Physics Archives – Page 7 of 142

A new probe of radial flow

Radial-flow fluctuations — **Fig. 1.** Schematic illustration of how radial-flow fluctuations lead to correlations between the differential yield n(p_T) and transverse momentum p_T. Blue indicates a smaller-than-average initial area and a larger integral radial flow, whereas red indicates a larger-than-average initial area and a smaller integral radial flow. Credit: ATLAS Collab.

The ATLAS and ALICE collaborations have announced the first results of a new way to measure the “radial flow” of quark–gluon plasma (QGP). The two analyses offer a fresh perspective into the fluid-like behaviour of QCD matter under extreme conditions, such as those that prevailed after the Big Bang. The measurements are highly complementary, with ALICE drawing on their detector’s particle-identification capabilities and ATLAS leveraging the experiment’s large rapidity coverage.

At the Large Hadron Collider, lead–ion collisions produce matter at temperatures and densities so high that quarks and gluons momentarily escape their confinement within hadrons. The resulting QGP is believed to have filled the universe during its first few microseconds, before cooling and fragmenting into mesons and baryons. In the laboratory, these streams of particles allow researchers to reconstruct the dynamical evolution of the QGP, which has long been known to transform anisotropies of the initial collision geometry into anisotropic momentum distributions of the final-state particles.

Compelling evidence

Differential measurements of the azimuthal distributions of produced particles over the last decades have provided compelling evidence that the outgoing momentum distribution reflects a collective response driven by initial pressure gradients. The isotropic expansion component, typically referred to as radial flow, has instead been inferred from the slope of particle spectra (see figure 1). Despite its fundamental role in driving the QGP fireball, radial flow lacked a differential probe comparable to those of its anisotropic counterparts.

**Fig. 2.** ATLAS measurements of radial flow as a function of p_T, for various reference momenta (left), pseudorapidity gaps (middle) and centrality ranges from head-on (0–5% centrality) to peripheral (60–70% centrality) collisions (right). The scaling at low p_T suggests that radial flow is a collective phenomenon. Credit: ATLAS Collab. 2025 arXiv:2503.24125

That situation has now changed. The ALICE and ATLAS collaborations recently employed the novel observable v₀(p_T) to investigate radial flow directly. Their independent results demonstrate, for the first time, that the isotropic expansion of the QGP in heavy-ion collisions exhibits clear signatures of collective behaviour. The isotropic expansion of the QGP and its azimuthal modulations ultimately depend on the hydrodynamic properties of the QGP, such as shear or bulk viscosity, and can thus be measured to constrain them.

Traditionally, radial flow has been inferred from the slope of p_T-spectra, with the p_T-integrated radial-flow extracted via fits to “blast wave” models. The newly introduced differential observable v₀(p_T) captures fluctuations in spectral shape across p_T bins. v₀(p_T) retains differential sensitivity, since it is defined as the correlation (technically the normalised covariance) between the fraction of particles in a given p_T-interval and the mean transverse momentum of the collision products within a single event, [p_T]. Roughly speaking, a fluctuation raising [p_T] produces a positive v₀(p_T) at high p_T due to the fractional yield increasing; conversely, the fractional yield decreasing at low p_T causes a negative v₀(p_T). A pseudorapidity gap between the measurement of mean p_T and the particle yields is used to suppress short-range correlations and isolate the long-range, collective signal. Previous studies observed event-by-event fluctuations in [p_T], related to radial flow over a wide p_T range and quantified by the coefficient v₀^ref, but they could not establish whether these fluctuations were correlated across different p_T intervals – a crucial signature of collective behaviour.

Origins

The ATLAS collaboration performed a measurement of v₀(p_T) in the 0.5 to 10 GeV range, identifying three signatures of the collective origin of radial flow (see figure 2). First, correlations between the particle yield at fixed p_T and the event-wise mean [p_T] in a reference interval show that the two-particle radial flow factorises into single-particle coefficients as v₀(p_T) × v₀^ref for p_T < 4 GeV, independent of the reference choice (left panel). Second, the data display no dependence on the rapidity gap between correlated particles, suggesting a long-range effect intrinsic to the entire system (middle panel). Finally, the centrality dependence of the ratio v₀(p_T)/v₀^ref followed a consistent trend from head-on to peripheral collisions, effectively cancelling initial geometry effects and supporting the interpretation of a collective QGP response (right panel). At higher p_T, a decrease in v₀(p_T) and a splitting with respect to centrality suggest the onset of non-thermal effects such as jet quenching. This may reveal fluctuations in jet energy loss – an area warranting further investigation.

**Fig. 3.** ALICE measurements of radial flow as a function of p_T for pions, kaons and protons in mid-central (left) and peripheral (right) Pb–Pb collisions at a centre-of-mass energy of 5.02 TeV. The measurements are compared to state-of-the-art hydrodynamic-based model calculations (Hydro) and a non-collective model (HIJING). Credit: ALICE Collab. 2025 arXiv:2504.04796

Using more than 80 million collisions at a centre-of-mass energy of 5.02 TeV, ALICE extracted v₀(p_T) for identified pions, kaons and protons across a broad range of centralities. ALICE observes v₀(p_T) to be negative at low p_T, reflecting the influence of mean-p_T fluctuations on the spectral shape (see figure 3). The data display a clear mass ordering at low p_T, from protons to kaons to pions, consistent with expectations from collective radial expansion. This mass ordering reflects the greater “push” heavier particles experience in the rapidly expanding medium. The picture changes above 3 GeV, where protons have larger v₀(p_T) values than pions and kaons, perhaps indicating the contribution of recombination processes in hadron production.

The results demonstrate that the isotropic expansion of the QGP in heavy-ion collisions exhibits clear signatures of collective behaviour

The two collaborations’ measurements of the new v₀(p_T) observable highlight its sensitivity to the bulk-transport properties of the QGP medium. Comparisons with hydrodynamic calculations show that v₀(p_T) varies with bulk viscosity and the speed of sound, but that it has a weaker dependence on shear viscosity. Hydrodynamic predictions reproduce the data well up to about 2 GeV, but diverge at higher momenta. The deviation of non-collective models like HIJING from the data underscores the dominance of final-state, hydrodynamic-like effects in shaping radial flow.

These results advance our understanding of one of the most extreme regimes of QCD matter, strengthening the case for the formation of a strongly interacting, radially expanding QGP medium in heavy-ion collisions. Differential measurements of radial flow offer a new tool to probe this fluid-like expansion in detail, establishing its collective origin and complementing decades of studies of anisotropic flow.

Neutron stars as fundamental physics labs

Neutron stars are truly remarkable systems. They pack between one and two times the mass of the Sun into a radius of about 10 kilometres. Teetering on the edge of gravitational collapse into a black hole, they exhibit some of the strongest gravitational forces in the universe. They feature extreme densities in excess of atomic nuclei. And due to their high densities they produce weakly interacting particles such as neutrinos. Fifty experts on nuclear physics, particle physics and astrophysics met at CERN from 9 to 13 June to discuss how to use these extreme environments as precise laboratories for fundamental physics.

Perhaps the most intriguing open question surrounding neutron stars is what is actually inside them. Clearly they are primarily composed of neutrons, but many theories suggest that other forms of matter should appear in the highest density regions near the centre of the star, including free quarks, hyperons and kaon or pion condensates. Diverse data can constrain these hypotheses, including astronomical inferences of the masses and radii of neutron stars, observations of the mergers of neutron stars by LIGO, and baryon production patterns and correlations in heavy-ion collisions at the LHC. Theoretical consistency is critical here. Several talks highlighted the importance of low-energy nuclear data to understand the behaviour of nuclear matter at low densities, though also emphasising that at very high densities and energies any description should fall within the realm of QCD – a theory that beautifully describes the dynamics of quarks and gluons at the LHC.

Another key question for neutron stars is how fast they cool. This depends critically on their composition. Quarks, hyperons, nuclear resonances, pions or muons would each lead to different channels to cool the neutron star. Measurements of the temperatures and ages of neutron stars might thereby be used to learn about their composition.

Research into neutron stars has progressed so rapidly in recent years that it allows key tests of fundamental physics

The workshop revealed that research into neutron stars has progressed so rapidly in recent years that it allows key tests of fundamental physics including tests of particles beyond the Standard Model, including the axion: a very light and weakly coupled dark-matter candidate that was initially postulated to explain the “strong CP problem” of why strong interactions are identical for particles and antiparticles. The workshop allowed particle theorists to appreciate the various possible uncertainties in their theoretical predictions and propagate them into new channels that may allow sharper tests of axions and other weakly interacting particles. An intriguing question that the workshop left open is whether the canonical QCD axion could condense inside neutron stars.

While many uncertainties remain, the workshop revealed that the field is open and exciting, and that upcoming observations of neutron stars, including neutron-star mergers or the next galactic supernova, hold unique opportunities to understand fundamental questions from the nature of dark matter to the strong CP problem.

Quantum theory returns to Helgoland

In June 1925, Werner Heisenberg retreated to the German island of Helgoland seeking relief from hay fever and the conceptual disarray of the old quantum theory. On this remote, rocky outpost in the North Sea, he laid the foundations of matrix mechanics. Later, his “island epiphany” would pass through the hands of Max Born, Wolfgang Pauli, Pascual Jordan and several others, and become the first mature formulation of quantum theory. From 9 to 14 June 2025, almost a century later, hundreds of researchers gathered on Helgoland to mark the anniversary – and to deal with pressing and unfinished business.

Alfred D Stone (Yale University) called upon participants to challenge the folklore surrounding quantum theory’s birth. Philosopher Elise Crull (City College of New York) drew overdue attention to Grete Hermann, who hinted at entanglement before it had a name and anticipated Bell in identifying a flaw in von Neumann’s no-go theorem, which had been taken as proof that hidden-variable theories are impossible. Science writer Philip Ball questioned Heisenberg’s epiphany itself: he didn’t invent matrix mechanics in a flash, claims Ball, nor immediately grasp its relevance, and it took months, and others, to see his contribution for what it was (see “Lend me your ears” image).

Building on a strong base

A clear takeaway from Helgoland 2025 was that the foundations of quantum mechanics, though strongly built on Helgoland 100 years ago, nevertheless remain open to interpretation, and any future progress will depend on excavating them directly (see “Four ways to interpret quantum mechanics“).

Does the quantum wavefunction represent an objective element of reality or merely an observer’s state of knowledge? On this question, Helgoland 2025 could scarcely have been more diverse. Christopher Fuchs (UMass Boston) passionately defended quantum Bayesianism, which recasts the Born probability rule as a consistency condition for rational agents updating their beliefs. Wojciech Zurek (Los Alamos National Laboratory) presented the Darwinist perspective, for which classical objectivity emerges from redundant quantum information encoded across the environment. Although Zurek himself maintains a more agnostic stance, his decoherence-based framework is now widely embraced by proponents of many-worlds quantum mechanics (see “The minimalism of many worlds“).

The foundations of quantum mechanics remain open to interpretation, and any future progress will depend on excavating them directly

Markus Aspelmeyer (University of Vienna) made the case that a signature of gravity’s long-speculated quantum nature may soon be within experimental reach. Building on the “gravitational Schrödinger’s cat” thought experiment proposed by Feynman in the 1950s, he described how placing a massive object in a spatial superposition could entangle a nearby test mass through their gravitational interaction. Such a scenario would produce correlations that are inexplicable by classical general relativity alone, offering direct empirical evidence that gravity must be described quantum-mechanically. Realising this type of experiment requires ultra-low pressures and cryogenic temperatures to suppress decoherence, alongside extremely low-noise measurements of gravitational effects at short distances. Recent advances in optical and optomechanical techniques for levitating and controlling nanoparticles suggest a path forward – one that could bring evidence for quantum gravity not from black holes or the early universe, but from laboratories on Earth.

Information insights

Quantum information was never far from the conversation. Isaac Chuang (MIT) offered a reconstruction of how Heisenberg might have arrived at the principles of quantum information, had his inspiration come from Shannon’s Mathematical Theory of Communication. He recast his original insights into three broad principles: observations act on systems; local and global perspectives are in tension; and the order of measurements matters. Starting from these ingredients, one could in principle recover the structure of the qubit and the foundations of quantum computation. Taking the analogy one step further, he suggested that similar tensions between memorisation and generalisation – or robustness and adaptability – may one day give rise to a quantum theory of learning.

Helgoland 2025 illustrated just how much quantum mechanics has diversified since its early days. No longer just a framework for explaining atomic spectra, the photoelectric effect and black-body radiation, it is at once a formalism describing high-energy particle scattering, a handbook for controlling the most exotic states of matter, the foundation for information technologies now driving national investment plans, and a source of philosophical conundrums that, after decades at the margins, has once again taken centre stage in theoretical physics.

Exceptional flare tests blazar emission models

Active galactic nuclei (AGNs) are extremely energetic regions at the centres of galaxies, powered by accretion onto a supermassive black hole. Some AGNs launch plasma outflows moving near light speed. Blazars are a subclass of AGNs whose jets are pointed almost directly at Earth, making them appear exceptionally bright across the electromagnetic spectrum. A new analysis of an exceptional flare of BL Lacertae by NASA’s Imaging X-ray Polarimetry Explorer (IXPE) has now shed light on their emission mechanisms.

The spectral energy distribution of blazars generally has two broad peaks. The low-energy peak from radio to X-rays is well explained by synchrotron radiation from relativistic electrons spiraling in magnetic fields, but the origin of the higher-energy peak from X-rays to γ-rays is a longstanding point of contention, with two classes of models, dubbed hadronic and leptonic, vying to explain it. Polarisation measurements offer a key diagnostic tool, as the two models predict distinct polarisation signatures.

Model signatures

In hadronic models, high-energy emission is produced by protons, either through synchrotron radiation or via photo-hadronic interactions that generate secondary particles. Hadronic models predict that X-ray polarisation should be as high as that in the optical and millimetre bands, even in complex jet structures.

Leptonic models are powered by inverse Compton scattering, wherein relativistic electrons “upscatter” low-energy photons, boosting them to higher energies with low polarisation. Leptonic models can be further subdivided by the source of the inverse-Compton-scattered photons. If initially generated by synchrotron radiation in the AGN (synchrotron self-Compton, SSC), modest polarisation (~50%) is expected due to the inherent polarisation of synchrotron photons, with further reductions if the emission comes from inhomogeneous or multiple emitting regions. If initially generated by external sources (external Compton, EC), isotropic photon fields from the surrounding structures are expected to average out their polarisation.

IXPE launched on 9 December 2021, seeking to resolve such questions. It is designed to have 100-fold better sensitivity to the polarisation of X-rays in astrophysical sources than the last major X-ray polarimeter, which was launched half a century ago (CERN Courier July/August 2022 p10). In November 2023, it participated in a coordinated multiwavelength campaign spanning radio, millimetre and optical, and X-ray bands targeted the blazar BL Lacertae, whose X-ray emission arises mostly from the high-energy component, with its low-energy synchrotron component mainly at infrared energies. The campaign captured an exceptional flare, providing a rare opportunity to test competing emission models.

Optical telescopes recorded a peak optical polarisation of 47.5 ± 0.4%, the highest ever measured in a blazar. The short-mm (1.3 mm) polarisation also rose to about 10%, with both bands showing similar trends in polarisation angle. IXPE measured no significant polarisation in the 2 to 8 keV X-ray band, placing a 3σ upper limit of 7.4%.

The striking contrast between the high polarisation in optical and mm bands, and a strict upper limit in X-rays, effectively rules out all single-zone and multi-region hadronic models. Had these processes dominated, the X-ray polarisation would have been comparable to the optical. Instead, the observations strongly support a leptonic origin, specifically the SSC model with a stratified or multi-zone jet structure that naturally explains the low X-ray polarisation.

A key feature of the flare was the rapid rise and fall of optical polarisation

A key feature of the flare was the rapid rise and fall of optical polarisation. Initially, it was low, of order 5%, and aligned with the jet direction, suggesting the dominance of poloidal or turbulent fields. A sharp increase to nearly 50%, while retaining alignment, indicates the sudden injection of a compact, toroidally dominated magnetic structure.

The authors of the analysis propose a “magnetic spring” model wherein a tightly wound toroidal field structure is injected into the jet, temporarily ordering the magnetic field and raising the optical polarisation. As the structure travels outward, it relaxes, likely through kink instabilities, causing the polarisation to decline over about two weeks. This resembles an elastic system, briefly stretched and then returning to equilibrium.

A magnetic spring would also explain the multiwavelength flaring. The injection boosted the total magnetic field strength, triggering an unprecedented mm-band flare powered by low-energy electrons with long cooling times. The modest rise in mm-wavelength polarisation (green points) suggests emission from a large, turbulent region. Meanwhile, optical flaring (black points) was suppressed due to the rapid synchrotron cooling of high-energy electrons, consistent with the observed softening of the optical spectrum. No significant γ-ray enhancement was observed, as these photons originate from the same rapidly cooling electron population.

Turning point

These findings mark a turning point in high-energy astrophysics. The data definitively favour leptonic emission mechanisms in BL Lacertae during this flare, ruling out efficient proton acceleration and thus any associated high-energy neutrino or cosmic-ray production. The ability of the jet to sustain nearly 50% polarisation across parsec scales implies a highly ordered, possibly helical magnetic field extending far from the supermassive black hole.

The results cement polarimetry as a definitive tool in identifying the origin of blazar emission. The dedicated Compton Spectrometer and Imager (COSI) γ-ray polarimeter is soon set to complement IXPE at even higher energies when launched by NASA in 2027. Coordinated campaigns will be crucial for probing jet composition and plasma processes in AGNs, helping us understand the most extreme environments in the universe.

Fermilab’s final word on muon g-2

Fermilab’s Muon g-2 collaboration has given its final word on the magnetic moment of the muon. The new measurement agrees closely with a significantly revised Standard Model (SM) prediction. Though the experimental measurement will likely now remain stable for several years, theorists expect to make rapid progress to reduce uncertainties and resolve tensions underlying the SM value. One of the most intriguing anomalies in particle physics is therefore severely undermined, but not yet definitively resolved.

The muon g-2 anomaly dates back to the late 1990s and early 2000s, when measurements at Brookhaven National Laboratory (BNL) uncovered a possible discrepancy by comparison to theoretical predictions of the so-called muon anomaly, a_μ = (g-2)/2. a_μ expresses the magnitude of quantum loop corrections to the leading-order prediction of the Dirac equation, which multiplies the classical gyromagnetic ratio of fundamental fermions by a “g-factor” of precisely two. Loop corrections of a_μ ~ 0.1% quantify the extent to which virtual particles emitted by the muon further increase the strength of its interaction with magnetic fields. Were measurements to be shown to deviate from SM predictions, this would indicate the influence of virtual fields beyond the SM.

Move on up

In 2013, the BNL experiment’s magnetic storage ring was transported from Long Island, New York, to Fermilab in Batavia, Illinois. After years of upgrades and improvements, the new experiment began in 2017. It now reports a final precision of 127 parts per billion (ppb), bettering the experiment’s design precision of 140 ppb, and a factor of four more sensitive than the BNL result.

“First and foremost, an increase in the number of stored muons allowed us to reduce our statistical uncertainty to 98 ppb compared to 460 ppb for BNL,” explains co-spokesperson Peter Winter of Argonne National Laboratory, “but a lot of technical improvements to our calorimetry, tracking, detector calibration and magnetic-field mapping were also needed to improve on the systematic uncertainties from 280 ppb at BNL to 78 ppb at Fermilab.”

This formidable experimental precision throws down the gauntlet to the theory community

The final Fermilab measurement is (116592070.5 ± 11.4 (stat.) ± 9.1(syst.) ± 2.1 (ext.)) × 10^–11, fully consistent with the previous BNL measurement. This formidable precision throws down the gauntlet to the Muon g-2 Theory Initiative (TI), which was founded to achieve an international consensus on the theoretical prediction.

The calculation is difficult, featuring contributions from all sectors of the SM (CERN Courier March/April 2025 p21). The TI published its first whitepaper in 2020, reporting a_μ = (116591810 ± 43) × 10^–11, based exclusively on a data-driven analysis of cross-section measurements at electron–positron colliders (WP20). In May, the TI updated its prediction, publishing a value a_μ = (116592033 ± 62) × 10^–11, statistically incompatible with the previous prediction at the level of three standard deviations, and with an increased uncertainty of 530 ppb (WP25). The new prediction is based exclusively on numerical SM calculations. This was made possible by rapid progress in the use of lattice QCD to control the dominant source of uncertainty, which arises due to the contribution of so-called hadronic vacuum polarisation (HVP). In HVP, the photon representing the magnetic field interacts with the muon during a brief moment when a virtual photon erupts into a difficult-to-model cloud of quarks and gluons.

Significant shift

“The switch from using the data-driven method for HVP in WP20 to lattice QCD in WP25 results in a significant shift in the SM prediction,” confirms Aida El-Khadra of the University of Illinois, chair of the TI, who believes that it is not unreasonable to expect significant error reductions in the next couple of years. “There still are puzzles to resolve, particularly around the experimental measurements that are used in the data-driven method for HVP, which prevent us, at this point in time, from obtaining a new prediction for HVP in the data-driven method. This means that we also don’t yet know if the data-driven HVP evaluation will agree or disagree with lattice–QCD calculations. However, given the ongoing dedicated efforts to resolve the puzzles, we are confident we will soon know what the data-driven method has to say about HVP. Regardless of the outcome of the comparison with lattice QCD, this will yield profound insights.”

We are making plans to improve experimental precision beyond the Fermilab experiment

On the experimental side, attention now turns to the Muon g-2/EDM experiment at J-PARC in Tokai, Japan. While the Fermilab experiment used the “magic gamma” method first employed at CERN in the 1970s to cancel the effect of electric fields on spin precession in a magnetic field (CERN Courier September/October 2024 p53), the J-PARC experiment seeks to control systematic uncertainties by exercising particularly tight control of its muon beam. In the Japanese experiment, antimatter muons will be captured by atomic electrons to form muonium, ionised using a laser, and reaccelerated for a traditional precession measurement with sensitivity to both the muon’s magnetic moment and its electric dipole moment (CERN Courier July/August 2024 p8).

“We are making plans to improve experimental precision beyond the Fermilab experiment, though their precision is quite tough to beat,” says spokesperson Tsutomu Mibe of KEK. “We also plan to search for the electric dipole moment of the muon with an unprecedented precision of roughly 10^–21 e cm, improving the sensitivity of the last results from BNL by a factor of 70.”

With theoretical predictions from high-order loop processes expected to be of the order 10^–38 e cm, any observation of an electric dipole moment would be a clear indication of new physics.

“Construction of the experimental facility is currently ongoing,” says Mibe. “We plan to start data taking in 2030.”

STAR hunts QCD critical point

**Phases of QCD** Conjectured QCD phase diagram showing hadronisation trajectories as a function of centre-of-mass energy (√s). The baryon chemical potential is a measure of baryon density. Credit: Adapted from L Du *et al.* 2024 *Int. J. Mod. Phys.* 33 2430008

Just as water takes the form of ice, liquid or vapour, QCD matter exhibits distinct phases. But while the phase diagram of water is well established, the QCD phase diagram remains largely conjectural. The STAR collaboration at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC) recently completed a new beam-energy scan (BES-II) of gold–gold collisions. The results narrow the search for a long-sought-after “critical point” in the QCD phase diagram.

“BES-II precision measurements rule out the existence of a critical point in the regions of the QCD phase diagram accessed at LHC and top RHIC energies, while still allowing the possibility at lower collision energies,” says Bedangadas Mohanty of the National Institute of Science Education and Research in India, who co-led the analysis. “The results refine earlier BES-I indications, now with much reduced uncertainties.”

At low temperatures and densities, quarks and gluons are confined within hadrons. Heating QCD matter leads to the formation of a deconfined quark–gluon plasma (QGP), while increasing the density at low temperatures is expected to give rise to more exotic states such as colour superconductors. Above a certain threshold in baryon density, the transition from hadron gas to QGP is expected to be first-order – a sharp, discontinuous change akin to water boiling. As density decreases, this boundary gives way to a smooth crossover where the two phases blend. A hypothetical critical point marks the shift between these regimes, much like the endpoint of the liquid–gas coexistence line in the phase diagram of water (see “Phases of QCD” figure).

Heavy-ion collisions offer a way to observe this phase transition directly. At the Large Hadron Collider, the QGP created in heavy-ion collisions transitions smoothly to a hadronic gas as it cools, but the lower energies explored by RHIC probe the region of phase space where the critical point may lie.

To search for possible signatures of a critical point, the STAR collaboration measured gold–gold collisions at centre-of-mass energies between 7.7 and 27 GeV per nucleon pair. The collaboration reports that their data deviate from frameworks that do not include a critical point, including the hadronic transport model, thermal models with canonical ensemble treatment, and hydrodynamic approaches with excluded-volume effects. Depending on the choice of observable and non-critical baseline model, the significance of the deviations ranges from two to five standard deviations, with the largest effects seen in head-on collisions when using peripheral collisions as a reference.

“None of the existing theoretical models fully reproduce the features observed in the data,” explains Mohanty. “To interpret these precision measurements, it is essential that dynamical model calculations that include critical-point physics be developed.” The STAR collaboration is now mapping lower energies and higher baryon densities using a fixed target (FXT) mode, wherein a 1 mm gold foil sits 2 cm below the beam axis.

“The FXT data are a valuable opportunity to explore QCD matter at high baryon density,” says Mohanty. “Data taking will conclude later this year when RHIC transitions to the Electron–Ion Collider. The Compressed Baryonic Matter experiment at FAIR in Germany will then pick up the study of the QCD critical point towards the end of the 2020s.”

Plotting the discovery of Higgs pairs on Elba

Precise measurements of the Higgs self-coupling and its effects on the Higgs potential will play a key role in testing the validity of the Standard Model (SM). 150 physicists discussed the required experimental and theoretical manoeuvres on the serene island of Elba from 11 to 17 May at the Higgs Pairs 2025 workshop.

The conference mixed updates on theoretical developments in Higgs-boson pair production, searches for new physics in the scalar sector, and the most recent results from Run 2 and Run 3 of the LHC. Among the highlights was the first Run 3 analysis released by ATLAS on the search for di-Higgs production in the bbγγ final state – a particularly sensitive channel for probing the Higgs self-coupling. This result builds on earlier Run 2 analyses and demonstrates significantly improved sensitivity, now comparable to the full Run 2 combination of all channels. These gains were driven by the use of new b-tagging algorithms, improved mass resolution through updated analysis techniques, and the availability of nearly twice the dataset.

Complementing this, CMS presented the first search for ttHH production – a rare process that would provide additional sensitivity to the Higgs self-coupling and Higgs–top interactions. Alongside this, ATLAS presented first experimental searches for triple Higgs boson production (HHH), one of the rarest processes predicted by the SM. Work on more traditional final states such as bbττ and bbbb is ongoing at both experiments, and continues to benefit from improved reconstruction techniques and larger datasets.

Beyond current data, the workshop featured discussions of the latest combined projection study by ATLAS and CMS, prepared as part of the input to the upcoming European Strategy Update. It extrapolates results of the Run 2 analyses to expected conditions of the High-Luminosity LHC (HL-LHC), estimating future sensitivities to the Higgs self-coupling and di-Higgs cross-section in scenarios with vastly higher luminosity and upgraded detectors. Under these assumptions, the combined sensitivity of ATLAS and CMS to di-Higgs production is projected to reach a significance of 7.6σ, firmly establishing the process.

These projections provide crucial input for analysis strategy planning and detector design for the next phase of operations at the HL-LHC. Beyond the HL-LHC, efforts are already underway to design experiments at future colliders that will enhance sensitivity to the production of Higgs pairs, and offer new insights into electroweak symmetry breaking.

Hadronic decays confirm long-lived Ω_c⁰ baryon

LHCb figure 1 — **Fig. 1.** Average charm baryon lifetimes determined by the PDG in 2018, and the three lifetime measurements performed by the LHCb experiment in 2018 and 2019 (semileptonic), 2021 (prompt) and 2025 (hadronic). The coloured bands indicate the average of the LHCb measurements. Credit: LHCb Collab. 2025 LHCb-PAPER-2025-013

In 2018 and 2019, the LHCb collaboration published surprising measurements of the Ξ_c⁰ and Ω_c⁰ baryon lifetimes, which were inconsistent with previous results and overturned the established hierarchy between the two. A new analysis of their hadronic decays now confirms this observation, promising insights into the dynamics of baryons.

The Λ_c⁺, Ξ_c⁺, Ξ_c⁰ and Ω_c⁰ baryons – each composed of one charm and two lighter up, down or strange quarks – are the only ground-state singly charmed baryons that decay predominantly via the weak interaction. The main contribution to this process comes from the charm quark transitioning into a strange quark, with the other constituents acting as passive spectators. Consequently, at leading order, their lifetimes should be the same. Differences arise from higher-order effects, such as W-boson exchange between the charm and spectator quarks and quantum interference between identical particles, known as “Pauli interference”. Charm hadron lifetimes are more sensitive to these effects than beauty hadrons because of the smaller charm quark mass compared to the bottom quark, making them a promising testing ground to study these effects.

Measurements of the Ξ_c⁰ and Ω_c⁰ lifetimes prior to the start of the LHCb experiment resulted in the PDG averages shown in figure 1. The first LHCb analysis, using charm baryons produced in semi-leptonic decays of beauty baryons, was in tension with the established values, giving a Ω_c⁰ lifetime four times larger than the previous average. The inconsistencies were later confirmed by another LHCb measurement, using an independent data set with charm baryons produced directly (prompt) in the pp collision (CERN Courier July/August 2021 p17). These results changed the ordering of the four single-charm baryons when arranged according to their lifetimes, triggering a scientific discussion on how to treat higher-order effects in decay rate calculations.

Using the full Run 1 and 2 datasets, LHCb has now measured the Ξ_c⁰ and Ω_c⁰lifetimes with a third independent data sample, based on fully reconstructed Ξ_b^– → Ξ_c⁰ (→ pK^–K^–π⁺)π^– and Ω^–_b → Ω_c⁰ (→ pK^–K^–π⁺)π^– decays. The selection of these hadronic decay chains exploits the long lifetime of the beauty baryons, such that the selection efficiency is almost independent of the charm baryon decay time. To cancel out the small remaining acceptance effects, the measurement is normalised to the kinematically and topologically similar B^– → D⁰(→ K⁺K^–π⁺π^–)π^– channel, minimising the uncertainties with only a small additional correction from simulation.

The signal decays are separated from the remaining background by fits to the Ξ_c⁰ π^– and Ω_c⁰ π^– invariant mass spectra, providing 8260 ± 100 Ξ_c⁰ and 355 ± 26 Ω_c⁰ candidates. The decay time distributions are obtained with two independent methods: by determining the yield in each of a specific set of decay time intervals, and by employing a statistical technique that uses the covariance matrix from the fit to the mass spectra. The two methods give consistent results, confirming LHCb’s earlier measurements. Combining the three measurements from LHCb, while accounting for their correlated uncertainties, gives τ(Ξ_c⁰) = 150.7 ± 1.6 fs and τ(Ω_c⁰) = 274.8 ± 10.5 fs. These new results will serve as experimental guidance on how to treat higher-order effects in weak baryon decays, particularly regarding the approach-dependent sign and magnitude of Pauli interference terms.

Decoding the Higgs mechanism with vector bosons

CMS figure 1 — **Fig. 1.** Measured VBS signal strengths (μ) from the combined data, compared to Standard Model predictions. Left: μ values with 1σ (thick) and 2σ (thin) intervals; the red and blue bands show systematic and statistical uncertainties, respectively. Right: observed (black) and expected (grey) significances of the electroweak signal relative to the background-only hypothesis. Credit: CMS Collab. 2025 CMS-PAS-SMP-24-013

The discovery of the Higgs boson at the LHC in 2012 provided strong experimental support for the Brout–Englert–Higgs mechanism of spontaneous electroweak symmetry breaking (EWSB) as predicted by the Standard Model. The EWSB explains how the W and Z bosons, the mediators of the weak interaction, acquire mass: their longitudinal polarisation states emerge from the Goldstone modes of the Higgs field, linking the mass generation of vector bosons directly to the dynamics of the process.

Yet, its ultimate origins remain unknown and the Standard Model may only offer an effective low-energy description of a more fundamental theory. Exploring this possibility requires precise tests of how EWSB operates, and vector boson scattering (VBS) provides a particularly sensitive probe. In VBS, two electroweak gauge bosons scatter off one another. The cross section remains finite at high energies only because there is an exact cancellation between the pure gauge-boson interactions and the Higgs-boson mediated contributions, an effect analogous to the role of the Z boson propagator in WW production at electron–positron colliders. Deviations from the expected behaviour could signal new dynamics, such as anomalous couplings, strong interactions in the Higgs sector or new particles at higher energy scales.

This result lays the groundwork for future searches for new physics hidden within the electroweak sector

VBS interactions are among the rarest observed so far at the LHC, with cross sections as low as one femtobarn. To disentangle them from the background, researchers rely on the distinctive experimental signature of two high-energy jets in the forward detector regions produced by the initial quarks that radiate the bosons, with minimal hadronic activity between them. Using the full data set from Run 2 of the LHC at a centre-of-mass energy of 13 TeV, the CMS collaboration carried out a comprehensive set of VBS measurements across several production modes: WW (with both same and opposite charges), WZ and ZZ, studied in five final states where both bosons decay leptonically and in two semi-leptonic configurations where one boson decays into leptons and the other into quarks. To enhance sensitivity further, the data from all the measurements have now been combined in a single joint fit, with a complete treatment of uncertainty correlations and a careful handling of events selected by more than one analysis.

All modes, one analysis

To account for possible deviations from the expected predictions, each process is characterised by a signal strength parameter (μ), defined as the ratio of the measured production rate to the cross section predicted by the Standard Model. A value of μ near unity indicates consistency with the Standard Model, while significant deviations may suggest new physics. The results, summarised in figure 1, display good agreement with the Standard Model predictions: all measured signal strengths are consistent with unity within their respective uncertainties. A mild excess with respect to the leading-order theoretical predictions is observed across several channels, highlighting the need for more accurate modelling, in particular for the measurements that have reached a level of precision where systematic effects dominate. By presenting the first evidence for all charged VBS production modes from a single combined statistical analysis, this CMS result lays the groundwork for future searches for new physics hidden within the electroweak sector.

Hadrons in Porto Alegre

The 16th International Workshop on Hadron Physics (Hadrons 2025) welcomed 135 physicists to the Federal University of Rio Grande do Sul (UFRGS) in Porto Alegre, Brazil. Delayed by four months due to a tragic flood that devastated the city, the triennial conference took place from 10 to 14 March, despite adversity maintaining its long tradition as a forum for collaboration among Brazilian and international researchers at different stages of their careers.

The workshop’s scientific programme included field theoretical approaches to QCD, the behaviour of hadronic and quark matter in astrophysical contexts, hadronic structure and decays, lattice QCD calculations, recent experimental developments in relativistic heavy-ion collisions, and the interplay of strong and electroweak forces within the Standard Model.

Fernanda Steffens (University of Bonn) explained how deep-inelastic-scattering experiments and theoretical developments are revealing the internal structure of the proton. Kenji Fukushima (University of Tokyo) addressed the theoretical framework and phase structure of strongly interacting matter, with particular emphasis on the QCD phase diagram and its relevance to heavy-ion collisions and neutron stars. Chun Shen (Wayne State University) presented a comprehensive overview of the state-of-the-art techniques used to extract the transport properties of quark–gluon plasma from heavy-ion collision data, emphasising the role of Bayesian inference and machine learning in constraining theoretical models. Li-Sheng Geng (Beihang University) explored exotic hadrons through the lens of hadronic molecules, highlighting symmetry multiplets such as pentaquarks, the formation of multi-hadron states and the role of femtoscopy in studying unstable particle interactions.

This edition of Hadrons was dedicated to the memory of two individuals who left a profound mark on the Brazilian hadronic-physics community: Yogiro Hama, a distinguished senior researcher and educator whose decades-long contributions were foundational to the development of the field in Brazil, and Kau Marquez, an early-career physicist whose passion for science remained steadfast despite her courageous battle with spinal muscular atrophy. Both were remembered with deep admiration and respect, not only for their scientific dedication but also for their personal strength and impact on the community.

Its mission is to cultivate a vibrant and inclusive scientific environment

Since its creation in 1988, the Hadrons workshop has played a central role in developing Brazil’s scientific capacity in particle and nuclear physics. Its structure facilitates close interaction between master’s and doctoral students, and senior researchers, thus enhancing both technical training and academic exchange. This model continues to strengthen the foundations of research and collaboration throughout the Brazilian scientific community.

This is the main event for the Brazilian particle- and nuclear-physics communities, reflecting a commitment to advancing research in this highly interactive field. By circulating the venue across multiple regions of Brazil, each edition further renews its mission to cultivate a vibrant and inclusive scientific environment. This edition was closed by a public lecture on QCD by Tereza Mendes (University of São Paolo), who engaged local students with the foundational questions of strong-interaction physics.

The next edition of the Hadrons series will take place in Bahia in 2028.

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