one of progress against all odds.”
Today, at a pivotal moment for particle physics, we are redefining what we believe is possible. Plucked from the ATLAS and CMS collaborations’ inputs to the 2026 update to the European Strategy for Particle Physics (CERN Courier November/December 2025 p23), the analyses described in this article are just a snapshot of what will be possible at the HL-LHC. In close collaboration with the theory community, experimentalists will use the unmatched datasets and detector capabilities of the HL-LHC and allow the field to explore a rich landscape of anticipated phenomena, including many signatures yet to be imagined.
The HL-LHC will deliver proton–proton collisions at least five times more intensely than the LHC’s original design. By the end of its lifetime, the HL-LHC is expected to accumulate an integrated dataset of around 3 ab–1 of proton–proton collisions – about six times the data collected during the LHC era.
ATLAS and CMS are undergoing extensive upgrades to cope with the intense environment created by a “pileup” of up to 200 simultaneous proton–proton interactions per bunch crossing. For this, researchers are building ever more precise particle detectors and developing faster, more intelligent software.
The ATLAS and CMS collaborations will implement a full upgrade of their tracking systems, providing extended detector coverage and improved spatial resolution (see “Tracking upgrades” figure). New capabilities are added to either or both experiments, such as precision timing layers outside the tracker, a more performant high-granularity forward calorimeter, new muon detectors designed to handle the increased particle flux, and modernised front- and back-end electronics across the calorimeter and muon systems, among other improvements.
Major advances are also being made in data readout, particle reconstruction and event selection. These include track reconstruction capabilities in the trigger and a significantly increased latency, allowing for more advanced decisions about which collisions to keep for offline analysis. Novel selection techniques are also emerging to handle very high event rates with minimal event content, along with AI-assisted methods for identifying anomalous events already in the first stages of the trigger chain.
Finally, detector advancements go hand-in-hand with innovation in algorithms. The reconstruction of physics objects is being revolutionised by higher detector granularity, precise timing, and the integration of machine learning and hardware accelerators such as modern GPUs. These developments will significantly enhance the identification of charged-particle tracks, interaction vertices, b-quark-initiated jets, tau leptons and other signatures – far surpassing the capabilities foreseen when the HL-LHC was first conceived.
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