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

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

Compelling evidence

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

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

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

Origins

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

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

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

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

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

The post A new probe of radial flow appeared first on CERN Courier.

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.”

The post STAR hunts QCD critical point appeared first on CERN Courier.

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.

The post Hadrons in Porto Alegre appeared first on CERN Courier.

Heavy-ion collisions usually have very high multiplicities due to colour flow and multiple nucleon interactions. However, when the ions are separated by greater than about twice their radii in so-called ultra-peripheral collisions (UPC), electromagnetic-induced interactions dominate. In these colour-neutral interactions, the ions remain intact and a central system with few particles is produced whose summed transverse momenta, being the Fourier transform of the distance between the ions, is typically less than 100 MeV/c.

In the photoproduction of vector mesons, a photon, radiated from one of the ions, fluctuates into a virtual vector meson long before it reaches the target and then interacts with one or more nucleons in the other ion. The production of ρ mesons has been measured at the LHC by ALICE in PbPb and XeXe collisions, while J/ψ mesons have been measured in PbPb collisions by ALICE, CMS and LHCb. Now, LHCb has isolated a precisely measured, high-statistics sample of di-pions with backgrounds below 1% in which several vector mesons are seen.

Figure 1 shows the invariant mass distribution of the pions, and the fit to the data requires contributions from the ρ meson, continuum ππ, the ω meson and two higher mass resonances at about 1.35 and 1.80 GeV, consistent with excited ρ mesons. The higher structure was also discernible in previous measurements by STAR and ALICE. Since its discovery in 1961, the ρ meson has proved challenging to describe because of its broad width and because of interference effects. More data in the di-pion channel, particularly when practically background-free down almost to production threshold, are therefore welcome. These data may help with hadronic corrections to the prediction of muon g-2: the dip and bump structure at high masses seen by LHCb is qualitatively similar to that observed by BaBar in e⁺e^– → π⁺π^– scattering (CERN Courier March/April 2025 p21). From the invariant mass spectrum, LHCb has measured the cross-sections for ρ, ω, ρ′ and ρ′′ as a function of rapidity in photoproduction on lead nuclei.

Naively, comparison of the photo­production on the nucleus and on the proton should simply scale with the number of nucleons, and can be calculated in the impulse approximation that only takes into account the nuclear form factor, neglecting all other potential nuclear effects.

However, nuclear shadowing, caused by multiple interactions as the meson passes through the nucleus, leads to a suppression (CERN Courier January/February 2025 p31). In addition, there may be further non-linear QCD effects at play.

Elastic re-scattering is usually described through a Glauber calculation that takes account of multiple elastic scatters. This is extended in the GKZ model using Gribov’s formalism to include inelastic scatters. The inset in figure 1 shows the measured differential cross-section for the ρ meson as a function of rapidity for LHCb data compared to the GKZ prediction, to a prediction for the STARlight generator, and to ALICE data at central rapidities. Additional suppression due to nuclear effects is observed above that predicted by GKZ.

The post Clean di-pions reveal vector mesons appeared first on CERN Courier.

The 31st Quark Matter conference took place from 6 to 12 April at Goethe University in Frankfurt, Germany. This edition of the world’s flagship conference for ultra-relativistic heavy-ion physics was the best attended in the series’ history, with more than 1000 participants.

A host of experimental measurements and theoretical calculations targeted fundamental questions in many-body QCD. These included the search for a critical point along the QCD phase diagram, the extraction of the properties of the deconfined quark–gluon plasma (QGP) medium created in heavy-ion collisions, and the search for signatures of the formation of this deconfined medium in smaller collision systems.

Probing thermalisation

New results highlighted the ability of the strong force to thermalise the out-of-equilibrium QCD matter produced during the collisions. Thermalisation can be probed by taking advantage of spatial anisotropies in the initial collision geometry which, due to the rapid onset of strong interactions at early times, result in pressure gradients across the system. These pressure gradients in turn translate into a momentum-space anisotropy of produced particles in the bulk, which can be experimentally measured by taking a Fourier transform of the azimuthal distribution of final-state particles with respect to a reference event axis.

An area of active experimental and theoretical interest is to quantify the degree to which heavy quarks, such as charm and beauty, participate in this collective behaviour, which informs on the diffusion properties of the medium. The ALICE collaboration presented the first measurement of the second-order coefficient of the momentum anisotropy of charm baryons in Pb–Pb collisions, showing significant collective behaviour and suggesting that charm quarks undergo some degree of thermalisation. This collective behaviour appears to be stronger in charm baryons than charm mesons, following similar observations for light flavour.

A host of measurements and calculations targeted fundamental questions in many-body QCD

Due to the nature of thermalisation and the long hydrodynamic phase of the medium in Pb–Pb collisions, signatures of the microscopic dynamics giving rise to the thermalisation are often washed out in bulk observables. However, local excitations of the hydrodynamic medium, caused by the propagation of a high-energy jet through the QGP, can offer a window into such dynamics. Due to coupling to the coloured medium, the jet loses energy to the QGP, which in turn re-excites the thermalised medium. These excited states quickly decay and dissipate, and the local perturbation can partially thermalise. This results in a correlated response of the medium in the direction of the propagating jet, the distribution of which allows measurement of the thermalisation properties of the medium in a more controlled manner.

In this direction, the CMS collaboration presented the first measurement of an event-wise two-point energy–energy correlator, for events containing a Z boson, in both pp and Pb–Pb collisions. The two-point correlator represents the energy-weighted cross section of the angle between particle pairs in the event and can separate out QCD effects at different scales, as these populate different regions in angular phase space. In particular, the correlated response of the medium is expected to appear at large angles in the correlator in Pb–Pb collisions.

The use of a colourless Z boson, which does not interact in the QGP, allows CMS to compare events with similar initial virtuality scales in pp and Pb–Pb collisions, without incurring biases due to energy loss in the QCD probes. The collaboration showed modifications in the two-point correlator at large angles, from pp to Pb–Pb collisions, alluding to a possible signature of the correlated response of the medium to the traversing jets. Such measurements can help guide models into capturing the relevant physical processes underpinning the diffusion of colour information in the medium.

Looking to the future

The next addition of this conference series will take place in 2027 in Jeju, South Korea, and the new results presented there should notably contain the latest complement of results from the upgraded Run 3 detectors at the LHC and the newly commissioned sPHENIX detector at RHIC. New collision systems like O–O at the LHC will help shed light on many of the properties of the QGP, including its thermalisation, by varying the lifetime of the pre-equilibrium and hydrodynamic phases in the collision evolution.

The post Colour information diffuses in Frankfurt appeared first on CERN Courier.

Since the discovery of the electron and proton over 100 years ago, physicists have observed a “zoo” of different types of particles. While some of these particles have been fundamental, like neutrinos and muons, many are composite hadrons consisting of quarks bound together by the exchange of gluons. Studying the zoo of hadrons – their compositions, masses, lifetimes and decay modes – allows physicists to understand the details of the strong interaction, one of the fundamental forces of nature.

The Ω(2012) was discovered by the Belle Collaboration in 2018. The ALICE collaboration recently released an observation of a signal consistent with it with a significance of 15σ in proton–proton (pp) collisions at a centre-of-mass energy of 13 TeV. This is the first observation of the Ω(2012) by another experiment.

While the details of its internal structure are still up for debate, the Ω(2012) consists, at minimum, of three strange quarks bound together. It is a heavier, excited version of the ground-state Ω baryon discovered in 1964, which also contains three strange quarks. Multiple theoretical models predicted a spectrum of excited Ω baryons, with some calling for a state with a mass around 2 GeV. Following the discovery of the Ω(2012), theoretical work has attempted to describe its internal structure, with hypotheses including a simple three-quark baryon or a hadronic molecule.

Using a sample of a billion pp collisions, ALICE has measured the decay of Ω(2012) baryons to Ξ^–K⁰_S pairs. After traveling a few centimetres, these hadrons decay in turn, eventually producing a proton and four charged pions that are tracked by the ALICE detector.

ALICE’s measurements of the mass and width of the Ω(2012) are consistent with Belle’s, and superior precision on the mass. ALICE has also confirmed the rather narrow width of around 6 MeV, which indicates that the Ω(2012) is fairly long-lived for a particle that decays via the strong interaction. Belle and ALICE’s width measurements also lend support to the conclusion that the Ω(2012) has a spin-parity configuration of J^P = 3/2^–.

ALICE also measured the number of Ω(2012) decays to Ξ^–K⁰_S pairs. By comparing this to the total Ω(2012) yield based on statistical thermal model calculations, ALICE has estimated the absolute branching ratio for the Ω(2012) → Ξ^–K⁰ decay. A branching ratio is the probability of decay to a given mode. The ALICE results indicate that Ω(2012) undergoes two-body (ΞK–) decays more than half the time, disfavouring models of the Ω(2012) structure that require large branching ratios for three-body decays.

The present ALICE results will help to improve the theoretical description of the structure of excited baryons. They can also serve as baseline measurements in searches for modifications of Ω-baryon properties in nucleus–nucleus collisions. In the future, Ω(2012) bary­ons may also serve as new probes to study the strangeness enhancement effect observed in proton–proton and nucleus–nucleus collisions.

The post ALICE measures a rare Ω baryon appeared first on CERN Courier.

CERN’s Large Hadron Collider continues to deliver surprises. While searching for additional Higgs bosons, the CMS collaboration may have instead uncovered evidence for the smallest composite particle yet observed in nature – a “quasi-bound” hadron made up of the most massive and shortest-lived fundamental particle known to science and its antimatter counterpart. The findings, which do not yet constitute a discovery claim and could also be susceptible to other explanations, were reported this week at the Rencontres de Moriond conference in the Italian Alps.

Almost all of the Standard Model’s shortcomings motivate the search for additional Higgs bosons. Their properties are usually assumed to be simple. Much as the 125 GeV Higgs boson discovered in 2012 appears to interact with each fundamental fermion with a strength proportional to the fermion’s mass, theories postulating additional Higgs bosons generally expect them to couple more strongly to heavier quarks. This puts the singularly massive top quark at centre stage. If an additional Higgs boson has a mass greater than about 345 GeV and can therefore decay to a top quark–antiquark pair, this should dominate the way it decays inside detectors. Hunting for bumps in the invariant mass spectrum of top–antitop pairs is therefore often considered to be the key experimental signature of additional Higgs bosons above the top–antitop production threshold.

The CMS experiment has observed just such a bump. Intriguingly, however, it is located at the lower limit of the search, right at the top-quark pair production threshold itself, leading CMS to also consider an alternative hypothesis long considered difficult to detect: a top–antitop quasi-bound state known as toponium (see “Threshold excess” figure).

The toponium hypothesis is very exciting as we previously did not expect to be able to see it at the LHC

“When we started the project, toponium was not even considered as a background to this search,” explains CMS physics coordinator Andreas Meyer (DESY). “In our analysis today we are only using a simplified model for toponium – just a generic spin-0 colour-singlet state with a pseudoscalar coupling to top quarks. The toponium hypothesis is very exciting as we previously did not expect to be able to see it at the LHC.”

Though other explanations can’t be ruled out, CMS finds the toponium hypothesis to be sufficient to explain the observed excess. The size of the excess is consistent with the latest theoretical estimate of the cross section to produce pseudoscalar toponium of around 6.4 pb.

“The cross section we obtain for our simplified hypothesis is 8.8 pb with an uncertainty of about 15%,” explains Meyer. “One can infer that this is significantly above five sigma.”

The smallest hadron

If confirmed, toponium would be the final example of quarkonium – a term for quark–antiquark states formed from heavy charm, bottom and perhaps top quarks. Charmonium (charm–anticharm) mesons were discovered at SLAC and Brookhaven National Laboratory in the November Revolution of 1974. Bottomonium (bottom–antibottom) mesons were discovered at Fermilab in 1977. These heavy quarks move relatively slowly compared to the speed of light, allowing the strong interaction to be modelled by a static potential as a function of the separation between them. When the quarks are far apart, the potential is proportional to their separation due to the self-interacting gluons forming an elongating flux tube, yielding a constant force of attraction. At close separations, the potential is due to the exchange of individual gluons and is Coulomb-like in form, and inversely proportional to separation, leading to an inverse-square force of attraction. This is the domain where compact quarkonium states are formed, in a near perfect QCD analogy to positronium, wherein an electron and a positron are bound by photon exchange. The Bohr radii of the ground states of charmonium and bottomonium are approximately 0.3 fm and 0.2 fm, and bottomonium is thought to be the smallest hadron yet discovered. Given its larger mass, toponium’s Bohr radius would be an order of magnitude smaller.

For a long time it was thought that toponium bound states were unlikely to be detected in hadron–hadron collisions. The top quark is the most massive and the shortest-lived of the known fundamental particles. It decays into a bottom quark and a real W boson in the time it takes light to travel just 0.1 fm, leaving little time for a hadron to form. Toponium would be unique among quarkonia in that its decay would be triggered by the weak decay of one of its constituent quarks rather than the annihilation of its constituent quarks into photons or gluons. Toponium is expected to decay at twice the rate of the top quark itself, with a width of approximately 3 GeV.

CMS first saw a 3.5 sigma excess in a 2019 search studying the mass range above 400 GeV, based on 35.9 fb⁻¹ of proton–proton collisions at 13 TeV from 2016. Now armed with 138 fb^–1 of collisions from 2016 to 2018, the collaboration extended the search down to the top–antitop production threshold at 345 GeV. Searches are complicated by the possibility that quantum interference between background and Higgs signal processes could generate an experimentally challenging peak–dip structure with a more or less pronounced bump.

“The signal reported by CMS, if confirmed, could be due either to a quasi-bound top–antitop meson, commonly called ‘toponium’, or possibly an elementary spin-zero boson such as appears in models with additional Higgs bosons, or conceivably even a combination of the two,” says theorist John Ellis of King’s College London. “The mass of the lowest-lying toponium state can be calculated quite accurately in QCD, and is expected to lie just below the nominal top–antitop threshold. However, this threshold is smeared out by the short lifetime of the top quark, as well as the mass resolution of an LHC detector, so toponium would appear spread out as a broad excess of events in the final states with leptons and jets that generally appear in top decays.”

Quantum numbers

An important task of the analysis is to investigate the quantum numbers of the signal. It could be a scalar particle, like the Higgs boson discovered in 2012, or a pseudoscalar particle – a different type of spin-0 object with odd rather than even parity. To measure its spin-parity, CMS studied the angular correlations of the top-quark-pair decay products, which retain information on the original quantum state. The decays bear all the experimental hallmarks of a pseudoscalar particle, consistent with toponium (see “Angular analysis” figure) or the pseudoscalar Higgs bosons common to many theories featuring extended Higgs sectors.

“The toponium state produced at the LHC would be a pseudoscalar boson, whose decays into these final states would have characteristic angular distributions, and the excess of events reported by CMS exhibits the angular correlations expected for such a pseudoscalar state,” explains Ellis. “Similar angular correlations would be expected in the decays of an elementary pseudoscalar boson, whereas scalar-boson decays would exhibit different angular correlations that are disfavoured by the CMS analysis.”

Whatever the true cause of the excess, the analyses reflect a vibrant programme of sensitive measurements at the LHC – and the possibility of a timely discovery

Two main challenges now stand in the way of definitively identifying the nature of the excess. The first is to improve the modelling of the creation of top-quark pairs at the LHC, including the creation of bound states at the threshold. The second challenge is to obtain consistency with the ATLAS experiment. “ATLAS had similar studies in the past but with a more conservative approach on the systematic uncertainties,” says ATLAS physics coordinator Fabio Cerutti (LBNL). “This included, for example, larger uncertainties related to parton showers and other top-modelling effects. To shed more light on the CMS observation, be it a new boson, a top quasi-bound state, or some limited understanding of the modelling of top–antitop production at threshold, further studies are needed on our side. We have several analysis teams working on that. We expect to have new results with improved modelling of the top-pair production at threshold and additional variables sensitive to both a new pseudo-scalar boson or a top quasi-bounded state very soon.”

Whatever the true cause of the excess, the analyses reflect a vibrant programme of sensitive measurements at the LHC – and the possibility of a timely discovery.

“Discovering toponium 50 years after the November Revolution would be an unanticipated and welcome golden anniversary present for its charmonium cousin that was discovered in 1974,” concludes Ellis. “The prospective observation and measurement of the vector state of toponium in e⁺e^– collisions around 350 GeV have been studied in considerable theoretical detail, but there have been rather fewer studies of the observability of pseudoscalar toponium at the LHC. In addition to the angular correlations observed by CMS, the effective production cross section of the observed threshold effect is consistent with non-relativistic QCD calculations. More detailed calculations will be desirable for confirmation that another quarkonium family member has made its appearance, though the omens are promising.”

The post CMS observes top–antitop excess appeared first on CERN Courier.

Fundamental charged particles have spins that wobble in a magnetic field. This is just one of the insights that emerged from the equation Paul Dirac wrote down in 1928. Almost 100 years later, calculating how much they wobble – their “magnetic moment” – strains the computational sinews of theoretical physicists to a level rarely matched. The challenge is to sum all the possible ways in which the quantum fluctuations of the vacuum affect their wobbling.

The particle in question here is the muon. Discovered in cosmic rays in 1936, muons are more massive but ephemeral cousins of the electron. Their greater mass is expected to amplify the effect of any undiscovered new particles shimmering in the quantum haze around them, and measurements have disagreed with theoretical predictions for nearly 20 years. This suggests a possible gap in the Standard Model (SM) of particle physics, potentially providing a glimpse of deeper truths beyond it.

In the coming weeks, Fermilab is expected to present the final results of a seven-year campaign to measure this property, reducing uncertainties to a remarkable one part in 10¹⁰ on the magnetic moment of the muon, and 0.1 parts per million on the quantum corrections. Theorists are racing to match this with an updated prediction of comparable precision. The calculation is in good shape, except for the incredibly unusual eventuality that the muon briefly emits a cloud of quarks and gluons at just the moment it absorbs a photon from the magnetic field. But in quantum mechanics all possibilities count all the time, and the experimental precision is such that the fine details of “hadronic vacuum polarisation” (HVP) could be the difference between reinforcing the SM and challenging it.

Quantum fluctuations

The Dirac equation predicts that fundamental spin s = ½ particles have a magnetic moment given by g(eħ/2m)s, where the gyromagnetic ratio (g) is precisely equal to two. For the electron, this remarkable result was soon confirmed by atomic spectroscopy, before more precise experiments in 1947 indicated a deviation from g = 2 of a few parts per thousand. Expressed as a = (g-2)/2, the shift was a surprise and was named the magnetic anomaly or the anomalous magnetic moment.

This marked the beginning of an enduring dialogue between experiment and theory. It became clear that a relativistic field theory like the developing quantum electrodynamics (QED) could produce quantum fluctuations, shifting g from two. In 1948, Julian Schwinger calculated the first correction to be a = α/2π ≈ 0.00116, aligning beautifully with 1947 experimental results. The emission and absorption of a virtual photon creates a cloud around the electron, altering its interaction with the external magnetic field (see “Quantum fluctuation” figure). Soon, other particles would be seen to influence the calculations. The SM’s limitations suggest that undiscovered particles could also affect these calculations. Their existence might be revealed by a discrepancy between the SM prediction for a particle’s anomalous magnetic moment and its measured value.

As noted, the muon is an even more promising target than the electron, as its sensitivity to physics beyond QED is generically enhanced by the square of the ratio of their masses: a factor of around 43,000. In 1957, inspired by Tsung-Dao Lee and Chen-Ning Yang’s proposal that parity is violated in the weak interaction, Richard Garwin, Leon Lederman and Marcel Weinrich studied the decay of muons brought to rest in a magnetic field at the Nevis cyclotron at Columbia University. As well as showing that parity is broken in both pion and muon decays, they found g to be close to two for muons by studying their “precession” in the magnetic field as their spins circled around the field lines.

This iconic experiment was the prototype of muon-precession projects at CERN (see CERN Courier September/October 2024 p53), later at Brookhaven National Laboratory and now Fermilab (see “Precision” figure). By the end of the Brookhaven project, a disagreement between the measured value of “a_μ” – the subscript indicating g-2 for the muon rather than the electron – and the SM prediction was too large to ignore, motivating the present round of measurements at Fermilab and rapidly improving theory refinements.

g-2 and the Standard Model

Today, a prediction for a_μ must include the effects of all three of the SM’s interactions and all of its elementary particles. The leading contributions are from electrons, muons and tau leptons interacting electromagnetically. These QED contributions can be computed in an expansion where each successive term contributes only around 1% of the previous one. QED effects have been computed to fifth order, yielding an extraordinary precision of 0.9 parts per billion – significantly more precise than needed to match measurements of the muon’s g-2, though not the electron’s. It took over half a century to achieve this theoretical tour de force.

The weak interaction gives the smallest contribution to a_μ, a million times less than QED. These contributions can also be computed in an expansion. Second order suffices. All SM particles except gluons need to be taken into account.

Gluons are responsible for the strong interaction and appear in the third and last set of contributions. These are described by QCD and are called “hadronic” because quarks and gluons form hadrons at the low energies relevant for the muon g-2 (see “Hadronic contributions” figure). HVP is the largest, though 10,000 times smaller than the corrections due to QED. “Hadronic light-by-light scattering” (HLbL) is a further 100 times smaller due to the exchange of an additional photon. The challenge is that the strong-interaction effects cannot be approximated by a perturbative expansion. QCD is highly nonlinear and different methods are needed.

Data or the lattice?

Even before QCD was formulated, theorists sought to subdue the wildness of the strong force using experimental data. In the case of HVP, this triggered experimental investigations of e⁺e^– annihilation into hadrons and later hadronic tau–lepton decays. Though apparently disparate, the production of hadrons in these processes can be related to the clouds of virtual quarks and gluons that are responsible for HVP.

A more recent alternative makes use of massively parallel numerical simulations to directly solve the equations of QCD. To compute quantities such as HVP or HLbL, “lattice QCD” requires hundreds of millions of processor-core hours on the world’s largest supercomputers.

In preparation for Fermilab’s first measurement in 2021, the Muon g-2 Theory Initiative, spanning more than 120 collaborators from over 80 institutions, was formed to provide a reference SM prediction that was published in a 2020 white paper. The HVP contribution was obtained with a precision of a few parts per thousand using a compilation of measurements of e⁺e^– annihilation into hadrons. The HLbL contribution was determined from a combination of data-driven and lattice–QCD methods. Though even more complex to compute, HLbL is needed only to 10% precision, as its contribution is smaller.

After summing all contributions, the prediction of the 2020 white paper sits over five standard deviations below the most recent experimental world average (see “Landscape of muon g-2” figure). Such a deviation would usually be interpreted as a discovery of physics beyond the SM. However, in 2021 the result of the first lattice calculation of the HVP contribution with a precision comparable to that of the data-driven white paper was published by the Budapest–Marseille–Wuppertal collaboration (BMW). The result, labelled BMW 2020 as it was uploaded to the preprint archive the previous year, is much closer to the experimental average (green band on the figure), suggesting that the SM may still be in the race. The calculation relied on methods developed by dozens of physicists since the seminal work of Tom Blum (University of Connecticut) in 2002 (see CERN Courier May/June 2021 p25).

In 2020, the uncertainties on the data-driven and lattice-QCD predictions for the HVP contribution were still large enough that both could be correct, but BMW’s 2021 paper showed them to be explicitly incompatible in an “intermediate-distance window” accounting for approximately 35% of the HVP contribution, where lattice QCD is most reliable.

This disagreement was the first sign that the 2020 consensus had to be revised. To move forward, the sources of the various disagreements – more numerous now – and the relative limitations of the different approaches must be understood better. Moreover, uncertainty on HVP already dominated the SM prediction in 2020. As well as resolving these discrepancies, its uncertainty must be reduced by a factor of three to fully leverage the coming measurement from Fermilab. Work on the HVP is therefore even more critical than before, as elsewhere the theory house is in order: Sergey Volkov (KITP) recently verified the fifth-order QED calculation of Tatsumi Aoyama, Toichiro Kinoshita and Makiko Nio, identifying an oversight not numerically relevant at current experimental sensitivities; new HLbL calculations remain consistent; and weak contributions have already been checked and are precise enough for the foreseeable future.

News from the lattice

Since BMW’s 2020 lattice results, a further eight lattice-QCD computations of the dominant up-and-down-quark (u + d) contribution to HVP’s intermediate-distance window have been performed with similar precision, with four also including all other relevant contributions. Agreement is excellent and the verdict is clear: the disagreement between the lattice and data-driven approaches is confirmed (see “Intermediate window” figure).

Work on the short-distance window (about 10% of the HVP contribution) has also advanced rapidly. Seven computations of the u + d contribution have appeared, with four including all other relevant contributions. No significant disagreement is observed.

The long-distance window (around 55% of the total) is by far the most challenging, with the largest uncertainties. In recent weeks three calculations of the dominant u + d contribution have appeared, by the RBC–UKQCD, Mainz and FHM collaborations. Though some differences are present, none can be considered significant for the time being.

With all three windows cross-validated, the Muon g-2 Theory Initiative is combining results to obtain a robust lattice–QCD determination of the HVP contribution. The final uncertainty should be slightly below 1%, still quite far from the 0.2% ultimately needed.

The BMW–DMZ and Mainz collaborations have also presented new results for the full HVP contribution to a_μ, and the RBC–UKQCD collaboration, which first proposed the multi-window approach, is also in a position to make a full calculation. (The corresponding result in the “Landscape of muon g-2” figure combines contributions reported in their publications.) Mainz obtained a result with 1% precision using the three windows described above. BMW–DMZ divided its new calculation into five windows and replaced the lattice–QCD computation of the longest distance window – “the tail”, encompassing just 5% of the total – with a data-driven result. This pragmatic approach allows a total uncertainty of just 0.46%, with the collaboration showing that all e⁺e^– datasets contributing to this long-distance tail are entirely consistent. This new prediction differs from the experimental measurement of a_μ by only 0.9 standard deviations.

These new lattice results, which have not yet been published in refereed journals, make the disagreement with the 2020 data-driven result even more blatant. However, the analysis of the annihilation of e⁺e^– into hadrons is also evolving rapidly.

News from electron–positron annihilation

Many experiments have measured the cross-section for e⁺e^– annihilation to hadrons as a function of centre-of-mass energy (√s). The dominant contribution to a data-driven calculation of a_μ, and over 70% of its uncertainty budget, is provided by the e⁺e^– → π⁺π^– process, in which the final-state pions are produced via the ρ resonance (see “Two-pion channel” figure).

The most recent measurement, by the CMD-3 energy-scan experiment in Novosibirsk, obtained a cross-section on the peak of the ρ resonance that is larger than all previous ones, significantly changing the picture in the π⁺π^– channel. Scrutiny by the Theory Initiative has identified no major problem.

CMD-3’s approach contrasts that used by KLOE, BaBar and BESIII, which study e⁺e^– annihilation with a hard photon emitted from the initial state (radiative return) at facilities with fixed √s. BaBar has innovated by calibrating the luminosity of the initial-state radiation using the μ⁺μ^– channel and using a unique “next-to-leading-order” approach that accounts for extra radiation from either the initial or the final state – a necessary step at the required level of precision.

In 1997, Ricard Alemany, Michel Davier and Andreas Höcker proposed an alternative method that employs τ^– → π^–π⁰ν decay while requiring some additional theoretical input. The decay rate has been precisely measured as a function of the two-pion invariant mass by the ALEPH and OPAL experiments at LEP, as well as by the Belle and CLEO experiments at B factories, under very different conditions. The measurements are in good agreement. ALEPH offers the best normalisation and Belle the best shape measurement.

KLOE and CMD-3 differ by more than five standard deviations on the ρ peak, precluding a combined analysis of e⁺e^– → π⁺π^– cross-sections. BaBar and τ data lie between them. All measurements are in good agreement at low energies, below the ρ peak. BaBar, CMD-3 and τ data are also in agreement above the ρ peak. To help clarify this unsatisfactory situation, in 2023 BaBar performed a careful study of radiative corrections to e⁺e^– → π⁺π^–. That study points to the possible underestimate of systematic uncertainties in radiative-return experiments that rely on Monte Carlo simulations to describe extra radiation, as opposed to the in situ studies performed by BaBar.

The future

While most contributions to the SM prediction of the muon g-2 are under control at the level of precision required to match the forthcoming Fermilab measurement, in trying to reduce the uncertainties of the HVP contribution to a commensurate degree, theorists and experimentalists shattered a 20 year consensus. This has triggered an intense collective effort that is still in progress.

The prospect of testing the limits of the SM through high-precision measurements generates considerable impetus

New analyses of e⁺e^– are underway at BaBar, Belle II, BES III and KLOE, experiments are continuing at CMD-3, and Belle II is also studying τ decays. At CERN, the longer term “MUonE” project will extract HVP by analysing how muons scatter off electrons – a very challenging endeavour regarding the unusual accuracy required both in the control of experimental systematic uncertainties and also theoretically, for the radiative corrections.

At the same time, lattice-QCD calculations have made enormous progress in the last five years and provide a very competitive alternative. The fact that several groups are involved with somewhat independent techniques is allowing detailed cross checks. The complementarity of the data-driven and lattice-QCD approaches should soon provide a reliable value for the g-2 theoretical prediction at unprecedented levels of precision.

There is still some way to go to reach that point, but the prospect of testing the limits of the SM through high-precision measurements generates considerable impetus. A new white paper is expected in the coming weeks. The ultimate aim is to reach a level of precision in the SM prediction that allows us to fully leverage the potential of the muon anomalous magnetic moment in the search for new fundamental physics, in concert with the final results of Fermilab’s Muon g-2 experiment and the projected Muon g-2/EDM experiment at J-PARC in Japan, which will implement a novel technique.

The post Do muons wobble faster than expected? appeared first on CERN Courier.

Ever since quarks and gluons were discovered, scientists have been gathering clues about their nature and behaviour. When quarks and gluons – collectively called partons – are produced at particle colliders, they shower to form jets – sprays of composite particles called hadrons. The study of jets has been indispensable towards understanding quantum chromodynamics (QCD) and the description of the final state using parton shower models. Recently, particular focus has been on the study of the jet substructure, which provides further input about the modelling of parton showers.

Jets initiated by the heavy charm (c-jets) or bottom quarks (b-jets) provide insight into the role of the quark mass, as an additional energy scale in QCD calculations. Heavy-flavour jets are not only used to test QCD predictions, they are also a key part of the study of other particles, such as the top quark and the Higgs boson. Understanding the internal structure of heavy-quark jets is thus crucial for both the identification of these heavier objects and the interpretation of QCD properties. One such property is the presence of a “dead cone” around the heavy quark, where collinear gluon emissions are suppressed in the direction of motion of the quark.

CMS has shed light on the role of the quark mass in the parton shower with two new results focusing on c- and b-jets, respectively. Heavy-flavour hadrons in these jets are typically long-lived, and decay at a small but measurable distance from the primary interaction vertex. In c-jets, the D⁰ meson is reconstructed in the K^±π^∓ decay channel by combining pairs of charged hadrons that do not appear to come from the primary interaction vertex. In the case of b-jets, a novel technique is employed. Instead of reconstructing the b hadron in a given decay channel, its charged decay daughters are identified using a multivariate analysis. In both cases, the decay daughters are replaced by the mother hadron in the jet constituents.

CMS has shed light on the role of the quark mass in the parton shower

Jets are reconstructed by clustering particles in a pairwise manner, leading to a clustering tree that mimics the parton shower process. Substructure techniques are then employed to decompose the jet into two subjets, which correspond to the heavy quark and a gluon being emitted from it. Two of those algorithms are soft drop and late-k_T. They select the first and last emission in the jet clustering tree, respectively, capturing different aspects of the QCD shower. Looking at the angle between the two subjets (see figure 1), denoted as R_g for soft drop and θ_ℓ for late-k_T, demonstrates the dead-cone effect, as the small angle emissions of b-jets (left) and c-jets (right) are suppressed compared to the inclusive jet case. The effect is captured better by the late-k_T algorithm than soft drop in the case of c-jets.

These measurements serve to refine the tuning of Monte Carlo event generators relating to the heavy-quark mass and strong coupling. Identifying the onset of the dead cone in the vacuum also opens up possibilities for substructure studies in heavy-ion collisions, where emissions induced by the strongly interacting quark–gluon plasma can be isolated.

The post CMS peers inside heavy-quark jets appeared first on CERN Courier.

Collisions between lead ions at the LHC generate the hottest and densest system ever created in the laboratory. Under these extreme conditions, quarks and gluons are no longer confined inside hadrons but instead form a quark–gluon plasma (QGP). Being heavier than the more abundantly produced light quarks, charm quarks play a special role in probing the plasma since they are created in the collision before the plasma is formed and interact with the plasma as they traverse the collision zone. Charm jets, which are clusters of particles originating from charm quarks, have been investigated for the first time by the ALICE collaboration in Pb–Pb collisions at the LHC using the D⁰ mesons (that carry a charm quark) as tags.

The primary interest lies in measuring the extent of energy loss experienced by different types of particles as they traverse the plasma, referred to as “in-medium energy loss”. This energy loss specifically depends on the particle type and particle mass, varying between quarks and gluons. Due to their larger mass, charm quarks at low transverse momentum do not reach the speed of light and lose substantially less energy than light quarks through both collisional and radiative processes, as gluon radiation by massive quarks is suppressed: the so-called “dead-cone effect”. Additionally, gluons, which carry a larger colour charge than quarks, experience greater energy loss in the QGP as quantified by the Casimir factors C_A = 3 for gluons and C_F = 4/3 for quarks. This makes the charm quark an ideal probe for studying the QGP properties. ALICE is well suited to study the in-medium energy loss of charm quarks, which is dependent on the mass of the charm quark and its colour charge.

The production yield of charm jets tagged with fully reconstructed D⁰ mesons (D⁰ → K^–π⁺) in central Pb–Pb collisions at a centre-of-mass energy of 5.02 TeV per nucleon pair during LHC Run 2 was measured by ALICE. The results are reported in terms of nuclear modification factor (R_AA), which is the ratio of the particle production rate in Pb–Pb collisions to that in proton–proton collisions, scaled by the number of binary nucleon–nucleon collisions. A measured nuclear modification factor of unity would indicate the absence of final-state effects.

The results, shown in figure 1, show a clear suppression (R_AA < 1) for both charm jets and inclusive jets (that mainly originate from light quarks and gluons) due to energy loss. Importantly, the charm jets exhibit less suppression than the inclusive jets within the transverse momentum range of 20 to 50 GeV, which is consistent with mass and colour-charge dependence.

The measured results are compared with theoretical model calculations that include mass effects in the in-medium energy loss. Among the different models, LIDO incorporates both the dead-cone effect and the colour-charge effects, which are essential for describing the energy-loss mechanisms. Consequently, it shows reasonable agreement with experimental data, reproducing the observed hierarchy between charm jets and inclusive jets.

The present finding provides a hint of the flavour-dependent energy loss in the QGP, suggesting that charm jets lose less energy than inclusive jets. This highlights the quark-mass and colour-charge dependence of the in-medium energy-loss mechanisms.

The post Charm jets lose less energy appeared first on CERN Courier.

The four main LHC experiments played a prominent role at the conference, presenting a large set of newly published results from studies performed on data collected during LHC Run 2, as well as several new preliminary results performed on the new data samples from Run 3.

Jet modifications

A number of significant results on the modification of jets in heavy-ion collisions were presented. Splitting functions characterising the evolution of parton showers are expected to be modified in the presence of the QGP, providing experimental access to the medium properties. A more differential look at these modifications was presented through a correlated measurement of the shared momentum fraction and opening angle of the first splitting satisfying the “soft drop” condition in jets. Additionally, energy–energy correlators have recently emerged as promising observables where the properties of jet modification in the medium might be imprinted at different scales on the observable.

The first measurements of the two-particle energy–energy correlators in p–Pb and Pb–Pb collisions were presented, showing modifications in both the small- and large-angle correlations for both systems compared to pp collisions. A long-sought after effect of energy exchanges between the jet and the medium is a correlated response of the medium in the jet direction. For the first time, measurements of hadron–boson correlations in events containing photons or Z bosons showed a clear depletion of the bulk medium in the direction of the Z boson, providing direct evidence of a medium response correlated to the propagating back-to-back jet. In pp collisions, the first direct measurement of the dead cone of beauty quarks, using novel machine-learning methods to reconstruct the beauty hadron from partial decay information, was also shown.

Several new results from studies of particle production in ultraperipheral heavy-ion collisions were discussed. These studies allow us to investigate the possible onset of gluon saturation at low Bjorken-x values. In this context, new results of charm photoproduction, with measurements of incoherent and coherent J/ψ mesons, as well as of D⁰ mesons, were released. Photonuclear production cross-sections of di-jets, covering a large interval of photon energies to scan over different regions of Bjorken-x, were also presented. These measurements pave the way for setting constraints on the gluon component of nuclear parton distribution functions at low Bjorken-x values, over a wide Q² range, in the absence of significant final-state effects.

New experiments will explore higher-density regions of the QCD–matter phase diagram

During the last few years, a significant enhancement of charm and beauty-baryon production in proton–proton collisions was observed, compared to measurements in e⁺e^– and ep collisions. These observations have challenged the assumption of the universality of heavy-quark fragmentation across different collision systems. Several intriguing measurements on this topic were released at the conference. In addition to an extended set of charm meson-to-meson and baryon-to-meson production yield ratios, the first measurements of the production of Σ_c^0,++(2520) relative to Σ_c^0,++(2455) at the LHC, obtained exploiting the new Run 3 data samples, were discussed. New insights on the structure of the exotic χ_c1(3872) state and its hadronisation mechanism were garnered by measuring the ratio of its production yield to that of ψ(2S) mesons in hadronic collisions.

Additionally, strange-to-non-strange production-yield ratios for charm and beauty mesons as a function of the collision multiplicity were released, pointing toward an enhanced strangeness production in a higher colour-density environment. Several theoretical approaches implementing modified hadronisation mechanisms with respect to in-vacuum fragmentation have proven to be able to reproduce at least part of the measurements, but a comprehensive description of the heavy-quark hadronisation, in particular for the baryonic sector, is still to be reached.

A glimpse into the future of the experimental opportunities in this field was also provided. A new and intriguing set of physics observables for a complete characterisation of the QGP with hard probes will become accessible with the planned upgrades of the ALICE, ATLAS, CMS and LHCb detectors, both during the next long LHC shutdown and in the more distant future. New experiments at CERN, such as NA60+, or in other facilities like the Electron–Ion Collider in the US and J-PARC-HI in Japan, will explore higher-density regions of the QCD–matter phase diagram.

The next edition of this conference series is scheduled to be held in Nashville, US, from 1 to 5 June 2026.

The post Probing the quark–gluon plasma in Nagasaki appeared first on CERN Courier.

“When Wojciech got started, we thought it would be a trivial verification of the symmetry,” says Marek Gaździcki of Jan Kochanowski University of Kielce, spokesperson of NA61/SHINE at the time of the discovery. “We expected it to be closely obeyed – though we had previously measured discrepancies at NA49, they had large uncertainties and were not significant.”

Isospin symmetry is one facet of flavour symmetry, whereby the strong interaction treats all quark flavours identically, except for kinematic differences arising from their different masses. Strong interactions should therefore generate nearly equal yields of charged K⁺ (us) and K^– (us), and neutral K⁰ (ds) and K⁰ (ds), given the similar masses of the two lightest quarks. NA61/SHINE’s data contradict the hypothesis of equal yields with 4.7σ significance.

“I see two options to interpret the results,” says Francesco Giacosa, a theo­retical physicist at Jan Kochanowski University working with NA61/SHINE. “First, we substantially underestimate the role of electromagnetic interactions in creating quark–antiquark pairs. Second, strong interactions do not obey flavour symmetry – if so, this would falsify QCD.” Isospin is not a symmetry of the electromagnetic interaction as up and down quarks have different electric charges.

While the experiment routinely measures particle yields in nuclear collisions, finding a discrepancy in isospin symmetry was not something researchers were actively looking for. NA61/SHINE’s primary focus is studying the phase diagram of high-energy nuclear collisions using a range of ion beams. This includes looking at the onset of deconfinement, the formation of a quark-gluon plasma fireball, and the search for the hypothesised QCD critical point where the transition between hadronic matter and quark–gluon plasma changes from a smooth crossover to a first-order phase transition. Data is also shared with neutrino and cosmic-ray experiments to help refine their models.

The collaboration is now planning additional studies using different projectiles, targets and collision energies to determine whether this effect is unique to certain heavy-ion collisions or a more general feature of high-energy interactions. They have also put out a call to theorists to help explain what might have caused such an unexpectedly large asymmetry.

“The observation of the rather large isospin violation stands in sharp contrast to its validity in a wide range of physical systems,” says Rob Pisarski, a theoretical physicist from Brookhaven National Laboratory. “Any explanation must be special to heavy-ion systems at moderate energy. NA61/SHINE’s discrepancy is clearly significant, and shows that QCD still has the power to surprise our naive expectations.”

The post Isospin symmetry broken more than expected appeared first on CERN Courier.

Much remains to be understood about gluon dynamics. At present, the chief experimental challenge is to observe the onset of gluon saturation – a dynamic equilibrium between gluon splitting and recombination predicted by QCD. The experimental key looks likely to be a rare but intriguing type of LHC interaction known as an ultra­peripheral collision (UPC), and the breakthrough may come as soon as the next experimental run.

Gluon saturation is expected to end the rapid growth in gluon density measured at the HERA electron–proton collider at DESY in the 1990s and 2000s. HERA observed this growth as the energy of interactions increased and as the fraction of the proton’s momentum borne by the gluons (Bjorken x) decreased.

So gluons become more numerous in hadrons as their energy decreases – but to what end?

Gluonic hotspots are now being probed with unprecedented precision at the LHC and are central to understanding the high-energy regime of QCD

Nonlinear effects are expected to arise due to processes like gluon recombination, wherein two gluons combine to become one. When gluon recombination becomes a significant factor in QCD dynamics, gluon saturation sets in – an emergent phenomenon whose energy scale is a critical parameter to determine experimentally. At this scale, gluons begin to act like classical fields and gluon density plateaus. A dilute partonic picture transitions to a dense, saturated state. For recombination to take precedence over splitting, gluon momenta must be very small, corresponding to low values of Bjorken x. The saturation scale should also be directly proportional to the colour-charge density, making heavy nuclei like lead ideal for studying nonlinear QCD phenomena.

But despite strong theoretical reasoning and tantalising experimental hints, direct evidence for gluon saturation remains elusive.

Since the conclusion of the HERA programme, the quest to explore gluon saturation has shifted focus to the LHC. But with no point-like electron to probe the hadronic target, LHC physicists had to find a new point-like probe: light itself. UPCs at the LHC exploit the flux of quasi-real high-energy photons generated by ultra-relativistic particles. For heavy ions like lead, this flux of photons is enhanced by the square of the nuclear charge, enabling studies of photon-proton (γp) and photon-nucleus interactions at centre-of-mass energies reaching the TeV scale.

Keeping it clean

What really sets UPCs apart is their clean environment. UPCs occur at large impact parameters well outside the range of the strong nuclear force, allowing the nuclei to remain intact. Unlike hadronic collisions, which can produce thousands of particles, UPCs often involve only a few final-state particles, for example a single J/ψ, providing an ideal laboratory for gluon saturation. J/ψ are produced when a cc pair created by two or more gluons from one nucleus is brought on-shell by interacting with a quasi-real photon from the other nucleus (see “Sensitivity to saturation” figure).

Gluon saturation models predict deviations in the γp → J/ψp cross section from the power-law behaviour observed at HERA. The LHC experiments are placing a significant focus on investigating the energy dependence of this process to identify potential signatures of saturation, with ALICE and LHCb extending studies to higher γp centre-of-mass energies (W_γp) and lower Bjorken x than HERA. The results so far reveal that the cross-section continues to increase with energy, consistent with the power-law trend (see “Approaching the plateau?” figure).

The symmetric nature of pp collisions introduces significant challenges. In pp collisions, either proton can act as the photon source, leading to an intrinsic ambiguity in identifying the photon emitter. In proton–lead (pPb) collisions, the lead nucleus overwhelmingly dominates photon emission, eliminating this ambiguity. This makes pPb collisions an ideal environment for precise studies of the photoproduction of J/ψ by protons.

During LHC Run 1, the ALICE experiment probed W_γp up to 706 GeV in pPb collisions, more than doubling HERA’s maximum reach of 300 GeV. This translates to probing Bjorken-x values as low as 10^–5, significantly beyond the regime explored at HERA. LHCb took a different approach. The collaboration inferred the behaviour of pp collisions at high energies (“W+ solutions”) by assuming knowledge of their energy dependence at low energies (“W- solutions”), allowing LHCb to probe gluon energies as small as 10^–6 in Bjorken x and W_γp up to 2 TeV.

There is not yet any theoretical consensus on whether LHC data align with gluon-saturation predictions, and the measurements remain statistically limited, leaving room for further exploration. Theoretical challenges include incomplete next-to-leading-order calculations and the reliance of some models on fits to HERA data. Progress will depend on robust and model-independent calculations and high-quality UPC data from pPb collisions in LHC Run 3 and Run 4.

Some models predict a slowing increase in the γp → J/ψp cross section with energy at small Bjorken x. If these models are correct, gluon saturation will likely be discovered in LHC Run 4, where we expect to see a clear observation of whether pPb data deviate from the power law observed so far.

Gluonic hotspots

If a UPC photon interacts with the collective colour field of a nucleus – coherent scattering – it probes its overall distribution of gluons. If a UPC photon interacts with individual nucleons or smaller sub-nucleonic structures – incoherent scattering – it can probe smaller-scale gluon fluctuations.

These fluctuations, known as gluonic hotspots, are theorised to become more numerous and overlap in the regime of gluon saturation (see “Onset of saturation” figure). Now being probed with unprecedented precision at the LHC, they are central to understanding the high-energy regime of QCD.

Gluonic hotspots are used to model the internal transverse structure of colliding protons or nuclei (see “Hotspot snapshots” figure). The saturation scale is inherently impact-parameter dependent, with the densest colour charge densities concentrated at the core of the proton or nucleus, and diminishing toward the periphery, though subject to fluctuations. Researchers are increasingly interested in exploring how these fluctuations depend on the impact parameter of collisions to better characterise the spatial dynamics of colour charge. Future analyses will pinpoint contributions from localised hotspots where saturation effects are most likely to be observed.

The energy dependence of incoherent or dissociative photoproduction promises a clear signature for gluon saturation, independent of the coherent power-law method described above. As saturation sets in, all gluon configurations in the target converge to similar densities, causing the variance of the gluon field to decrease, and with it the dissociative cross section. Detecting a peak and a decline in the incoherent cross-section as a function of energy would represent a clear signature of gluon saturation.

The ALICE collaboration has taken significant steps in exploring this quantum terrain, demonstrating the possibility of studying different geometrical configurations of quantum fluctuations in processes where protons or lead nucleons dissociate. The results highlight a striking correlation between momentum transfer, which is inversely proportional to the impact parameter, and the size of the target structure. The observation that sub-nucleonic structures impart the greatest momentum transfer is compelling evidence for gluonic quantum fluctuations at the sub-nucleon level.

Into the shadows

In 1982 the European Muon Collaboration observed an intriguing phenomenon: nuclei appeared to contain fewer gluons than expected based on the contributions from their individual protons and neutrons. This effect, known as nuclear shadowing, was observed in experiments conducted at CERN at moderate values of Bjorken x. It is now known to occur because the interaction of a probe with one gluon reduces the likelihood of the probe interacting with other gluons within the nucleus – the gluons hiding behind them, in their shadow, so to speak. At smaller values of Bjorken x, saturation further suppresses the number of gluons contributing to the interaction.

The relationship between gluon saturation and nuclear shadowing is poorly understood, and separating their effects remains an open challenge. The situation is further complicated by an experimental reliance on lead–lead (PbPb) collisions, which, like pp collisions, suffer from ambiguity in identifying the interacting nucleus, unless the interaction is accompanied by an ejected neutron.

The ALICE, CMS and LHCb experiments have extensively studied nuclear shadowing via the exclusive production of vector mesons such as J/ψ in ultraperipheral PbPb
collisions. Results span photon–nucleus collision energies from 10 to 1000 GeV. The onset of nuclear shadowing, or another nonlinear QCD phenomenon like saturation, is clearly visible as a function of energy and Bjorken x (see “Nuclear shadowing” figure).

Multidimensional maps

While both saturation-based and gluon shadowing models describe the data reasonably well at high energies, neither framework captures the observed trends across the entire kinematic range. Future efforts must go beyond energy dependence by being differential in momentum transfer and studying a range of vector mesons with complementary sensitivities to the saturation scale.

Soon to be constructed at Brookhaven National Laboratory, the Electron-Ion Collider (EIC) promises to transform our understanding of gluonic matter. Designed specifically for QCD research, the EIC will probe gluon saturation and shadowing in unprecedented detail, using a broad array of reactions, collision species and energy levels. By providing a multidimensional map of gluonic behaviour, the EIC will address funda­mental questions such as the origin of mass and nuclear spin.

Before then, a tenfold increase in PbPb statistics in LHC Runs 3 and 4 will allow a transformative leap in low Bjorken-x physics. Though not originally designed for low Bjorken-x physics, the LHC’s unparalleled energy reach and diverse range of colliding systems offers unique opportunities to explore gluon dynamics at the highest energies.

Enhanced capabilities

Surpassing the gains from increased luminosity alone, ALICE’s new triggerless detector readout mode will offer a vast improvement over previous runs, which were constrained by dedicated triggers and bandwidth limitations. Subdetector upgrades will also play an important role. The muon forward tracker has already enhanced ALICE’s capabilities, and the high-granularity forward calorimeter set to be installed in time for Run 4 is specifically designed to improve sensitivity to small Bjorken-x physics (see “Saturation specific” figure).

Ultraperipheral-collision physics at the LHC is far more than a technical exploration of QCD. Gluons govern the structure of all visible matter. Saturation, hotspots and shadowing shed light on the origin of 99% of the mass of the visible universe. 

The post The other 99% appeared first on CERN Courier.

In high-energy collisions at the LHC, prompt photons are those that do not originate from particle decays and are instead directly produced by the hard scattering of quarks and gluons (partons). Due to their early production, they provide a clean method to probe the partons inside the colliding nucleons, and in particular the fraction of the momentum of the nucleon carried by each parton (Bjorken x). The distribution of each parton in Bjorken x is known as its parton distribution function (PDF).

Theoretical models of particle production rely on the precise knowledge of PDFs, which are derived from vast amounts of experimental data. The high centre-of-mass energies (√s) at the LHC probe very small values of the momentum fraction, Bjorken x. At “midrapidity”, when a parton scatters with a large angle with respect to the beam axis, and a prompt photon is produced in the final state, a useful approximation to Bjorken x is provided by the dimensionless variable x_T = 2p_T/√s, where p_T is the transverse momentum of the prompt photon.

Prompt photons can also be produced by next-to-leading order processes such as parton fragmentation or bremsstrahlung. A clean separation of the different prompt photon sources is difficult experimentally, but fragmentation can be suppressed by selecting “isolated photons”. For a photon to be considered isolated, the sum of the transverse energies or transverse momenta of the particles produced in a cone around the photon must be smaller than some threshold – a selection that can be done both in the experimental measurement and theoretical calculations. An isolation requirement also helps to reduce the background of decay photons, since hadrons that can decay to photons are often produced in jet fragmentation.

The ALICE collaboration now reports the measurement of the differential cross-section for isolated photons in proton–proton collisions at √s = 13 TeV at midrapidity. The photon measurement is performed by the electromagnetic calorimeter, and the isolated photons are selected by combining with the data from the central inner tracking system and time-projection chamber, requiring that the summed p_T of the charged particles in a cone of angular radius 0.4 radians centred on the photon candidate be smaller than 1.5 GeV/c. The isolated photon cross-sections are obtained within the transverse momentum range from 7 to 200 GeV/c, corresponding to 1.1 × 10^–3 < x_T < 30.8 × 10^–3.

Figure 1 shows the new ALICE results alongside those from ATLAS, CMS and prior measurements in proton–proton and proton–antiproton collisions at lower values of √s. The figure spans more than 15 orders of magnitude on the y-axis, representing the cross-section, over a wide range of x_T. The present measurement probes the smallest Bjorken x with isolated photons at midrapidity to date. The experimental data points show an agreement between all the measurements when scaled with the collision energy to the power n = 4.5. Such a scaling is designed to cancel the predicted 1/(p_T)ⁿ dependence of partonic 2 → 2 scattering cross-sections in perturbative QCD and reveal insights into the gluon PDF (see “The other 99%“).

This measurement will help to constrain the gluon PDF and will play a crucial role in exploring medium-induced modifications of hard probes in nucleus–nucleus collisions.

The post Isolating photons at low Bjorken x appeared first on CERN Courier.

Hypernuclei are exotic nuclei formed by a mix of protons, neutrons and hyperons, the latter being unstable particles containing one or more strange quarks. More than 70 years since their discovery in cosmic rays, hypernuclei remain a source of fascination for physicists due to their rarity in nature and the challenge of creating and studying them in the laboratory.

In heavy-ion collisions, hypernuclei are created in significant quantities, but only the lightest hypernucleus, hypertriton, and its antimatter partner, antihypertriton, have been observed. Hypertriton is composed of a proton, a neutron and a lambda hyperon containing one strange quark. Antihypertriton is made up of an antiproton, an antineutron and an antilambda.

Following hot on the heels of the observation of antihyperhydrogen-4 (a bound state of an antiproton, two antineutrons and an antilambda) earlier this year by the STAR collaboration at the Relativistic Heavy Ion Collider (RHIC), the ALICE collaboration at the LHC has now seen the first ever evidence for antihyperhelium-4, which is composed of two antiprotons, an antineutron and an antilambda. The result has a significance of 3.5 standard deviations. If confirmed, antihyper­helium-4 would be the heaviest antimatter hypernucleus yet seen at the LHC.

Hypernuclei remain a source of fascination due to their rarity in nature and the challenge of creating and studying them in the lab

The ALICE measurement is based on lead–lead collision data taken in 2018 at a centre-of-mass energy of 5.02 TeV for each colliding pair of nucleons, be they protons or neutrons. Using a machine-learning technique that outperforms conventional hypernuclei search techniques, the ALICE researchers looked at the data for signals of hyperhydrogen-4, hyperhelium-4 and their antimatter partners. Candidates for (anti)hyperhydrogen-4 were identified by looking for the (anti)helium-4 nucleus and the charged pion into which it decays, whereas candidates for (anti)hyperhelium-4 were identified via its decay into an (anti)helium-3 nucleus, an (anti)proton and a charged pion.

In addition to finding evidence of antihyperhelium-4 with a significance of 3.5 standard deviations, and evidence of antihyperhydrogen-4 with a significance of 4.5 standard deviations, the ALICE team measured the production yields and masses of both hypernuclei.

For both hypernuclei, the measured masses are compatible with the current world-average values. The measured production yields were compared with predictions from the statistical hadronisation model, which provides a good description of the formation of hadrons and nuclei in heavy-ion collisions. This comparison shows that the model’s predictions agree closely with the data if both excited hypernuclear states and ground states are included in the predictions. The results confirm that the statistical hadronisation model can also provide a good description of the production of hyper­nuclei modelled to be compact objects with sizes of around 2 femtometres.

The researchers also determined the antiparticle-to-particle yield ratios for both hypernuclei and found that they agree with unity within the experimental uncertainties. This agreement is consistent with ALICE’s observation of the equal production of matter and antimatter at LHC energies and adds to the ongoing research into the matter–antimatter imbalance in the universe.

The post First signs of antihyperhelium-4 appeared first on CERN Courier.

The Dirac equation predicts the magnetic moment of the muon (g) to be precisely two in units of the Bohr magneton. Virtual lines and loops add roughly 0.1% to this value, giving rise to a so-called anomalous contribution often quantified by a_μ = (g–2)/2. Countless electromagnetic loops dominate the calculation, spontaneous symmetry breaking is evident in the effect of weak interactions, and contributions from the strong force are non-perturbative. Despite this formidable complexity, theoretical calculations of a_μ have been experimentally verified to nine significant figures.

The devil is in the 10th digit. The experimental world average for a_μ currently stands more than 5σ above the Standard Model (SM) prediction published by the Muon g-2 Theory Initiative in a 2020 white paper. But two recent results may ease this tension in advance of a new showdown with experiment next year.

The first new input is data from the CMD-3 experiment at the Budker Institute of Nuclear Physics, which yields a_μconsistent with experimental data. Comparable electron–positron (e⁺e^–) collider data from the KLOE experiment at the National Laboratory of Frascati, the BaBar experiment at SLAC, the BESIII experiment at IHEP Beijing and CMD-3’s predecessor CMD-2, were the backbone of the 2020 theory white paper. With KLOE and CMD-3 now incompatible at the level of 5σ, theorists are exploring alternative bases for the theoretical prediction, such as an ab-initio approach based on lattice QCD and a data-driven approach using tau–lepton decays.

The second new result is an updated theory calculation of a_μ by the Budapest–Marseille–Wuppertal (BMW) collaboration. BMW’s ab-initio lattice–QCD calculation of 2020 was the first to challenge the data-driven consensus expressed in the 2020 white paper. The recent update now claims a superior precision, driven in part by the pragmatic implementation of a data-driven approach in the low-mass region, where experiments are in good agreement. Though only accounting for 5% of the hadronic contribution to a_μ, this “long distance” region is often the largest source of error in lattice–QCD calculations, and relatively insensitive to the use of finer lattices.

The new BMW result is fully compatible with the experimental world average, and incompatible with the 2020 white paper at the level of 4σ.

“It seems to me that the 0.9σ agreement between the direct experimental measurement of the magnetic moment of the muon and the ab-initio calculation of BMW has most probably postponed the possible discovery of new physics in this process,” says BMW spokesperson Zoltán Fodor (Wuppertal). “It is important to mention that other groups have partial results, too, so-called window results, and they all agree with us and in several cases disagree with the result of the data-driven method.”

These two analyses were among the many discussed at the seventh plenary workshop of the Muon g-2 Theory Initiative held in Tsukuba, Japan from 9 to 13 September. The theory initiative is planning to release an updated prediction in a white paper due to be published in early 2025. With multiple mature e⁺e^– and lattice–QCD analyses underway for several years, attention now turns to tau decays – the subject of a soon-to-be-announced mini-workshop to ensure their full availability for consideration as a possible basis for the 2025 white paper. Input data would likely originate from tau decays recorded by the Belle experiment at KEK and the ALEPH experiment at CERN, both now decommissioned.

I am hopeful we will be able to establish consolidation between independent lattice calculations at the sub-percent level

“From a theoretical point of view, the challenge for including the tau data is the isospin rotation that is needed to convert the weak hadronic tau decay to the desired input for hadronic vacuum polarisation,” explains theory-initiative chair Aida X El-Khadra (University of Illinois). Hadronic vacuum polarisation (HVP) is the most challenging part of the calculation of a_μ, accounting for the effect of a muon emitting a virtual photon that briefly transforms into a flurry of quarks and gluons just before it absorbs the photon representing the magnetic field (CERN Courier May/June 2021 p25).

Lattice QCD offers the possibility of a purely theoretical calculation of HVP. While BMW remains the only group to have published a full lattice-QCD calculation, multiple groups are zeroing in on its most sensitive aspects (CERN CourierSeptember/October 2024 p21).

“The main challenge in lattice-QCD calculations of HVP is improving the precision to the desired sub-percent level, especially at long distances,” continues El-Khadra. “With the new results for the long-distance contribution by the RBC/UKQCD and Mainz collaborations that were already reported this year, and the results that are still expected to be released this fall, I am hopeful that we will be able to establish consolidation between independent lattice calculations at the sub-percent level. In this case we will provide a lattice-only determination of HVP in the second white paper.”

The post Shifting sands for muon g–2 appeared first on CERN Courier.

Seventy-six new particles have been discovered at the Large Hadron Collider (LHC) so far: the Higgs boson, 52 conventional hadrons and a bestiary of 23 exotic hadrons whose structure cannot reliably be explained or their existence predicted.

The exotic states are varied and complex, displaying little discernible pattern at first glance. They represent a fascinating detective story: an experimentally driven quest to understand the exotic offspring of the strong interaction, motivating rival schools of thought among theorists.

This surge in new hadrons has been one of the least expected outcomes of the LHC (see “Unexpected” figure). With a tenfold increase in data at the High-Luminosity LHC (HL-LHC) on the horizon, and further new states also likely to emerge at the Belle II experiment in Japan, the BESIII experiment in China, and perhaps at a super charm–tau factory in the same country, their story is in its infancy, with twists and turns still to come.

Building blocks

Just as electric charges arrange themselves in neutral atoms, the colour charges that carry the strong interaction arrange themselves into colourless composite states. As fundamental particles with colour charge, quarks (q) and gluons (g) therefore cannot exist independently, but only in colour-neutral composite states called hadrons. Since the discovery of the pion in 1947, a rich phenomenology of mesons (qq) and baryons (qqq) inspired the quark model and eventually the theory of quantum chromodynamics (QCD), which serves as an impeccable description of the strong interaction to this day.

But why should nature not also contain exotic colour-neutral combinations such as tetraquarks (qqqq), pentaquarks (qqqqq), hexaquarks (qqqqqq or qqqqqq), hybrid hadrons (qqg or qqgg) and glueballs (gg or ggg)?

The existence of exotic hadrons was debated without consensus for decades, with interest growing in the early 2000s, when new states with unexpected features were observed. In 2003, the BaBar experiment at SLAC discovered the D^*_s0(2317)⁺ meson, with a mass close to the sum of the masses of a D meson and a kaon. A few months later that year, Belle discovered the χ_c1(3872) meson, then called X(3872) (see “What’s in a name?” panel), with a mass close to the sum of the masses of a D⁰ meson and a D^*0 meson. As well as their striking closeness to meson–meson thresholds, the “width” of their signals was much narrower than expected. (Measured in units of energy, such widths are reciprocal to particle lifetimes.)

Soon afterwards, in 2007, a number of other charmonium-like and bottomonium-like states were observed. Belle’s observation in 2007 of the electrically charged charmonium-like state Z(4430)⁺ (now called T_cc1(4430)⁺) was a pathfinder in theorising the existence of QCD exotics. Though these states exhibited the telltale signs of being excitations of a charm–anticharm (cc) system (see “The new particles”), their net electric charge indicated a system that could not be composed of only a quark–antiquark pair, as particles and antiparticles have opposite electric charges. Two additional quarks had to be present.

Exotic states at the LHC

The start-up of the LHC opened up the trail, with 23 new exotic hadrons observed there so far (see “The 23 exotic hadrons discovered at the LHC” table). The harvest of new states began in autumn 2013 with the CMS experiment at the LHC reporting the observation of the χ_c1(4140) state in the J/ψφ mass spectrum in B⁺ → J/ψφK⁺ decays, confirming a hint from the CDF experiment at Fermilab. Its minimal quark content is likely ccss. CMS also reported evidence for a state at a higher mass, observed by the LHCb experiment at the LHC in 2016 as the χ_c1(4274), alongside two more states at masses of 4500 and 4700 MeV.

In a 2021 analysis of the same B⁺ → J/ψφK⁺ decay mode including LHC Run 2 data, LHCb reported two more neutral states, χ_c1(4685) and X(4630), that do not correspond to cc states expected from the quark model. The analysis also reported two more resonances seen in the J/ψK⁺ mass spectrum, T_ccs1(4000)⁺ and T_ccs1(4220)⁺. Carrying charge and strangeness, these charmonia-like states are manifestly exotic, with a minimal quark content ccus.

For T_ccs1(4000)⁺, LHCb had sufficient data to produce an Argand diagram with the distinct signature of a resonance (see “Round resonances” panel). A possible isospin partner, T_ccs1(4000)⁰ was later found in B⁰ → J/ψφK⁰_s decays, lending further evidence that it is a resonance and not a kinematical feature. (According to an approximate symmetry of QCD, the strong interaction should treat a ccus state almost exactly like a ccds. state, as up and down quarks have the same colour charges and similar masses.) Other charmonium-like tetraquarks were later seen by LHCb in the decays χ_c0(3960) → D⁺_sD^–_s and χ_c1(4010) → D^*+D^–.

The world’s first pentaquarks were discovered by LHCb in 2015. Two pentaquarks appeared in the J/ψp spectrum by studying Λ⁰_b → J/ψpK^– decays: P_cc(4380)⁺, a rather broad resonance with a width of 200 MeV; and P_cc(4450)⁺, which is narrower at 40 MeV. The observed decay mode implied a minimal quark content ccuud, excluding any conventional interpretation.

These states were hiding in plain sight: they were spotted independently by several LHCb physicists, including a CERN summer student. In a 2019 analysis using more data, the heavier state was identified as the sum of two overlapping pentaquarks now called P_cc(4440)⁺ and P_cc(4457)⁺. Another narrow state was also seen at a mass of 4312 MeV. LHCb observed the first strange pentaquark in B^– → J/ψΛp decays in 2022, with a quark content ccuds.

Other manifestly exotic hadrons followed, with two exotic hadrons T_cccc(6600) and T_cccc(6900) observed by LHCb, CMS and ATLAS in the J/ψJ/ψ spectrum. They can be interpreted as a tetraquark made of two charm and two anti-charm quarks – a fully charmed tetraquark. When both J/ψ mesons decay to a muon pair, the final state consists of four muons, allowing the LHCb, ATLAS and CMS experiments to study the final spectrum in multiple acceptance regions and transverse momentum ranges. These states do not contain any light quarks, which eases their theoretical study and also implies a state with four bottom quarks that could be long-lived.

Doubly charming

The world’s first double-open-charm meson was discovered by LHCb in 2021: the T_cc(3875)⁺. With a charm of two, it cannot be accommodated in the conventional qq scheme. There is an intriguing similarity between the exotic T_cc(3875)⁺(ccud) and the charmonium-like (cc-like) χ_c1(3872) meson discovered by Belle in 2003, whose nature is still controversial. Both have similar masses and remarkably narrow widths. The jury is still out on their interpretation (see “Inside pentaquarks and tetraquarks“).

The discovery of a T_cc(3875)⁺ (ccud) meson also implies the existence of a T_bb state, with a bbud quark content, that should be stable except with regard to weak decays. The observation of the first long-lived exotic state, with a sizable flight distance, is an intriguing goal for future experiments. At the HL-LHC, the search for B⁺_c mesons displaced from the interaction point, could return the first evidence for a T_bb tetraquark given that the decays of weakly decaying double-beauty hadrons such as Ξ_bbq and T_bb are their only known sources.

Round resonances

Particles are most likely to be created in collisions when the centre-of-mass energy matches their mass. The longer the mean lifetime of the new particle, the greater the uncertainty on its decay time and, via Heisenberg’s uncertainty principle, the smaller the uncertainty on their energy. Such particles have narrow peaks in their energy spectra. Fast-decaying particles have broad peaks. Searching for such “resonances” can reveal new particles – but bumps can be deceiving. A more revealing analysis fits differential decay rates to measure the complex quantum amplitude A(s) describing the production of the particle. As the energy (√s) increases, the amplitude traces a circle counterclockwise in the complex plane, with the magnitude of the amplitude tracing the classic resonant peak observed in energy spectra (see figure above left).

Demonstrating this behaviour, as LHCb did in 2021 for theT_ccs1(4000)⁺ meson (above, centre) is a significant experimental achievement, which the collaboration also performed in 2018 for the pathfinding Z(4430)⁺ (T_cc1(4430)⁺) meson discovered by Belle in 2007 (black points, above right). The LHCb measurement confirmed its resonant character and resolved any controversy over whether it was a true exotic state. The simulated blue measurement illustrates the improvement such measurements stand to accrue with upgraded detectors and increased statistics at the HL-LHC.

There are also other exotic states predicted by QCD that are still missing in the particle zoo, such as meson–gluon hybrids and glueballs. Hybrid mesons could be identified by exotic spin-parity (J^P) quantum numbers not allowed in the qq scheme. Glueballs could be observed in gluon-enriched heavy-ion collisions. A potential candidate has recently been observed by the BESIII collaboration, which is another major player in exotic spectroscopy.

Exotic hadrons might even have been observed in the light quark sector without having been searched for. The scalar mesons are too numerous to fit in the conventional quark model, and some of them, for instance the f₀(980) and a₀(980) mesons, might be tetraquarks. Exotic light pentaquarks may also exist. Twenty years ago, the θ⁺ baryon caused quite some excitement, being apparently openly exotic, with a positive strangeness and a minimal quark content uudds. No fewer than 10 different experiments presented evidence for it, including several quoting 5σ significance, before it disappeared in blind analyses of larger data samples with better background subtraction (CERN Courier April 2004 p29). Its story is now material for historians of science, but its interpretation triggered many theory papers that are still useful today.

The challenge of understanding how quarks are bound inside exotic hadrons is the greatest outstanding question in hadron spectroscopy. Models include a cloud of light quarks and gluons bound to a heavy qq core by van-der-Waals-like forces (hadro-quarkonium); colour-singlet hadrons bound by residual nuclear forces (hadronic molecules); and compact tetraquarks [qq] [qq] and pentaquarks [qq][qq]q composed of diquarks [qq] and antidiquarks [qq], which masquerade as antiquarks and quarks, respectively.

Some exotic hadrons may also have been misinterpreted as resonant states when they are actually “threshold cusps” – enhancements caused by rescattering. For instance, the P_cc(4457)⁺ pentaquark seen in Λ⁰_b → J/ψpK^– decays could in fact be rescattering between the D⁰ and Λ_c(2595)⁺ decay products in Λ⁰ _b→ Λ_c(2595)⁺D⁰K^– to exchange a charm quark and form a J/ψp system. This hypothesis can be tested by searching for additional decay modes and isospin partners, or via detailed amplitude analyses – a process already completed for many of the aforementioned states, but not yet all.

Establishing the nature of the exotic hadrons will be challenging, and a comprehensive organisation of exotic hadrons in flavour multiples is still missing. Establishing whether exotic hadrons obey the same flavour symmetries as conventional hadrons will be an important step forward in understanding their composition.

Effective predictions

The dynamics of quarks and gluons can be described perturbatively in hard processes thanks to the smallness of the strong coupling constant at short distances, but the spectrum of stable hadrons is affected by non-perturbative effects and cannot be computed from the fundamental theory. Though lattice QCD attempts this by discretising space–time in a cubic lattice, the results are time consuming and limited in precision by computational power. Predictions rely on approximate analytical methods such as effective field theories.

The challenge of understanding how quarks are bound inside exotic hadrons is the greatest outstanding question in hadron spectroscopy

Hadron physics is therefore driven by empirical data, and hadron spectroscopy plays a pivotal role in testing the predictions of lattice QCD, which is itself an increasingly important tool in precision electroweak physics and searches for physics beyond the Standard Model.

Like Mendeleev and Gell-Mann, we are at the beginning of a new field, in the taxonomy stage, discovering, studying and classifying exotic hadrons. The deeper challenge is to explain and anticipate them. Though the underlying principles are fully known, we are still far from being able to do the chemistry of quantum chromodynamics.

The post A bestiary of exotic hadrons appeared first on CERN Courier.

Breakthroughs are like London buses. You wait a long time, and three turn up at once. In 1963 and 1964, Murray Gell-Mann, André Peterman and George Zweig independently developed the concept of quarks (q) and antiquarks (q) as the fundamental constituents of the observed bestiary of mesons (qq) and baryons (qqq).

But other states were allowed too. Additional qq pairs could be added at will, to create tetraquarks (qqqq), pentaquarks (qqqqq) and other states besides. In the
1970s, Robert L Jaffe carried out the first explicit calculations of multiquark states, based on the framework of the MIT bag model. Under the auspices of the new theory of quantum chromodynamics (QCD), this computationally simplified model ignored gluon interactions and considered quarks to be free, though confined in a bag with a steep potential at its boundary. These and other early theoretical efforts triggered many experimental searches, but no clear-cut results.

New regimes

Evidence for such states took nearly two decades to emerge. The essential precursors were the discovery of the charm quark (c) at SLAC and BNL in the November Revolution of 1974, some 50 years ago (p41), and the discovery of the bottom quark (b) at Fermilab three years later. The masses and lifetimes of these heavy quarks allowed experiments to probe new regimes in parameter space where otherwise inexplicable bumps in energy spectra could be resolved (see “Heavy breakthroughs” panel).

Heavy breakthroughs

With the benefit of hindsight, it is clear why early experimental efforts did not find irrefutable evidence for multiquark states. For a multiquark state to be clearly identifiable, it is not enough to form a multiquark colour-singlet (a mixture of colourless red–green–blue, red–antired, green–antigreen and blue–antiblue components). Such a state also needs to be narrow and long-lived enough to stand out on top of the experimental background, and has to have distinct decay modes that cannot be explained by the decay of a conventional hadron. Multiquark states containing only light quarks (up, down and strange) typically have many open decay channels, with a large phase space, so they tend to be wide and short-lived. Moreover, they share these decay channels with excited states of conventional hadrons and mix with them, so they are extremely difficult to pin down.

Multiquark states with at least one heavy quark are very different. Once hadrons are “dressed” by gluons, they acquire effective masses of the order of several hundred MeV, with all quarks coupling in the same way to gluons. For light quarks, the bare quark masses are negligible compared to the effective mass, and can be neglected to zeroth order. But for heavy quarks (c or b), the ratio of the bare quark masses to the effective mass of the hadron dramatically affects the dynamics and the experimental situation, creating narrow multiquark states that stand out. These states were not seen in the early searches simply because the relevant production cross sections are very small and particle identification requires very high spatial resolution. These features became accessible only with the advent of the huge luminosity and the superb spatial resolution provided by vertex detectors in bottom and charm factories such as BaBar, Belle, BESIII and LHCb.

The attraction between two heavy quarks scales like α²_s m_q, where α_s is the strong coupling constant and m_q is the mass of the quarks. This is because the Coulomb-like part of the QCD potential dominates, scaling as –α_s/r as a function of distance r, and yielding an analogue of the Bohr radius ~1/(α_sm_q). Thus, the interaction grows approximately linearly with the heavy quark mass. In at least one case (discussed below), the highly anticipated but as yet undiscovered bbud. tetraquark T_bb is expected to result in a state with a mass that is below the two-meson threshold, and therefore stable under strong interactions.

Exclusively heavy states are also possible. In 2020 and in 2024, respectively, LHCb and CMS discovered exotic states T_cccc(6900) and T_cccc(6600), which both decay into two J/ψ particles, implying a quark content (cccc). J/ψ does not couple to light quarks, so these states are unlikely to be hadronic molecules bound by light meson exchange. Though they are too heavy to be the ground state of a (cccc) compact tetraquark, they might perhaps be its excitations. Measuring their spin and parity would be very helpful in distinguishing between the various alternatives that have been proposed.

The first unambiguously exotic hadron, the X(3872) (dubbed χ_c1(3872) in the LHCb collaboration’s new taxonomy; see “What’s in a name?” panel), was discovered at the Belle experiment at KEK in Japan in 2003. Subsequently confirmed by many other experiments, its nature is still controversial. (More of that later.) Since then, there has been a rapidly growing body of experimental evidence for the existence of exotic multiquark hadrons. New states have been discovered at Belle, at the BaBar experiment at SLAC in the US, at the BESIII experiment at IHEP in China, and at the CMS and LHCb experiments at CERN (see “A bestiary of exotic hadrons“). In all cases with robust evidence, the exotic new states contain at least one heavy charm or bottom quark. The majority include two.

The key theoretical question is how the quarks are organised inside these multiquark states. Are they hadronic molecules, with two heavy hadrons bound by the exchange of light mesons? Or are they compact objects with all quarks located within a single confinement volume?

The compact and molecular interpretations each provide a natural explanation for part of the data, but neither explains all. Both kinds of structures appear in nature, and certain states may be superpositions of compact and molecular states.

In the molecular case the deuteron is a good mental image. (As a bound state of a proton and a neutron, it is technically a molecular hexaquark.) In the compact interpretation, the diquark – an entangled pair of quarks with well-defined spin, colour and flavour quantum numbers – may play a crucial role. Diquarks have curious properties, whereby, for example, a strongly correlated red–green pair of quarks can behave like a blue antiquark, opening up intriguing possibilities for the interpretation of qqqq and qqqqq states.

Compact states

A clearcut example of a compact structure is the T_bb tetraquark with quark content bbud. T_bb has not yet been observed experimentally, but its existence is supported by robust theoretical evidence from several complementary approaches. As for any ground-state hadron, its mass is given to a good approximation by the sum of its constituent quark masses and their (negative) binding energy. The constituent masses implied here are effective masses that also include the quarks’ kinetic energies. The binding energy is negative as it was released when the compact state formed.

In the case of T_bb, the binding energy is expected to be so large that its mass is below all two-meson decay channels: it can only decay weakly, and must be stable with respect to the strong interaction. No such exotic hadron has yet been discovered, making T_bb a highly prized target for experimentalists. Such a large binding energy cannot be generated by meson exchange and must be due to colour forces between the very heavy b quarks. T_bb is an iso­scalar with J^P = 1⁺. Its charmed analogue, T_cc = (ccud), also known as T_cc(3875)⁺, was observed by LHCb in 2021 to be a whisker away from stability, with a very small binding energy and width less than 1 MeV (CERN Courier September/October 2021 p7). The big difference between the binding energies of T_bb and T_cc, which make the former stable and the latter unstable, is due to the substantially greater mass of the b quark than the c quark, as discussed in the panel above. An intermediate case, T_bc = (bcud), is very likely also below threshold for strong decay and therefore stable. It is also easier to produce and detect than T_bb and therefore extremely tempting experimentally.

Molecular pentaquarks

At the other extreme, we have states that are most probably pure hadronic molecules. The most conspicuous examples are the P_c(4312), P_c(4440) and P_c(4457) pentaquarks discovered by LHCb in 2019, and labelled according to the convention adopted by the Particle Data Group as P_cc(4312)⁺, P_cc(4440)⁺ and P_cc(4457)⁺. All three have quark content (ccuud) and decay into J/ψp, with an energy release of order 300 MeV. Yet, despite having such a large phase space, all three have anomalously narrow widths less than about 10 MeV. Put more simply, the pentaquarks decay remarkably slowly, given how much energy stands to be released.

But why should long life count against the pentaquarks being tightly bound and compact? In a compact (ccuud) state there is nothing to prevent the charm quark from binding with the anticharm quark, hadronising as J/ψ and leaving behind a (uud) proton. It would decay immediately with a large width.

On the other hand, hadronic molecules such as Σ_cD and Σ_cD^* automatically provide a decay-suppression mechanism. Hadronic molecules are typically large, so the c quark inside the Σ_c baryon is typically far from the c quark inside the D or D^* meson. Because of this, the formation of J/ψ = (c c) has a low probability, resulting in a long lifetime and a narrow width. (Unstable particles decay randomly within fixed half-lives. According to Heisenberg’s uncertainty principle, this uncertainty on their lifetime yields a reciprocal uncertainty on their energy, which may be directly observed as the width of the peak in the spectrum of their measured masses when they are created in particle collisions. Long-lived particles exhibit sharply spiked peaks, and short-lived particles exhibit broad peaks. Though the lifetimes of strongly interacting particle are usually not measurable directly, they may be inferred from these “widths”, which are measured in units of energy.)

Additional evidence in favour of their molecular nature comes from the mass of P_c(4312) being just below the Σ_cD production threshold, and the masses of P_c(4440) and P_c(4457) being just below the Σ_cD^* production threshold. This is perfectly natural. Hadronic molecules are weakly bound, so they typically only form an S-wave bound state, with no orbital angular momentum. So Σ_cD, which combines a spin-1/2 baryon and a spin-0 negative-parity meson, can only form a single state with J^P = 1/2^–. By contrast, Σ_cD^*, which combines a spin-1/2 baryon and spin-1 negative-parity meson, can form two closely-spaced states with J^P = 1/2^– and 3/2^–, with a small splitting coming from a spin–spin interaction.

An example of a possible mixture of a compact state and a hadronic molecule is provided by the X(3872) meson

The robust prediction of the J^P quantum numbers makes it very straightforward in principle to kill this physical picture, if one were to measure J^P values different from these. Conversely, measuring the predicted values of J^P would provide a strong confirmation (see “The 23 exotic hadrons discovered at the LHC table”).

These predictions have already received substantial indirect support from the strange-pentaquark sector. The spin-parity of the P_ccs(4338), which also has a narrow width below 10 MeV, has been determined by LHCb to be 1/2^–, exactly as expected for a Ξ_c D molecule (see “Strange pentaquark” figure).

The mysterious X(3872)

An example of a possible mixture of a compact state and a hadronic molecule is provided by the already mentioned X(3872) meson. Its mass is so close to the sum of the masses of a D⁰ meson and a D^*0 meson that no difference has yet been established with statistical significance, but it is known to be less than about 1 MeV. It can decay to J/ψπ⁺π^– with a branching ratio (3.5 ± 0.9)%, releasing almost 500 MeV of energy. Yet its width is only of order 1 MeV. This is an even more striking case of relative stability in the face of naively expected instability than for the pentaquarks. At first sight, then, it is tempting to identify X(3872) as a clearcut D⁰D^*0 hadronic molecule.

The situation is not that simple, however. If X(3872) is just a weakly-bound hadronic molecule, it is expected to be very large, of the scale of a few fermi (10^–15 m). So it should be very difficult to produce it in hard reactions, requiring a large momentum transfer. Yet this is not the case. A possible resolution might come from X(3872) being a mixture of a D⁰D^*0molecular state and χ_c1(2P), a conventional radial excitation of P-wave charmonium, which is much more compact and is expected to have a similar mass and the same J^PC = 1⁺⁺ quantum numbers. Additional evidence in favour of such a mixing comes from comparing the rates of the radiative decays X(3872) → J/ψγ and X(3872) → ψ(2S)γ.

The question associated with exotic mesons and baryons can be posed crisply: is an observed state a molecule, a compact multiquark system or something in between? We have given examples of each. Definitive compact-multiquark behaviour can be confirmed if a state’s flavour-SU(3) partners are identified. This is because compact states are bound by colour forces, which are only weakly sensitive to flavour-SU(3) rotations. (Such rotations exchange up, down and strange quarks, and to a good approximation the strong force treats these light flavours equally at the energies of charmed and beautiful exotic hadrons.) For example, if X(3872) should in fact prove to be a compact tetraquark, it should have charged isospin partners that have not yet been observed.

On the experimental front, the sensitivity of LHCb, Belle II, BESIII, CMS and ATLAS have continued to reap great benefits to hadron spectroscopy. Together with the proposed super τ-charm factory in China, they are virtually guaranteed to discover additional exotic hadrons, expanding our understanding of QCD in its strongly interacting regime.

The post Inside pentaquarks and tetraquarks appeared first on CERN Courier.

According to the cosmological standard model, the first generation of nuclei was produced during the cooling of the hot mixture of quarks and gluons that was created shortly following the Big Bang. Relativistic heavy-ion collisions create a quark–gluon plasma (QGP) on a small scale, producing a “little bang”. In such collisions, the nucleosynthesis mechanism at play is different from the one of the Big Bang due to the rapid cool down of the fireball. Recently, the nucleosynthesis mechanism in heavy-ion collisions has been investigated via the measurement of hypertriton production by the ALICE collaboration.

The hypertriton, which consists of a proton, a neutron and a Λ hyperon, can be considered to be a loosely bound deuteron-Λ molecule (see “Inside pentaquarks and tetraquarks“). In this picture, the energy required to separate the Λ from the deuteron (B_Λ) is about 100 keV, significantly lower than the binding energy of ordinary nuclei. This makes hypertriton production a sensitive probe of the properties of the fireball.

In heavy-ion collisions, the formation of nuclei can be explained by two main classes of models. The statistical hadronisation model (SHM) assumes that particles are produced from a system in thermal equilibrium. In this model, the production rate of nuclei depends only on their mass, quantum numbers and the temperature and volume of the system. On the other hand, in coalescence models, nuclei are formed from nucleons that are close together in phase space. In these models, the production rate of nuclei is also sensitive to their nuclear structure and size.

For an ordinary nucleus like the deuteron, coalescence and SHM predict similar production rates in all colliding systems, but for a loosely bound molecule such as the hypertriton, the predictions of the two models differ significantly. In order to identify the mechanism of nuclear production, the ALICE collab­oration used the ratio between the production rates of hypertriton and helium-3 – also known as a yield ratio – as an observable.

ALICE measured hypertriton production as a function of charged-particle multiplicity density using Pb–Pb collisions collected at a centre-of-mass energy of 5.02 TeV per nucleon pair during LHC Run 2. Figure 1 shows the yield ratio of hypertriton to ³He across different multiplicity intervals. The data points (red) exhibit a clear deviation from the SHM (dashed orange line), but are well-described by the coalescence model (blue band), supporting the conclusion that hypertriton formation at the LHC is driven by the coalescence mechanism.

The ongoing LHC Run 3 is expected to improve the precision of these measurements across all collision systems, allowing us to probe the internal structure of hypertriton and even heavier hypernuclei, whose properties remain largely unknown. This will provide insights into the interactions between ordinary nucleons and hyperons, which are essential for understanding the internal composition of neutron stars.

The post Hypertriton and ‘little bang’ nucleosynthesis appeared first on CERN Courier.

Hadronic contributions

One of the highest profile topics in lattice QCD is the evaluation of hadronic contributions to the magnetic moment of the muon. For many years, the experimental measurements from Brookhaven and Fermilab have appeared to be in tension with the Standard Model (SM), based on theoretical predictions that rely on data from e⁺e^– annihilation to hadrons. Intense work on the lattice by multiple groups is now maturing rapidly and providing a valuable cross-check for data-driven SM calculations.

At the lowest order in quantum electrodynamics, the Dirac equation accounts for precisely two Bohr magnetons in the muon’s magnetic moment (g = 2) – a contribution arising purely from the muon interacting with a single real external photon representing the magnetic field. At higher orders in QED, virtual Standard Model particles modify that value, leading to a so-called anomalous magnetic moment g–2. The Schwinger term adds a virtual photon and a contribution to g-2 of approximately 0.2%. Adding individual virtual W, Z or Higgs bosons adds a well defined contribution a factor of a million or so smaller. The remaining relevant contributions are from hadronic vacuum polarisation (HVP) and hadronic light-by-light (HLBL) scattering. HVP and HLBL both add hadronic contributions integrated to all orders in the strong coupling constant to interactions between the muon and the external electric field, which also feature additional virtual photons. Though their contributions to g-2 are in the ballpark of the small electroweak contribution, they are more difficult to calculate, and dominate the error budget for the SM prediction of the muon’s g-2.

Christine Davies (University of Glasgow) gave a comprehensive survey of muon g–2 that stressed several high-level points: the small HLBL contribution looks to be settled, and is unlikely to be a key piece to the puzzle; recent tensions among the e⁺e^– experiments for HVP have emerged and need to be better understood; and in the most contentious region, all eight recent lattice–QCD calculations agree with each other and with the very recent e⁺e^– → hadrons experiment CMD 3 (2024 Phys. Rev. Lett. 132 231903), though not so much with earlier experiments. Thus, lattice QCD and CMD 3 suggest there is “almost certainly less new physics in muon g–2 than previously hoped, and perhaps none,” said Davies. We shall see: many groups are preparing results for the full HVP, targeting a new whitepaper from the Muon g–2 Theory Initiative by the end of this year, in anticipation of the final measurement from the Fermilab experiment sometime in 2025.

New directions

While the main focus of Lattice calculations is the study of QCD, lattice methods have been applied beyond that. There is a small but active community investigating systems that could be relevant to physics beyond the Standard Model, including composite Higgs models, supersymmetry and dark matter. These studies often inspire formal “theoretical” developments that are of interest beyond the lattice community. Particularly exciting directions this year were the development on emergent phases, non-invertible symmetries and their possible application to formulate chiral gauge theories, one of the outstanding theoretical issues in lattice gauge theories.

The lattice QCD community is one of the main users of high-performance computing resources

The lattice QCD community is one of the main users of high-performance computing resources, with its simulation efforts generating petabytes of Monte Carlo data. For more than 20 years, a community wide effort, the international lattice data grid (ILDG), has allowed this data to be shared. Since its inception, ILDG implemented the FAIR principles – data should be findable, accessible, interoperable and reusable – almost fully. The lattice QCD community is now discussing Open Science. Ed Bennett (Swansea) led a panel discussion that explored the benefits of ILDG embracing open science, such as higher credibility for published results, and not least the means to fulfill the expectations of funding bodies. Sustainably maintaining the infrastructure and employing the personnel required calls for national or even international community efforts to convince the funding agencies to provide corresponding funding lines, but also the researchers of the benefits of open science.

The Kenneth G. Wilson Award for Excellence in Lattice Field Theory was awarded to Michael Wagman (Fermi­lab) for his lattice-QCD studies of noise reduction in nuclear systems, the structure of nuclei and transverse-momentum-dependent hadronic structure functions. Fifty years on from Wilson’s seminal paper, two of the field’s earliest contributors, John Kogut (US Department of Energy) and Jan Smit (University of Amsterdam), reminisced about the birth of the lattice in a special session chaired by Liverpool pioneer Chris Michael. Both speakers gave fascinating insights into a time where physics was extracted from a handful of small-volume gauge configurations, compared to hundreds of thousands today.

Lattice 2025 will take place at the Tata Institute of Fundamental Research in Mumbai, India, from 3 to 8 November 2025.

The post Lattice calculations start to clarify muon g-2 appeared first on CERN Courier.

In ultra-peripheral collisions, two nuclei pass close to each other without colliding. With their impact parameter larger than the sum of their radii, one nucleus emits a photon that transforms into a virtual quark–antiquark pair. This pair interacts strongly with the other nucleus, resulting in the emission of a vector meson and the exchange of two gluons. Such vector-meson photoproduction is a well-established tool for probing the internal structure of colliding nuclei.

In vector-meson photoproduction involving symmetric systems, such as two lead nuclei, it is not possible to determine which of the nuclei emitted the photon and which emitted the two gluons. Crucially, however, due to the short range of the strong force between the virtual quark–antiquark pair and the nucleus, the vector mesons must have been produced within or close to one of the two well-separated nuclei. Because of this and their relatively short lifetime, the vector mesons decay quite rapidly into other particles. These decay products form a quantum-mechanically entangled state and generate an interference pattern akin to that of a double-slit interferometer.

In the photoproduction of the electrically neutral ρ⁰ vector meson, the interference pattern takes the form of a cos(2φ) modulation of the ρ⁰ yield, where φ is the angle between the two vectors formed by the sum and difference of the transverse momenta of the two oppositely charged pions into which the ρ⁰ decays. The strength of the modulation is expected to increase as the impact parameter decreases.

Using a dataset of 57,000 ρ⁰ mesons produced in lead–lead collisions at an energy of 5.02 TeV per nucleon pair during Run 2 of the LHC, the ALICE team measured the cos(2φ) modulation of the ρ⁰ yield for different values of the impact parameter. The measurements showed that the strength of the modulation varies strongly with the impact parameter. Theoretical calculations indicate that this behaviour is indeed the result of a quantum interference effect at the femtometre scale.

In the ongoing Run 3 of the LHC and in the next run, Run 4, ALICE is expected to collect more than 15 million ρ⁰ mesons from lead–lead collisions. This enhanced dataset will allow a more detailed analysis of the interference effect, further testing the validity of quantum mechanics at femtometre scales.

The post ALICE does the double slit appeared first on CERN Courier.

In high-energy hadronic and heavy-ion collisions, strange quarks are dominantly produced from gluon fusion. In contrast to u and d quarks, they are not present in the colliding particles. Since strangeness is a conserved quantity in QCD, the net number of strange and anti-strange particles must equal zero, making them prime observable to study the dynamics of these collisions. Various experimental results from high-multiplicity pp collisions at the LHC demonstrate striking similarities to Pb–Pb collision results. Notably, the fraction of hadrons carrying one or more strange quarks smoothly increases as a function of particle multiplicity in pp and p–Pb collisions to values consistent with those measured in peripheral Pb–Pb collisions. Multi-particle correlations in pp collisions also closely resemble those in Pb–Pb collisions.

Explaining such observations requires understanding the hadronisation mechanism, which governs how quarks and gluons rearrange into bound states (hadrons). Since there are no first-principle calculations of the hadronisation process available, phenomenological models are used, based on either the Lund string fragmentation (Pythia 8, HIJING) or a statistical approach assuming a system of hadrons and their resonances (HRG) at thermal and chemical equilibrium. Despite having vastly different approaches, both models successfully describe the enhanced production of strange hadrons. This similarity calls for new observables to decisively discriminate between these two approaches.

The data indicate a weaker opposite-sign strangeness correlation than that predicted by string fragmentation

In a recently published study, the ALICE collaboration measured correlations between particles arising from the conservation of quantum numbers to further distinguish the two models. In the string fragmentation model, the quantum numbers are conserved locally through the creation of quark–antiquark pairs from the breaking of colour strings. This leads to a short-range rapidity correlation between strange and anti-strange hadrons. On the other hand, in the statistical hadronisation approach, quantum numbers are conserved globally over a finite volume, leading to long-range correlations between both strange–strange and strange–anti-strange hadron pairs. Quantum-number conservation leads to correlated particle production that is probed by measuring the yields of charged kaons (with one strange quark) and multistrange baryons (Ξ^– and Ξ⁺) on an event-by-event basis. In ALICE, charged kaons are directly tracked in the detectors, while Ξ baryons are reconstructed via their weak decay to a charged pion and a Λ-baryon, which is itself identified via its weak decay into a proton and a charged pion.

Figure 1 shows the first measurement of the correlation between the “net number” of Ξ baryons and kaons, as a function of the charged-particle multiplicity at midrapidity in pp, p–Pb and Pb–Pb collisions, where the net number is the difference between particle and antiparticle multiplicities. The experimental results deviate from the uncorrelated baseline (dashed line), and string fragmentation models that mainly correlate strange hadrons with opposite strange quark content over a small rapidity range fail to describe both observables. At the same time, the measurements agree with the statistical hadronisation model description that includes opposite-sign and same-sign strangeness correlations over large rapidity intervals. The data indicate a weaker opposite-sign strangeness correlation than that predicted by string fragmentation, suggesting that the correlation volume for strangeness conservation extends to about three units of rapidity.

The present study will be extended using the recently collected data during LHC Run 3. The larger data samples will enable similar measurements for the triply strange Ω baryon, as well as the study of higher cumulants.

The post Strange correlations benchmark hadronisation appeared first on CERN Courier.

Quantum chromodynamics (QCD) is one of the pillars of the Standard Model of particle physics, but much remains to be understood about its emergent behaviours, and theoretical calculations often disagree. A new result from the ALICE collaboration has now added fresh intrigue to interpretations of hadronisation – the process by which quarks and gluons become confined inside colour-neutral groupings such as baryons and mesons.

The production of heavy charm and beauty quarks in proton–proton collisions at the LHC is a rather fast process (~7 × 10^–24 s) and subject to perturbative QCD calculations. On the other hand, the transformation of heavy quarks into hadrons requires substantially more time (~3 × 10^–23 s). This separation of time scales has motivated the idea that the hadronisation process of heavy quarks is independent of the colliding system and collision energy. However, the production of baryons carrying a heavy quark in proton–proton collisions at the LHC has been found to be enhanced compared to more elementary e⁺e^– collisions. This surprising finding seems to invalidate the concept of universal hadronisation of heavy quarks, which is an important basis for calculations of particle production in QCD.

A new dimension

Heavy-flavour baryons carrying charm and strange quarks add a new dimension to these measurements. Such measurements are challenging because they suffer from low production rates. Due to the short lifetime of charm baryons (typically a fraction of a picosecond), they are usually observed through the detection of their decay products. The probability of how often they decay into a particular set of daughter particles, known as the branching ratio (BR), is poorly known for many of the strange-charm baryons. Knowledge of the precise branching ratio is crucial for interpreting the production results of these baryons.

Recently, the ALICE collaboration has measured the production of Ω_c⁰ (css) baryons via the semileptonic decay channel  Ω_c⁰ → Ω^–e⁺ν_e (and its charge-conjugate modes) as a function of transverse momentum (p_T) in proton–proton collisions at 13 TeV at midrapidity (|y| < 0.8). The Ω_c⁰ candidates are built by pairing an electron or positron candidate track with an Ω baryon candidate using a Kalman Filter vertexing algorithm. The Ω candidates are reconstructed via the cascading decay chain Ω^– → ΛK^–, followed by the decay Λ → pπ^–. The missing momentum of the neutrino was corrected by using an unfolding technique. Figure 1 shows the invariant-mass distribution of the Ω_c⁰ candidates.

Figure 2 compiles measurements of the decay by CLEO, Belle and now ALICE. Due to the lack of an absolute BR, results are quoted relative to the BR of Ω_c⁰ → Ω^–π⁺. Combined with the earlier measurement of Ω_c⁰ → Ω^–π⁺, the relative probability of the two decay modes is obtained: BR(Ω_c⁰ → Ω^–e⁺ν_e)/BR(Ω_c⁰ → Ω^–π⁺) = 1.12 ± 0.22 (stat.) ± 0.27 (syst.). The Belle and CLEO collaborations have measured this ratio to be 1.98 ± 0.13 (stat.) ± 0.08 (syst.) and 2.4 ± 1.1 (stat.) ± 0.2 (syst.). Model predictions using the light-front approach and light-cone sum rules predict values of 1.1 ± 0.2 and 0.71, respectively. Another approach calculates decay modes and probabilities of charmed-baryon decays based on SU(3)_f flavour symmetry in the quark model, resulting in a computed branching fraction ratio of 1.35.

The ALICE result is consistent with theory calculations and is 2.3σ lower than the more precise value reported by the Belle collaboration. The present measurement provides constraints on the decay probabilities of the Ω_c⁰ baryons. It demonstrates that such measurements are now possible at the LHC with a precision similar to that at e⁺e^– colliders.

With the ongoing Run 3 at the LHC and thanks to the recent upgrades, ALICE is on the way to collecting a data sample that is about a thousand times larger for these types of analyses, which will enable more precise measurements of other decay modes. Thanks to these data, we expect to resolve the question of universal hadronisation in the near future.

The post Intrigue in charm hadronisation appeared first on CERN Courier.

In the past two decades, it has become clear that three-quark baryons and quark–antiquark mesons cannot describe the full spectrum of hadrons. Dozens of exotic states have been observed in the charm sector alone. These states are either interpreted as compact objects with four or five valence quarks or as hadron molecules, however, their inner structures remain uncertain due to the complexity of calculations in quantum chromodynamics (QCD) and the lack of direct experimental measurements of the residual strong interaction between charm and light hadrons. New femtoscopy measurement by the ALICE collaboration challenge theoretical expectations and the current understanding of QCD.

Femtoscopy is a well-established method for studying the strong interactions between hadrons. Experimentally, this is achieved by studying particle pairs with small relative momentum. In high-energy collisions of protons at the LHC, the distance between such hadrons at the time of production is about one femtometre, which is within the range of the strong nuclear force. From the momentum correlations of particle pairs, one extracts the scattering length, a₀, which quantifies the final-state strong interaction between the two hadrons. By studying the momentum correlations of emitted particle pairs, it is possible to access the final-state interactions of even short-lived hadrons such as D mesons.

The scattering lengths are significantly smaller than the theoretical predictions

The ALICE collaboration has now, for the first time, measured the interaction of open-charm mesons (D⁺ and D^*+) with charged pions and kaons for all the charge combinations. The momentum correlation functions of each system were measured in proton–proton collisions in the LHC at a centre-of-mass energy of 13 TeV. As predicted by heavy-quark spin symmetry, the scattering lengths of Dπ and D^*π agree with each other, but they are found to be significantly smaller than the theoretical predictions (figure 1). This implies that the interaction between these mesons can be fully explained by the Coulomb force, and the contribution from strong interactions is negligible within experimental precision. The small measured values of the scattering length challenge our understanding of the residual strong force of heavy-flavour hadrons in the non-perturbative limit of QCD.

These results also have an important impact on the study of the quark–gluon plasma (QGP) – a deconfined state of matter created in ultra-relativistic heavy-ion collisions. The rescattering of D mesons with the other hadrons (mostly pions and kaons) created in such collisions was thought to modify the D-meson spectra, in addition to the modification expected from the QGP formation. The present ALICE measurement demonstrates, however, that the effect of rescattering is expected to be very small.

More precise and systematic studies of charm–hadron interactions will be carried out with the upgraded ALICE detector in the upcoming years.

The post Shy charm mesons confound predictions appeared first on CERN Courier.

The conference highlighted the impressive progress and diversity of UPC physics, which goes far beyond the initial studies of exclusive pro­-cesses. UPC23 covered the latest results from experiments at RHIC and the LHC, and prospects for the future Electron-Ion Collider (EIC) at Brookhaven National Laboratory. Discussions delved into the intricacies of inelastic photo-nuclear events, including the exciting programme of open charm that is yet to be explored, and examined how UPCs serve as a novel lens for investigating the quark–gluon plasma and other final-state nuclear effects. Lots of attention was devoted to the physics of low-x parton densities – a fundamental aspect of protons and nuclei that photons can probe in a unique way.

Enriched understanding

Among the conference’s theoretical highlights, Farid Salazar (UCLA) showed how vector–meson photoproduction could be a powerful method to detect gluon saturation across different collision systems, from proton–nucleus to electron–nucleus to UPCs. Zaki Panjsheeri (Virginia) put forth innovative ideas to study double-parton correlations, linking UPC vector–meson studies to generalised parton distributions, enhancing our understanding of the proton’s structure. Ashik Ikbal (Kent State), meanwhile, introduced exciting proposals to investigate quantum entanglement through exclusive J/ψ photoproduction at RHIC.

The conference also provided a platform for discussing the active exploration of light-by-light scattering and two-photon processes for probing fundamental physics and searches for axion-like particles, and for putting constraints on the anomalous magnetic moment of the tau lepton (see CMS closes in on tau g–2).

Energy exploration

Physicists at the LHC have effectively repurposed the world’s most powerful particle accelerator into a high-energy photon collider. This innovative approach, traditionally the domain of electron beams in colliders like LEP and HERA, and anticipated at the EIC, allows the LHC to explore photon-induced interactions at energies never before achieved. David Grund (Czech Technical University in Prague), Georgios Krintiras (Kansas) and Cesar Luiz Da Silva (Los Alamos) shared the latest LHC findings on the energy dependence of UPC J/ψ events. These results are crucial for understanding the onset of gluon saturation – a state where gluons become so dense reaching saturation, the dynamical equilibrium where the emission and recombination occurs. However, the data also align with the nuclear phenomenon known as gluon shadowing, which arises from multiple-scattering processes. David Tlusty (Creighton) presented the latest findings from the STAR Collaboration, which has recently expanded its UPC programme, complementing the energy exploration at the LHC. Klaudia Maj (AGH University of Krakow) presented the latest results on two-photon interactions and photonuclear jets from the ATLAS collaboration, including measurements that may be probing the quark-gluon plasma. 

Delegates discussed the future opportunities for UPC physics with the large integrated luminosity expected for Runs 3 and 4 at the LHC

Carlos Bertulani (Texas A&M) paid tribute to Gerhard Baur, who passed away on June 16 last year. Bertulani and Baur co-authored “Electromagnetic processes in relativistic heavy ion collisions” – a seminal paper with more than 1000 citations. Bertulani invited delegates to consider the untapped potential of UPCs in the study of anti-atoms and exotic atoms.

Delegates also discussed the future opportunities for UPC physics with the large integrated luminosity expected for Run 3 and Run 4 at the LHC, with the planned detector upgrades for Run 4 such as FoCal, the recent upgrades by STAR, the sPHENIX programme and at the EIC. Delegates are expecting event selection and instrumentation close to the beam line, for example using “zero degree” calorimeters, to offer the greatest experimental opportunities in the coming years.

The next edition of the UPC conference will take place in Saariselka, Finland in June 2025.

The post Ultra-peripheral conference debuts in Mexico appeared first on CERN Courier.

Among MENU 2023’s highlights on nucleon structure, a preliminary analysis by the NNPDF collaboration suggests that the proton contains more charm than anticharm, with Niccolò Laurenti (Università degli Studi di Milano) showing evidence of a non-vanishing intrinsic valence charm contribution to the proton’s wavefunction. Meanwhile, Michael Kohl (Hampton University) concluded that the proton–radius puzzle is still not resolved. To make progress, form-factor measurements in electron scattering must be scrutinised, and the use of atomic spectroscopy data clarified, he said.

Hadron physics

A large part of this year’s conference was dedicated to hadron spectroscopy, with updates from Belle II, BESIII, GlueX, Jefferson Lab, JPAC, KLOE/KLOE-2 and LHCb, as well as theoretical overviews covering everything from lattice quantum chromodynamics to effective-field theories. Special emphasis was also given to future directions in hadron physics at future facilities such as FAIR, the Electron-Ion Collider and the local Mainz Energy-Recovering Superconducting Accelerator (MESA) facility – a future low-energy but high-intensity electron accelerator that will make it possible to carry out experiments in nuclear astrophysics, dark-sector searches and tests of the SM. Among upgrade plans at Jefferson Lab, Eric Voutier (Paris-Saclay) presented a future experimental programme with positron beams at CEBAF, the institute’s Continuous Electron Beam Accelerator Facility. The upgrade will allow for a rich physics programme covering two-photon exchange, generalised polarisabilities, generalised parton distribution functions and direct dark-matter searches.

Highlights on nucleon structure include a preliminary analysis suggesting that the proton contains more charm than anticharm

Hadron physics is also closely related to searches for new physics, as precision observables of the Standard Model are in many cases limited by the non-perturbative regime of quantum chromodynamics. A prime example is the physics of the anomalous magnetic moment of the muon, for which a puzzling discrepancy between data-driven dispersive and lattice–quantum chromodynamics calculations of hadronic contributions to the Standard Model prediction persists (CERN Courier May/June 2021 p25). The upcoming collaboration meeting of the Muon g-2 Theory Initiative in September 2024 at KEK will provide important new insights from lattice QCD and e⁺e^– experiments. It remains to be seen whether the eventual theoretical consensus will confirm a significant deviation from the experimental value, which is currently being updated by Fermilab’s Muon g-2 experiment using their last three years of data.

The post Slim, charming protons on the menu in Mainz appeared first on CERN Courier.

The LHC’s dedicated heavy-ion experiment, ALICE, is to be equipped with an upgraded inner tracking system and a new forward calorimeter to extend its physics reach. The upgrades have been approved for installation during the next long shutdown from 2026 to 2028.

With 10 m² of active silicon and nearly 13 billion pixels, the current ALICE inner tracker, which has been in place since 2021, is the largest pixel detector ever built. It is also the first detector at the LHC to use monolithic active pixel sensors (MAPS) instead of the more traditional hybrid pixels and silicon microstrips. The new inner tracking system, ITS3, uses a novel stitching technology to construct MAPS of 50 µm thickness and up to 26 × 10 cm² in area that can be bent around the beampipe in a truly cylindrical shape. The first layer will be placed just 2 mm from the beampipe and 19 mm from the interaction point, with a much lighter support structure that significantly reduces the material volume and therefore its effect on particle trajectories. Overall, the new system will boost the pointing resolution of the tracks by a factor of two compared to the present ITS detector, strongly enhancing measurements of thermal radiation emitted by the quark–gluon plasma and enabling insights into the interactions of charm and beauty quarks as they propagate through it.

The new forward calorimeter, FoCal, is optimised for photon detection in the forward direction. It consists of a highly granular electromagnetic calorimeter, composed of 18 layers of 1 × 1 cm² silicon-pad sensors paired with tungsten converter plates and two additional layers of 30 × 30 μm² pixels, and a hadronic calorimeter made of copper capillary tubes and scintillating fibres. By measuring inclusive photons and their correlations with neutral mesons, as well as the production of jets and charmonia, FoCal will add new capabilities to explore the small Bjorken-x parton structure of nucleons and nuclei.

Technical design reports for the ITS3 and FoCal projects were endorsed by the relevant CERN review committees in March. The construction phase has now started, with the detectors due to be installed in early 2028 in order to be ready for data taking in 2029. The upgrades, in particular ITS3, are also an important step on the way to ALICE 3 – a major proposed upgrade of ALICE that, if approved, would enter operation in the mid-2030s.

The post New subdetectors to extend ALICE’s reach appeared first on CERN Courier.

Pentaquarks, bound states of five quarks predicted in the first formulation of the quark model in 1964, have had a troubled history. Following disputed claims of the discovery of light-flavour species over 20 years ago, pentaquarks with hidden charm are now well-established members of the hadronic spectrum. The breakthrough was achieved by the LHCb experiment in 2015 with the observation of P_c⁺ states in the J/ψ p system.

The P_c⁺ quark content (uudcc) implies that decays to two open-charm hadrons, such as Λ_c⁺ D⁰ or Λ_c⁺ D*⁰, are possible. The rates of such decays are important for understanding more about the nature of the P_c⁺ states, as different models predict rates that differ by orders of magnitude. Distinguishing between the proposed mechanisms by which pentaquarks, and excited hadrons in general, are produced and bound allows a better understanding of the dynamics of the strong interaction in the non-perturbative regime.

A new analysis by LHCb of the open-charm hadrons in Λ_b decays was presented at the International Conference on Meson-Nucleon Physics and the Structure of the Nucleon, held in Mainz in October. It concerns the first observation and measurement of the branching fractions of Λ_b⁰ → Λ_c⁺ D(*)⁰ K^– and Λ_b⁰ → Λ_c⁺ D_s^*– decays using proton–proton collision data collected during LHC Run 2.

All branching fractions are measured relative to the known Λ_b⁰ → Λ_c⁺ D_s^– decay mode, which is reconstructed with the same set of six final-state hadrons: p K^– π⁺ K⁺ π^– K^–. Many systematic uncertainties in the measured ratios therefore cancel out, making the precision on the relative branching fraction of Λ_b⁰ → Λ_c⁺ D⁰ K^– statistically limited. For Λ_b⁰ → Λ_c⁺ D^0* K^– and Λ_b⁰ → Λ_c⁺ D*^– the resulting branching fractions are systematically limited. This is because either a photon or neutral pion is not reconstructed, so their shape in the invariant mass spectrum of the reconstructed particles is more difficult to describe and more affected by the backgrounds (see figure 1, where the components with a missing photon for which a branching fraction is calculated are shown in orange and those with a missing neutral pion in green).

The partially reconstructed Λ_b⁰ → Λ_c⁺ D_s^*– decay cannot be used directly to search for pentaquarks, but it is an important input to model calculations. In addition, as a two-body decay, it is a powerful test of factorisation assumptions in heavy-quark effective theory.

In the Λ_b⁰ → Λ_c⁺ D(*)⁰  K^– decay, the production process of the P_c⁺ pentaquarks is the same as in the discovery channel, Λ_b⁰ → J/ψ p K^–. A comparison between the measured branching fractions and observed signal yields can thus be used to estimate the expected sensitivity for observing P_c⁺ signals in the open-charm channels. In particular, the rate of a Λ_b⁰ decay to Λ_c⁺ D⁰ K^– is about six times greater than to J/ψ p K^–; however, more than 60 times as much data would be needed to match the currently available Λ_b⁰ → J/ψ p K^– signal yield.

A factor of about 24 in this calculation comes from the branching fractions ratio of J/ψ and open-charm hadrons, given their reconstructed decay modes. The rest is from reconstruction and selection inefficiencies, which favour the four-prong μ⁺μ^– p K^– over the fully hadronic six-body final state. With the upgraded Run 3 detector and now triggerless detector readout, a large part of the inefficiency for fully hadronic final states is recoverable, making pentaquark searches in double open-charm final states more favourable compared to the situation in Run 2.

The post New pentaquark searches in beauty decays appeared first on CERN Courier.

When lead ions collide head-on at the LHC they deposit most of their kinetic energy in the collision zone, forming new matter at extremely high temperatures and energy densities. The hot and dense zone quickly expands and cools down, leading to the production of approximately equal numbers of particles and antiparticles at mid-rapidity. However, in reality the balance between matter and antimatter can be slightly distorted.

The collision starts with matter only, i.e. protons and neutrons from the incoming beam. During the collision process, incoming lead nuclei interact while penetrating each other, and most of their quantum numbers are carried away by particles travelling close to the beam direction. Due to strong interactions among the quarks and gluons, quantum numbers of the colliding ions are transported to mid-rapidity rather than to the ions themselves. This leads to an imbalance of baryons originating from the initial state, which has more baryons than antibaryons.

This matter–antimatter imbalance can be quantified by determining two global system properties: the chemical potentials associated with the electric charge and baryon number (denoted μ_Q and μ_B, respectively). In a thermodynamic description, the chemical potentials determine the net electric-charge and baryon-number densities of the system. Thus, μ_B measures the imbalance between matter and antimatter, with a vanishing value indicating a perfect balance.

In a new, high-precision measurement, the ALICE collaboration reports the most precise characterisation so far of the imbalance between matter and antimatter in collisions between lead nuclei at a centre-of-mass energy per nucleon pair of 5.02 TeV. The study was carried out by measuring the antiparticle-to-particle yield ratios of light-flavour hadrons, which make up the bulk of particles produced in heavy-ion collisions. The measurement using the ALICE central barrel detectors included identified charged pions, protons and multi- strange Ω^– baryons, in addition to light nuclei, ³He, triton and the hypertriton (a bound state of a proton, a neutron and a Λ-baryon). The larger baryon content of these light nuclei makes them more sensitive to baryon-asymmetry effects.

The medium created in lead–lead collisions at the LHC is nearly electrically neutral and baryon-number-free at mid-rapidity

The analysis reveals that in head-on lead–ion collisions, for every 1000 produced protons, approximately 986 ± 6 antiprotons are produced. The chemical potentials extracted from the experimental data are μ_Q = -0.18 ± 0.90 MeV and μ_B = 0.71 ± 0.45 MeV. These values are compatible with zero, showing that the medium created in lead–lead collisions at the LHC is nearly electrically neutral and baryon-number-free at mid-rapidity. This observation holds for the full centrality range, from collisions where the incoming ions peripherally interact with each other up to the most violent head-on processes, indicating that quantum-number transport at the LHC is independent of the size of the system formed.

The values of μ_B are shown in figure 1 as a function of the centre-of-mass energy of the colliding nuclei, along with lower-energy measurements at other facilities. The recent ALICE result is indicated by the red solid circle, along with a phenom­enological parametrisation of μ_B. The decreasing trend of μ_B observed as a function of increasing collision energy indicates that different net-baryon-number density conditions can be explored by varying the beam energy, reaching almost vanishing net-baryon content at the LHC. The inset gives the μ_B values extracted at two LHC energies. It shows that the new ALICE result is almost one order of magnitude more precise than the previous estimate (violet), thanks to a more refined study of systematic uncertainties.

The present study with improved precision characterises the vanishing baryon-asymmetry at the LHC, posing stringent limits to models describing baryon-number transport effects. Using the data samples collected in LHC Run 3, these studies will be extended to the strangeness sectors, enabling a full characterisation of quantum-number transport at the LHC.

The post Balancing matter and antimatter in Pb–Pb collisions appeared first on CERN Courier.

The very-high-energy densities reached in heavy-ion collisions at the LHC result in the production of an extremely hot form of matter, known as the quark-gluon plasma (QGP), consisting of freely roaming quarks and gluons. This medium undergoes a dynamic evolution before eventually transitioning to a collection of hadrons. But the details of this temporal evolution and phase transition are very challenging to calculate from first principles using quantum chromodynamics. The experimental study of the final-state hadrons produced in heavy-ion collisions therefore provides important insights into the nature of these processes. In particular, measurements of the pseudorapidity (η) distributions of charged hadrons help in understanding the initial energy density of the produced QGP and how this energy is transported throughout the event. These measurements involve different classes of collisions, sorted according to the degree of overlap between the two colliding nuclei; collisions with the largest overlap have the highest energy densities.

In 2022 the LHC entered Run 3, with higher collision energies and integrated luminosities than previous running periods. The CMS collaboration has now reported the first measurement using Run 3 heavy-ion data. Charged hadrons produced in lead–lead collisions at the record nucleon–nucleon centre-of-mass collision energy of 5.36 TeV were reconstructed by exploiting the pixel layers of the silicon tracker. At mid-rapidity and in the 5% most central collisions (which have the largest overlap between the two colliding nuclei), 2032 ± 91 charged hadrons are produced per unit of pseu­dorapidity. The data-to-theory comparisons show that models can successfully predict either the total charged-hadron multiplicity or the shape of its η distribution, but struggle to simultaneously describe both aspects.

Previous measurements have shown that the mid-rapidity yield of charged hadrons in proton-proton and heavy-ion collisions are comparable when scaled by the average number of nucleons parti­cipating in the collisions,〈N_part〉. Figure 1 shows measurements of this quantity in several collision systems as a function of collision energy. It was previously observed that central nucleus–nucleus collisions exhibit a power-law scaling, as illustrated by the blue dashed curve; the new CMS result agrees with this trend. In addition, the measurement is about two times larger than the values of proton–proton collisions at similar energies, indicating that heavy-ion collisions are more efficient at converting initial-state energy into final-state hadrons at mid-rapidity.

This measurement opens a new chapter in the CMS heavy-ion programme. At the end of 2023 the LHC delivered an integrated luminosity of around 2 nb^–1 to CMS, and more data will be collected in the coming years, enabling more precise analyses of the QGP features.

The post QGP production studied at record energies appeared first on CERN Courier.

Collisions between lead ions at the LHC produce the hottest system ever created in the lab, exceeding those in stellar interiors by about a factor of 10⁵. At such temperatures, nucleons no longer exist and quark–gluon plasma (QGP) is formed. Yet, a precise measurement of the initial temperature of the QGP created in these collisions remains challenging. Information about the early stage of the collision gets washed out because the system constituents continue to interact as it evolves. As a result, deriving the initial temperature from the hadronic final state requires a model-dependent extrapolation of system properties (such as energy density) by more than an order of magnitude.

In contrast, electromagnetic radiation in the form of real and virtual photons escapes the strongly interacting system. Moreover, virtual photons – emerging in the final state as electron–positron pairs (dielectrons) – carry mass, which allows early and late emission stages to be separated.

Radiation from the late hadronic phase dominates the thermal dielectron spectrum at invariant masses below 1 GeV. The yield and spectral shape in this mass window reflects the in-medium properties of vector mesons, mainly the ρ, and can be connected to the restoration of chiral symmetry in hot and dense matter. In the intermediate-mass region (IMR) between about 1 and 3 GeV, thermal radiation is expected to originate predominantly from the QGP, and an estimate of the initial QGP temperature can be derived from the slope of the exponential spectrum. This makes dielectrons a unique tool to study the properties of the system at its hottest and densest stage.

A new approach to separate the heavy-flavour contribution experimentally has been employed for the first time at the LHC

At the LHC, this measurement is challenging because the expected thermal dielectron yield in the IMR is outshined by a physical background that is about 10 times larger, mainly from semileptonic decays of correlated pairs of cc or bb hadrons. In ALICE, the electron and positron candidates are selected in the central barrel using complementary information provided by the inner tracking system (ITS), time projection chamber and time-of-flight measurements. Figure 1 (left) shows the dielectron invariant-mass spectrum in central lead–lead (Pb–Pb) collisions. The measured distribution is compared with a “cocktail” of all known contributions from hadronic decays. At masses below 0.5 GeV, an enhancement of the dielectron yield over the cocktail expectation is observed, which is consistent with calculations that include thermal radiation from the hadronic phase and an in-medium modification of the ρ-meson. Between 0.5 GeV and the ρ mass (0.77 GeV) a small discrepancy between the data and calculations is observed.

In the IMR, however, systematic uncertainties on the cocktail contributions from charm and beauty prevent any conclusion being drawn about thermal radiation from QGP. To overcome this limitation, a new approach to separate the heavy-flavour contribution experimentally has been employed for the first time at the LHC. This approach exploits the high-precision vertexing capabilities of the ITS to measure the displaced vertices of heavy-quark pairs. Figure 1 (right) shows the dielectron distribution in the IMR compared to template distributions from Monte Carlo simulations. The best fit includes templates from heavy-quark pairs and an additional prompt dielectron contribution, presumably from thermal radiation. This is the first experimental hint of thermal radiation from the QGP in Pb–Pb collisions at the LHC, albeit with a significance of 1σ.

Ongoing measurements with the upgraded ALICE detector will provide an unprecedented improvement in precision, paving the way for a detailed study of thermal radiation from hot QGP.

The post Dielectrons take the temperature of Pb–Pb collisions appeared first on CERN Courier.

The participants were treated to many LHC Run 2 legacy results, as well as brand-new ones using freshly analysed Run 3 data. A large chunk of these results comprised precision measurements of the Higgs boson in view of gaining a deeper understanding of the origin of electroweak symmetry breaking. As the Higgs boson is deeply connected to many open questions potentially linked to physics beyond the Standard Model (SM), such as the origin of particle masses and flavour, studying it in the context of effective field theory is a particularly hot topic. A rich potential programme of “simplified” models for Higgs physics that can better quantify the reach of the LHC and offer new observables is also under development.

New frontiers

The ATLAS and CMS collaborations presented no fewer than 37 and 27 new preliminary results, respectively. Besides Higgs-sector physics, the experiments revealed their latest results of searches for physics beyond the SM, including new limits on the existence of supersymmetric and dark-matter particles. At the intensity frontier, the latest search for the ultra-rare decay K⁺ → π⁺e⁺e^–e⁺e^– from the NA62 experiment placed upper limits on dark-boson candidate masses, underlining the powerful complementarity between CERN’s fixed-target and LHC programmes. The Belle II collaboration presented first evidence of the decay B⁺ → K⁺ νν, as well as the result of their R(X) = Br(B → Xτν_τ)/Br(B → Xℓν_ℓ) measurement – the first at a B factory. The LHCb collaboration also presented an update of its recent R(D^*) = Br(B → D^*τν_τ)/Br(B → D^*ℓν_ℓ) measurement. Another highlight was LHCb’s observation of the hypernuclei antihypertriton and hypertriton.

Intense discussions took place on novel and potentially game-changing accelerator concepts

The state of the art in neutrino physics was presented, covering the vast landscape of experiments seeking to shed light on the three-flavour paradigm as well as the origin of the neutrino masses and mixings. So far, analyses by T2K and NOvA show a weak preference for a normal mass ordering, while the inverted mass ordering is not yet ruled out. With a joint analysis between T2K and NOvA in progress, updates are expected next year. At CERN the FASER experiment, which made the first observation of muon neutrinos at a collider earlier this year, presented the first observation of collider electron neutrinos. Looking outwards, a long-awaited discovery of galactic neutrinos was presented by IceCube.

The current FCC feasibility status was presented, along with that of other proposed colliders that could serve as Higgs factories. The overarching need to join forces between the circular- and linear-collider communities and to use all the gained knowledge for getting at least one accelerator approved was reflected during the discussions and many talks, as were the sustainability and energy consumption of detector and accelerator concepts. Intense discussions took place on novel and potentially game-changing accelerator concepts, such as energy recovery technologies or plasma acceleration. While not yet ready to be used on a large scale, they promise to have a big impact on the way accelerators are built in the future. Beyond colliders, the community also looked ahead to the DUNE and Hyper-Kamiokande experiments, and to proposed experiments such as the Einstein Telescope and those searching for axions.

A rich social programme included a public lecture by Andreas Hoecker (CERN) about particle physics at the highest energies, a concert with an introduction to the physics of the organ by Wolfgang Hillert (University of Hamburg), as well as an art exhibition called “High Energy” and a Ukrainian photo exhibition depicting science during times of war.

The next EPS-HEP conference will take place in 2025 in Marseille.

The post Setting sail for HEP in Hamburg appeared first on CERN Courier.

At the end of two pedagogical seminars that Pietro Faccioli gave in April 2013 at CERN and HEPHY, Vienna, on the topic “Angular momentum and decay distributions in high energy physics: an introduction and use cases for the LHC”, several people, including myself, encouraged him to turn his slides into a textbook on particle polarisation. Ten years later I received a copy of Particle Polarization in High Energy Physics: An Introduction and Case Studies on Vector Particle Production at the LHC from Carlos Lourenço, co-author and Pietro’s long-term colleague. During this decade, much has been learned about particle polarisation and related topics, in particular thanks to measurements made at the LHC. As someone who had a front-row seat to observe this progress in the context of polarisation measurements in the CMS experiment, I can attest to the importance and timeliness of this book.

Throughout the first four chapters, the authors guide the reader through a mosaic of relatively easy paths that introduce important concepts, including among others: helicity conservation, parity properties, polarisation frames and their transformations, frame-independent polarisation, and the Lam–Tung relation. Throughout the narrative, they often present real or simulated examples of caveats that can induce irreversible distortions in the measured distributions, potentially biasing the experimental results or their interpretation. The second half of the book (running to another 150 pages) targets a more expert audience, interested, for example, in acquiring the background knowledge needed to study cascade decays to vector particles or smearing effects of higher-order QCD (“non-planar”) processes. Appendix B, in particular, with page-long equations and no figures, must have been prepared “on demand” for people studying rare Z and W radiative decays with LHC data.

The pedagogical style of the text and the quality of the figures have clearly benefitted from the multiple interactions that the authors had with many people through physics schools, university seminars and workshops. The reader can also easily appreciate that the authors contributed to the field of particle polarisation with several original ideas, both regarding the development of robust data-analysis methods and their phenomenological interpretations. It is particularly eye-opening to see how easy it is to obtain biased experimental results if the analysis methods follow simplified approaches, ignoring the intrinsic multidimensionality of polarisation measurements. While thetext is very well written, the aspect that most distinguishes this book from others on similar topics is the presence of several beautiful figures, providing a welcome visual presentation of non-trivial concepts.

The authors contributed to the field of particle polarisation with several original ideas

Thanks to the CERN-supported open-access publication, the book can be directly downloaded by anyone who is interested. Although many readers will prefer a paper copy, the PDF file has the advantage that the reader can very easily navigate within the book by clicking on the many links connecting the text to figures, equations, cited references, and even to words in the very useful index. It is particularly practical to be one click away from an equation shown, sometimes, a hundred pages earlier.

Given the steady increase in the size of data samples being collected by the LHC experiments and the role that the polarisation aspects play in precision measurements of Standard Model processes, as well as in improving the efficiency of searches for new particles, the authors may soon be tempted to write a sequel. In such a future edition, it would be good to include a list of exercises for the interested reader, based on the authors’ behind-the-scenes knowledge and including realistic “traps” that readers should avoid. This would strengthen even further the role of the book as a guide for students and researchers involved in analysis of experimental data or in the interpretation of results.

The post Particle Polarization in High Energy Physics appeared first on CERN Courier.

At the European Physical Society Conference on High Energy Physics, held in Hamburg in August, the LHCb collaboration announced first results on the production of antihelium and antihypertriton nuclei in proton–proton (pp) collisions at the LHC. These promising results open a new research field, that up to now has been pioneered by ground-breaking work from the ALICE collaboration on the central rapidity interval |y| < 0.5. By extending the measurements into the so-far unexplored forward region 1.0 < y < 4.0, the LHCb results provide new experimental input to derive the production cross sections of antimatter particles formed in pp collisions, which are not calculable from first principles.

LHCb’s newly developed helium-identification technique mainly exploits information from energy losses through ionisation in the silicon sensors upstream (VELO and TT stations) and downstream (Inner Tracker) of the LHCb magnet. The amplitude measurements from up to ~50 silicon layers are combined for each subdetector into a log-likelihood estimator. In addition, timing information from the Outer Tracker and velocity measurements from the RICH detectors are used to improve the separation power between heavy helium nuclei (with charge Z = 2) and lighter, singly charged particles (mostly charged pions). With a signal efficiency of about 50%, a nearly background-free sample of 1.1 × 10⁵ helium and antihelium nuclei is identified in the data collected during LHC Run 2 from 2016 to 2018 (see figure, inset).

The helium identification method proves the feasibility of new research fields at LHCb

As a first step towards a light-nuclei physics programme in LHCb, hypertritons are reconstructed via their two-body decay into a now-identified helium nucleus and a charged pion. Hypertriton (³_ΛH) is a bound state of a proton, a neutron and a Λ hyperon that can be produced via coalescence in pp collisions. These states provide experimental access to the hyperon–nucleon interaction through the measurement of their lifetime and of their binding energy. Hyperon–nucleon interactions have significant implications for the understanding of astrophysical objects such as neutron stars. For example, the presence of hypernuclei in the dense inner core can significantly suppress the formation of high-mass neutron stars. As a result, there is some tension between the observation of neutron stars heavier than two solar masses and corresponding hypertriton results from the STAR collaboration at Brookhaven. ALICE seems to have resolved the tension between hypertriton measurements at colliders and neutron stars. An independent confirmation of the ALICE result has up to now been missing, and can be provided by LHCb.

The invariant-mass distribution of hypertriton and antihypertriton candidates is shown in figure 1. More than 100 signal decays are reconstructed, with a statistical uncertainty on the mass of 0.16 MeV, similar to that of STAR. In a next step, corrections for efficiencies and acceptance obtained from simulation, as well as systematic uncertainties on the mass scale and lifetime measurement, will be derived.

The new helium identification method from LHCb summarised here proves the feasibility of a rich programme of measurements in QCD and astrophysics involving light antinuclei in the coming years. The collaboration also plans to apply the method to other LHCb Run 2 datasets, such as proton–ion, ion–ion and SMOG collision data.

The post Antinuclei production in pp collisions appeared first on CERN Courier.

About 90 physicists attended the sixth plenary workshop of the Muon g-2 Theory Initiative, held in Bern from 4 to 8 September, to discuss the status and strategies for future improvements of the Standard Model (SM) prediction for the anomalous magnetic moment of the muon. The meeting was particularly timely given the recent announcement of the results from runs two and three of the Fermilab g-2 experiment (Muon g-2 update sets up showdown with theory), which reduced the uncertainty of the world average to 0.19 ppm, in dire need of a SM prediction at commensurate precision. The main topics of the workshop were the two hadronic contributions to g-2, hadronic vacuum polarisation (HVP) and hadronic light-by-light scattering (HLbL), evaluated either with a lattice–QCD or data-driven approach.

Hadronic vacuum polarisation

The first one-and-a-half days were devoted to the evaluation of HVP – the largest QCD contribution to g-2, whereby a virtual photon briefly transforms into a hadronic “blob” before being reabsorbed – from e⁺e^– data. The session started with a talk from the CMD-3 collaboration at the VEPP-2000 collider, whose recent measurement of the e⁺e^– → π⁺π^– cross section generated shock waves earlier this year by disagreeing (at the level of 2.5–5σ) with all previous measurements used in the Theory Initiative’s 2020 white paper. The programme also featured a comparison with results from the earlier CMD-2 experiment, and a report from seminars and panel discussions organised by the Theory Initiative in March and July on the details of the CMD-3 result. While concerns remain regarding the estimate of certain systematic effects, no major shortcomings could be identified.

Further presentations from BaBar, Belle II, BESIII, KLOE and SND detailed their plans for new measurements of the 2π channel, which in the case of BaBar and KLOE involve large data samples never analysed before for this measurement. Emphasis was put on the role of radiative corrections, including a recent paper by BaBar on additional radiation in initial-state-radiation events and, in general, the development of higher-order Monte Carlo generators. Intensive discussions reflected a broad programme to clarify the extent to which tensions among the experiments can be due to higher-order radiative effects and structure-dependent corrections. Finally, updated combined fits were presented for the 2π and 3π channels, for the former assessing the level of discrepancy among datasets, and for the latter showing improved determinations of isospin-breaking contributions.

CMD-3 generated shock waves by disagreeing with all previous measurements at the level of 2.5-5σ

Six lattice collaborations (BMW, ETMC, Fermilab/HPQCD/MILC, Mainz, RBC/UKQCD, RC^*) presented updates on the status of their respective HVP programmes. For the intermediate-window quantity (the contribution of the region of Euclidean time between about 0.4–1.0 fm, making up about one third of the total), a consensus has emerged that differs from e⁺e^–-based evaluations (prior to CMD-3) by about 4σ, while the short-distance window comes out in agreement. Plans for improved evaluations of the long-distance window and isospin-breaking corrections were presented, leading to the expectation of new, full computations for the total HVP contribution in addition to the BMW result in 2024. Several talks addressed detailed comparisons between lattice-QCD and data-driven evaluations, which will allow physicists to better isolate the origin of the differences once more results from each method become available. A presentation on possible beyond-SM effects in the context of the HVP contribution showed that it seems quite unlikely that new physics can be invoked to solve the puzzles.

Light-by-light scattering

The fourth day of the workshop was devoted to the HLbL contribution, whereby the interaction of the muon with the magnetic field is mediated by a hadronic blob connected to three virtual photons. In contrast to HVP, here the data-driven and lattice-QCD evaluations agree. However, reducing the uncertainty by a further factor of two is required in view of the final precision expected from the Fermilab experiment. A number of talks discussed the various contributions that feed into improved phenomenological evaluations, including sub-leading contributions such as axial-vector intermediate states as well as short-distance constraints and their implementation. Updates on HLbL from lattice QCD were presented by the Mainz and RBC/UKQCD groups, as were results on the pseudoscalar transition form factor by ETMC and BMW. The latter in particular allow cross checks of the numerically dominant pseudoscalar- pole contributions between lattice QCD and data-driven evaluations.

It is critical that the Theory Initiative work continues beyond the lifespan of the Fermilab experiment

On the final day, the status of alternative methods to determine the HVP contribution were discussed, first from the MUonE experiment at CERN, then from τ data (by Belle, CLEOc, ALEPH and other LEP experiments). First MUonE results could become available at few-percent precision with data taken in 2025, while a competitive measurement would proceed after Long Shutdown 3. For the τ data, new input is expected from the Belle II experiment, but the critical concern continues to be control over isospin-breaking corrections. Progress in this direction from lattice QCD was presented by the RBC/UKQCD collaboration, together with a roadmap showing how, potentially in combination with data-driven methods, τ data could lead to a robust, complementary determination of the HVP contribution.

The workshop concluded with a discussion on how to converge on a recommendation for the SM prediction in time for the final Fermilab result, expected in 2025, including new information expected from lattice QCD, the BaBar 2π analysis and radiative corrections. A final decision for the procedure for an update of the 2020 white paper is planned to be taken at the next plenary meeting in Japan in September 2024. In view of the long-term developments discussed at the workshop – not least the J-PARC Muon g-2/EDM experiment, due to start taking data in 2028 – it is critical that the work by the Theory Initiative continues beyond the lifespan of the Fermilab experiment, to maximise the amount of information on physics beyond the SM that can be inferred from precision measurements of the anomalous magnetic moment of the muon.

The post Getting to the bottom of muon g-2 appeared first on CERN Courier.

High-energy heavy-ion collisions at the LHC exhibit strong collective flow effects in the azimuthal angle distribution of final-state particles. Since these effects are governed by the initial collision geometry of the two colliding nuclei and the hydrodynamic evolution of the collision, the study of anisotropic flow is a powerful way to characterise the production of the quark–gluon plasma (QGP) – an extreme state of matter expected to have existed in the early universe.

To their surprise, researchers on the ALICE experiment have now revealed similar flow signatures in small systems encompassing proton–proton (pp) and proton–lead (pPb) collisions, where QGP formation was previously assumed not to occur. The origin of the flow signals in small systems (and in particular whether the mechanisms behind these correlations in small systems share commonalities with heavy-ion collisions) are not yet fully understood. To better interpret these results, and thus to understand the limit of the system size that exhibits fluid-like behaviour, it is important to carefully single out possible scenarios that can mimic the effect of collective flow. 

Anisotropic-flow measurements become more difficult in small systems because non-flow effects, such as the presence of jets, become more dominant. Thus, it is important to examine methods where non-flow effects are properly subtracted first. One of the methods, the so-called low-multiplicity template fit, has been widely used in several experiments to determine and subtract the non-flow elements.

The origin of the flow signals in small systems is not yet fully understood

The ALICE collaboration studied long-range angular correlations for pairs of charged particles produced in pp and pPb collisions at centre-of-mass energies of 13 TeV and 5.02 TeV, respectively. Flow coefficients were extracted from these correlations using the template-fit method in samples of events with different charged-particle multiplicities. This method considers that the yield of jet fragments increases as a function of particle multiplicity and allows physicists to examine assumptions made in the low-multiplicity template fit for the first time – demonstrating their validity, including a possible jet-shape modification.

Figure 1 shows the measurement of two components of anisotropic flow – elliptic (v₂) and triangular (v₃) – as a function of charged-particle multiplicity at midrapidity (N_ch). The data show decreasing trends towards lower multiplicities. In pp collisions, the results suggest that the v₂ signal disappears below N_ch = 10. The results are then compared with hydrodynamic models. To accurately describe the data, especially for events with low multiplicities, a better understanding of initial conditions is needed.

These results can help to constrain the modelling of initial-state simulations, as the significance of initial-state effects increases for collisions resulting in low multiplicities. The measurements with larger statistics from Run 3 data will push down this multiplicity limit and reduce the associated uncertainties.

The post Collectivity in small systems produced at the LHC appeared first on CERN Courier.

Quarks and gluons are the only known elementary particles that cannot be seen in isolation. Once produced, they immediately start a cascade of radiation (the parton shower), followed by confinement, when the partons bind into (colour-neutral) hadrons. These hadrons form the jets that we observe in detectors. The different phases of jet formation can help physicists understand various aspects of quantum chromodynamics (QCD), from parton interactions to hadron interactions – including the confinement transition leading to hadron formation, which is particularly difficult to model. However, jet formation cannot be directly observed. Recently, theorists proposed that the footprints of jet formation are encoded in the energy and angular correlations of the final particles, which can be probed through a set of observables called energy correlators. These observables record the largest angular distance between N particles within a jet (x_L), weighted by the product of their energy fractions.

The CMS collaboration recently reported a measurement of the energy correlators between two (E2C) and three (E3C) particles inside a jet, using jets with p_T in the 0.1–1.8 TeV range. Figure 1 (top) shows the measured E2C distribution. In each jet p_T range, three scaling regions can be seen, corresponding to three stages in jet-formation evolution: parton shower, colour confinement and free hadrons (from right to left). The opposite E2C trends in the low and high x_L regions indicate that the interactions between partons and those between hadrons are rather different; the intermediate region reflects the confinement transition from partons to hadrons.

Theorists have recently calculated the dynamics of the parton shower with unprecedented precision. Given the high precision of the calculations and of the measurements, the CMS team used the E3C over E2C ratio, shown in figure 1 (bottom), to evaluate the strong coupling constant α_S. The ratio reduces the theoretical and experimental uncertainties, and therefore minimises the challenge of distinguishing the effects of α_S variations from those of changes in quark–gluon composition. Since α_S depends on the energy scale of the process under consideration, the measured value is given for the Z-boson mass: α_S = 0.1229 with an uncertainty of 4%, dominated by theory uncertainties and by the jet-constituent energy-scale uncertainty. This value, which is consistent with the world average, represents the most precise measurement of α_S using a method based on jet evolution.

The post Measuring energy correlators inside jets appeared first on CERN Courier.

Entanglement is an extraordinary feature of quantum mechanics: if two particles are entangled, the state of one particle cannot be described independently from the other. It has been observed in a wide variety of systems, ranging from microscopic particles such as photons or atoms to macroscopic diamonds, and over distances ranging from the nanoscale to hundreds of kilometres. Until now, however, entanglement has remained largely unexplored at the high energies accessible at hadron colliders, such as the LHC.

At the TOP 2023 workshop, which took place in Michigan this week, the ATLAS collaboration reported a measurement of entanglement using top-quark pairs with one electron and one muon in the final state selected from proton–proton collision data collected during LHC Run 2 at a centre-of-mass energy of 13 TeV, opening new ways to test the fundamental properties of quantum mechanics.

Two-qubit system
The simplest system which gives rise to entanglement is a pair of qubits, as in the case of two spin-1/2 particles. Since top quarks are typically generated in top-antitop pairs (tt) at the LHC, they represent a unique high-energy example of such a two-qubit system. The extremely short lifetime of the top (10^-25 s, which is shorter than the timescale for hadronisation and spin decorrelation) means that its spin information is directly transferred to its decay products. Close to threshold, the tt pair produced through gluon fusion is almost in a spin-singlet state, maximally entangled. By measuring the angular distributions of the tt decay products close to threshold, one can therefore conclude whether the tt pair is in an entangled state.

For this purpose, a single observable can be used as an entanglement witness, D. This can be measured from the distribution of cos?, where ? is the angle between the charged lepton directions in each of the parent top and anti-top rest frames, with D = −3⋅⟨cos?⟩. The entanglement criterion is given by D = tr(C)/3 < −1/3, where tr(C) is the sum of the diagonal elements of the spin-correlation matrix C of the tt̄ pair before hadronisation effects occur. Intuitively, this criterion can be understood from the fact that tr(C) is the expectation value of the product of the spin polarizations, tr(C) =〈σ⋅σ〉, with σ, σ being the t,t polarizations, respectively (classically tr(C) ≤ 1, since spin polarizations are unit vectors).  D is measured in a region where the invariant mass is approximately twice the mass of the top quark, 340 < m_tt < 380 GeV, and is performed at particle level, after hadronisation effects occur.

This constitutes the first observation of entanglement between a pair of quarks and the highest-energy measurement of entanglement

The shape of cos? is distorted by detector and event-selection effects for which it has to be corrected. A calibration curve connecting the value of D before and after the event reconstruction is extracted from simulation and used to derive D from the corresponding measurement, which is then compared to predictions from state-of-the-art Monte Carlo simulations. The measured value D = -0.547 ± 0.002 (stat.) ± 0.021 (syst.) is well beyond 5σ from the non-entanglement hypothesis. This constitutes the first-ever observation of entanglement between a pair of quarks and the highest-energy measurement of entanglement.

Apart from the intrinsic interest of testing entanglement under unprecedented conditions, this measurement paves the way to use the LHC as a novel facility to study quantum information. Prime examples are quantum discord, which is the most basic form of quantum correlations; quantum steering, which is how one subsystem can steer the state of the other one; and tests of Bell’s inequalities, which explore non-locality.  Furthermore, borrowing concepts from quantum information theory inspires new approaches to search for physics beyond the Standard Model.

The post Highest-energy observation of quantum entanglement appeared first on CERN Courier.

Henri Navelet died on 3 July 2023 in Bordeaux, at the age of 84. Born on 28 October 1938, he studied at the École Normale Supérieure in Paris. He went on to become a specialist in strong interactions and was a leading member of the Service de Physique Théorique (SPhT, now Institut de Physique Théorique) of CEA Saclay since its creation in 1963. Henri stood out for his theoretical rigour and remarkable computational skills, which meant a great deal to his many collaborators.

In the 1960s, Henri was a member of the famous “CoMoNav” trio with two other SPhT researchers, Gilles Cohen-Tannoudji and André Morel. The trio was famous in particular for introducing the so-called Regge-pole absorption model into the phenomenology of high-energy (at the time!) strong interactions. This model was used by many physicists to untangle the multitude of reactions studied at CERN. Henri’s other noteworthy contributions include his work with Alfred H Mueller on very-high-energy particle jets, today commonly referred to as “Mueller-Navelet jets”, which are still the subject of experimental research and theoretical calculations in quantum chromodynamics.

Henri had a great sense of humour and human qualities that were highly motivating for his colleagues and the young researchers who met him during his long career. He was not only a great theoretical physicist, but also a passionate sportsman, training the younger generations. In particular, he ran the marathon in two hours, 59 minutes and 59 seconds. A valued researcher and friend has left us.

The post Henry Navelet 1938–2023 appeared first on CERN Courier.

To advance our understanding of gluonic saturated matter at the LHC, the ALICE collaboration has presented a new study using photon-induced interactions in ultra-peripheral collisions (UPCs). In this type of collision, one beam emits a very high energetic photon that strikes the other beam, giving rise to photon–proton, photon–nucleus and even photon–photon collisions. 

While we know that the proton – and most of the visible matter of the universe – is made of quarks bound together by gluons, quantum chromodynamics (QCD)  has not yet provided a complete understanding of the rich physics phenomena that occur in high-energy interactions involving hadrons. For example, it is not known how the distribution of gluons evolve at low values of Bjorken-x. The rapid increase in gluon density observed with decreasing x cannot continue forever as it would eventually violate unitarity. At some point “gluon saturation” must set in to curb this growth.

So far, it has been challenging to experimentally establish when saturation sets in. One can expect, however, that it should occur at lower energies for heavy nuclei than for protons. Thus, the ALICE Collaboration has studied the energy dependence of UPC processes for both protons and heavy nuclei. At the same time, other physics phenomena, such as gluon shadowing originating from multi-scattering processes, can exist with similar experimental signatures. The interplay between these phenomena is still an open problem in QCD.

ALICE has presented new results on J/ψ meson-production UPC, where the photon probes the whole nucleus. The new ALICE results, analysed using LHC Run 1 and Run 2 data, probe a wide range of photon-nucleus collision energies from around 10 GeV to 1000 GeV. These results confirm previous measurements by ALICE, obtained at lower energies, that indicated a strong nuclear suppression when such photon–nucleus data are compared to expectations from photon–proton interactions. The present analysis employs novel methods for extracting the energy dependence, providing new information to test theo­retical models. The present data at high energies can be described by both saturation-based and gluon shadowing models. The coherent J/ψ meson production at low energy, in the anti-shadowing region, is not described by these models, nor can available models fully describe the energy dependence of this process over the explored energy range.

ALICE will continue to investigate these phenomena in LHC Runs 3 and 4, where high-precision measurements with larger data samples and upgraded detectors will provide more powerful tools to better understand gluonic saturated matter.

The post Probing gluonic saturated matter appeared first on CERN Courier.

The primary goal of high-energy heavy-ion physics is the study of a new state of nuclear matter, quark–gluon plasma, a thermalised system of quarks and gluons. The study of proton–proton (pp) and proton–nucleus (pA) collisions provides the baseline for the interpretation of results from heavy-ion collisions. The study of pA collisions also helps researchers understand the effects of cold nuclear matter on the production of final-state particles.

Global observables, such as the number of produced particles (particle multiplicity) and their distribution in pseudorapidity (η), provide key information about particle-production mechanisms in these collisions. The total multiplicity is mostly determined by soft interactions, i.e. processes with small momentum transfer, which cannot be calculated using perturbative techniques and are instead modelled using non-perturbative phenomenological descriptions. For example, the distribution of the number of produced particles can be used to disentangle relative contributions to particle production from hard and soft processes using a two-component model.

ALICE has recently completed the measurement of the multiplicity and pseudorapidity density distributions of inclusive photons at forward rapidity, spanning the range η = 2.3 to 3.9, by using the photon multiplicity detector (PMD) in pp, pPb and Pbp collisions at a centre-of-mass energy of 5.02 TeV per nucleon pair using LHC Run 1 and 2 data. Since photons mostly originate from decays of neutral pions, this result complements existing measurements of charged-particle production. A comparative study of charged particles and inclusive photons can reveal possible similarities and differences in the underlying production mechanisms for charged and neutral particles.

The PMD uses the preshower technique, where a three-radiation-length-thick lead converter is sandwiched between two planes comprising an array of 184,320 gas-filled proportional counters. Photons are distinguished from hadrons in the PMD’s preshower plane by applying suitable thresholds on the number of detector cells and the energy deposited in reconstructed clusters.

The measured distributions are corrected for instrumental effects using a Bayesian unfolding method. This is the first time that the dependence of the inclusive photon production on the number of nucleons participating in the pPb collision and its scaling behaviour has been studied at the LHC.

Figure 1 (left) compares the pseudorapidity density distribution of inclusive photons in minimum bias pp, pPb and Pbp collisions measured at forward rapidity to that of charged particles at midrapidity. The pseudorapidity distribution of inclusive photons at forward rapidity smoothly matches that of charged particles at midrapidity, indicating that the production mechanisms for charged and neutral pions are similar. Figure 1 (right) shows the pseudorapidity density distribution of inclusive photons in pPb collisions for different multiplicity classes as estimated using the energy deposited in the zero-degree calorimeter (ZNA) at beam rapidity. The multiplicity in the most central collisions reaches values twice as large as those in minimum bias events. The data and model agree within one sigma of the measurement uncertainties.

These results of inclusive photon production in pp, pPb and Pbp collisions provide valuable input for the development of theoretical models and Monte Carlo event generators, and help to establish the baseline measurements for the interpretation of PbPb collision data.

The post Inclusive photon production at forward rapidities appeared first on CERN Courier.

A crucial missing piece in our understanding of quantum chromodynamics (QCD) is a complete description of hadronisation in hard scattering processes with a large momentum transfer, which has now been investigated by the LHCb collaboration in proton–lead (pPb) collisions. While perturbative QCD describes reasonably well the transverse momentum (p_T) dependence of heavy-quark production in proton–proton (pp) collisions, the situation is different in heavy-ion collisions due to the formation of quark–gluon plasma (QGP), which affects the behaviour of particles traversing the medium. In particular, hadronisation can be affected, modifying the relative abundance of hadrons compared to pp collisions. Several models predict an enhanced strange-quark production. Thus an abundance of strange baryons is seen as a signature of QGP formation.

The role that QGP may play in pPb collisions is currently unclear. Some models predict the formation of “QGP droplets”, which could partially induce the same behaviour, albeit less pronounced, as in PbPb collisions. In addition, in pPb interactions, “cold nuclear matter” (CNM) effects are also present that can mimic the behaviour caused by QGP but via different mechanisms. For all these reasons, a strangeness enhancement in pPb collisions would strongly indicate the formation of a deconfined medium in small systems, providing crucial information about QGP properties and formation once the CNM effects are under control.

The LHCb collaboration recently analysed pPb data for QGP effects with the twofold purpose of searching for strangeness enhancement and providing a precise understanding of the CNM effects. This search was performed by measuring the production ratio of the strange baryon Ξ⁺_c, which has never been observed in pPb collisions before, to the strangeless baryon Λ⁺_c. Using an earlier pPb sample, LHCb has also studied the ratios of the D⁺_s, D⁺ and D⁰ , the first being measured for the first time down to zero p_T in the forward region, precisely addressing CNM effects. All measurements are performed differentially in p_T and the rapidity of the produced particle, and compared to the latest theory predictions. The Ξ⁺_c cross section has been measured for the first time in pPb collisions, giving strong indications on the factorisation scale μ₀ of the theory model. This result allows to set the absolute scale of the theoretical computations in terms of strangeness production, a trend confirmed with even higher precision by comparing the measurement to the Λ⁺_c production-cross section evaluated in the same decay mode. Moreover, the ratio is roughly constant as a function of p_T  and behaves in the same way at positive (pPb) and negative (Pbp) rapidities (see figure 1). The measurement is consistent with models incorporating initial-state effects due to gluon-shadowing in nuclei, suggesting that QGP formation and the resulting strangeness enhancement have little or no effect on Ξ⁺_c production in pPb collisions.

This interpretation is confirmed by the measurement of the D⁺_s, D⁺ and D⁰ cross sections and corresponding ratios in different rapidity regions. While the ratios show little enhancement within the statistical uncertainty, a large asymmetry is observed in the forward-backward production. This strongly indicates CNM effects and provides detailed constraints on models of nuclear parton distribution functions and hadron production in a very wide range of Bjorken-x (10^–2– 10^–5). A strong suppression is observed for the D mesons, giving insight into the nature of the CNM effects involved. An explanation via additional final-state effects is challenged by the Ξ⁺_c data that are well described by models not including them. The production ratios of Ξ⁺_c, D⁺_s, D⁺ and D⁰ measured as a function of p_T in pPb collisions confirm these findings. All these studies will profit from the increased statistics in pPb collisions that are expected from future LHC runs.

The post Charm production in proton–lead collisions appeared first on CERN Courier.

Although the charge and spin of the proton have been extensively studied for decades, relatively little is known about its mass distribution. This is because gluons, which despite being massless provide a sizeable contribution to the proton’s mass, are neutral and thus cannot be studied directly using electromagnetic probes. The Jefferson team instead used the gluonic gravitational form factors (GFFs). Similar to electromagnetic form factors, which provide information about a hadron’s charge and magnetisation distributions, the GFFs (technically the matrix elements of the proton’s energy–momentum tensor) encode mechanical properties of the proton such as its mass, density, pressure and shear distributions.

To access the GFFs, the team measured the threshold cross section of exclusive J/ψ photoproduction at different energies by forcing photons with energies between 9.1 and 10.6 GeV to interact with a liquid hydrogen target. Gluons dominate the production of J/ψ at small momentum transfer since J/ψ mesons share no valence quarks with the proton. Due to the J/ψ’s vector quantum numbers, this process can occur at certain energies by gluons in scalar (dilaton-like) and tensor (graviton-like) states. The researchers fed their cross-section results into QCD models describing the gluonic GFFs and extracted the parameters defining the GFFs, enabling them to deduce one mass radius and one scalar radius.

We need a new generation of high-precision J/ψ experiments to get a better picture

Zein-Eddine Meziani

The analysis revealed a scalar proton radius of 1 fm, which is substantially larger than both the charge radius (around 0.85 fm) and the proton mass radius (0.75 fm). This led the team to propose that the proton structure consists of three distinct regions: an inner core that makes up most of the mass radius and is dominated by the tensor gluonic field structure, followed by the charge radius resulting from the relativistic motion of quarks, all enveloped in a larger confining scalar gluon density.

“Given that the proton’s scalar gluon radius is the largest we need to understand how this converts to our understanding of the gluonic structure of nuclei. For example, what would be the scalar radius of ⁴He compared to its charge radius?” says study leader Zein-Eddine Meziani of Argonne. The team plans to extend its studies to include the J/ψ muon final state decay, doubling the statistics of the current measurement, and to extract the gluon pressure distribution. “It is hard to say much right now, but this is a field in its infancy and the direct role of gluons in nuclei is not well understood,” adds Meziani. “We need a new generation of high-precision J/ψ experiments to get a better picture.”

The post Proton structure consists of three distinct regions appeared first on CERN Courier.

Measurements of the production of hadrons containing heavy quarks (i.e. charm or beauty) in proton–proton (pp) collisions provide an important test of the accuracy of perturbative quantum chromodynamics (pQCD) calculations. The production of heavy quarks occurs in initial hard scatterings of quarks and gluons, whereas the production of light quarks in the underlying event is dominated by soft processes. Thus, measuring heavy-quark hadron production as a function of the charged-particle multiplicity provides insights into the interplay between soft and hard mechanisms of particle production.

Measurements in high-multiplicity pp collisions have shown features that resemble those associated with the formation of quark–gluon plasma in heavy-ion collisions, such as the enhancement of the production of particles with strangeness content and the modification of the baryon-to-meson production ratio as a function of transverse momentum (p_T). These effects can be explained by two different types of models: statistical hadronisation models, which evaluate the population of hadron states according to statistical weights governed by the masses of the hadrons and a universal temperature, or models that include hadronisation via coalescence (or recombination) of quarks and gluons which are close in phase space. Both predict an enhancement of the baryon-to-meson and strange-to-non-strange hadron ratios as a function of charged-particle multiplicity.

In the charm sector, the ALICE collaboration has recently observed a multiplicity dependence of the p_T-differential Λ_c⁺/D⁰ ratio, smoothly evolving from pp to lead–lead collisions, while no dependence was observed for the D_s⁺-meson production yield compared to the one of the D⁰ meson. Measurements of these phenomena in the beauty sector are needed to shed further light on the hadronisation mechanism.

To investigate beauty-quark production as a function of multiplicity and to put it in relation with that of charm quarks, ALICE measured for the first time the fraction of D⁰ and D⁺ originating from beauty-hadron decays (denoted as non-prompt) as a function of transverse momentum and charged-particle multiplicity in pp collisions at 13 TeV, using the Run 2 dataset. The measurement exploits different decay-vertex topologies of prompt and non-prompt D mesons with machine-learning classification techniques. The fractions of non-prompt D mesons were observed to somewhat increase with p_T from about 5 to 10%, as expected by pQCD calculations (figure 1). Similar fractions were measured in different charged-particle multiplicity intervals, suggesting either no or only mild multiplicity dependence. This suggests a similar production mechanism of charm and beauty quarks as a function of multiplicity.

The possible influence of the hadronisation mechanism was investigated by comparing the measured D-meson non-prompt fractions with predictions based on Monte Carlo generators such as PYTHIA 8. A good agreement was observed with different PYTHIA tunes, with and without the inclusion of the colour-reconnection mechanism beyond the leading colour approximation (CR-BLC), which was introduced to describe the production of charm baryons in pp collisions. Only the CR-BLC “Mode 3” tune that predicts an increase (decrease) of hadronisation in baryons for beauty (charm) quarks at high multiplicity is disfavoured by the current data.

The measurements of non-prompt D⁰ and D⁺ mesons represent an important test of production and hadronisation models in the charm and beauty sectors, and pave the way for future measurements of exclusive reconstructed beauty hadrons in pp collisions as a function of charged-particle multiplicity.

The post Beauty quark production versus particle multiplicity appeared first on CERN Courier.

Half a century since its inception, quantum chromodynamics (QCD) continues to prove itself as the correct description of the strong interaction between quarks and gluons. At low energies, however, perturbative calculations in QCD are not possible. Therefore, understanding the properties of hadrons usually requires the development of phenomenological models.

The study of exotic hadrons made up of more than three quarks offers a powerful way to gain a deeper understanding of the non-perturbative behaviour of QCD. The LHC has so far discovered no fewer than 23 new exotic hadrons, most of which were first observed by the LHCb experiment. In March 2021 the LHCb collaboration reported the observation of two tetraquarks with the c c u s quark content – named T^θ_ψs1(4000)⁺ and T^θ_ψs1(4220)⁺ – in the decay B⁺ → J/ψφK⁺. Now, based on a study of the isospin-symmetry-related decay B⁰ → J/ψφK⁰_S using a sample of about 2000 candidate events, the collaboration has found evidence for a new tetraquark state, T^θ_ψs1(4000)⁰, with a minimal quark content c c d s . The name of the new state follows a convention introduced by LHCb in 2022 to help simplify the exotic- hadron vista.

The T^θ_ψs1(4000)⁰ state was found as a resonance in the J/ψK⁰_S mass spectrum through an amplitude analysis, and is characterised as a horizontal band in the Dalitz plot (figure 1). Imposing isospin symmetry for all intermediate states except for the T^θ_ψs1(4000)^+/0 in the two B-meson decays, the signal significance is measured to be 4.0σ. The mass and width are found to be equal to those of the T^θ_ψs1(4000)⁺ within uncertainties, which is consistent with the new state being an isospin partner of the T^θ_ψs1(4000)⁰. If isospin symmetry between the two states is further applied, the T^θ_ψs1(4000)⁰ significance increases to 5.4σ.

The T^θ_ψs1(4000)⁺ and T^θ_ψs1(4000)⁰ states are not the only pair of isospin partners of hidden-charm tetraquark candidates with strangeness. Recently the BESIII collaboration reported signals of the T_ψs(3985)⁺ and T_ψs(3985)⁰ states with a minimal quark content of c c u s and c c d s , respectively. Although the tetraquark candidates seen by BESIII and LHCb have similar masses, the natural widths measured by each experiment are significantly different, indicating that they are distinct states.

Further studies and theoretical inputs are needed to determine the inner structure of such hidden-charm tetraquark candidates, for example whether they are compact tetraquarks, hadron molecules or produced due to kinematic effects. Despite continuous efforts, the detailed mechanisms responsible for binding multi-quark states have remained mysterious. With the start of LHC Run 3 and a new upgraded detector, the LHCb collaboration can look forward to finding further exotic states that shed light on the low-energy behaviour of QCD relevant to hadronic matter.

The post LHCb sees evidence for a new tetraquark state appeared first on CERN Courier.

Due to asymptotic freedom, the theory of quantum chromodynamics (QCD), which describes the strong interaction acting on quarks and gluons, becomes weaker at short-distance (high-energy) scales. The value of the strong coupling constant α_s at different scales can be determined using experimental observables that characterise the geometrical distribution of the outgoing hadrons from particle collisions. Among such observables are energy–energy correlations, which are obtained by computing the angular difference between all the possible pairs of final-state particles, weighted by the product of the normalised energies of the two particles involved. 

Precision tests

Energy–energy correlations had a significant impact on early precision tests of QCD, such as those performed at the PETRA e⁺e^– collider at DESY, where the gluon was discovered, and on the determination of α_s. At hadron colliders such as the LHC, where the longitudinal momentum of the colliding partons is unknown, longitudinally invariant quantities are defined instead. The transverse energy–energy correlation (TEEC) function is defined as the transverse energy-weighted azimuthal-angular distribution of jet pairs, covering the full cos(φ) range, where φ is the azimuthal angular difference between the two jets. The TEEC distribution peaks at the edges of the cos(φ) range due to self-correlations, i.e. the angle between a jet and itself, and due to di-jet back-to-back configurations arising from momentum conservation between both jets in the transverse plane. Moreover, due to additional gluon radiation, a central plateau whose height is sensitive to the value of α_s arises.

ATLAS has recently measured the TEEC distributions in multi-jet events and used them to determine α_s. The analysis is performed using the full data sample (139 fb^–1) of proton–proton collisions at a centre-of-mass energy of 13 TeV recorded during LHC Run 2. The measurements are presented in bins of the scalar sum of the transverse momenta of the two leading jets in the collision event (H_T2), and are corrected for detector effects. By fitting theoretical calculations to the experimental data, the value of α_s is determined in different kinematic regions, thereby testing the running of the strong coupling strength at high energy scales.

The results are 30% more precise than previous ATLAS measurements at 7 and 8 TeV, with a total systematic uncertainty of the order of 2.5% for the TEEC functions. The level of precision used for the theoretical predictions is equally important for the extraction of α_s as that of the data. Finite predictions as a function of the value of α_s(m_Z), where m_Z is the Z-boson mass, are calculated at next-to-next-to-leading order (NNLO) in perturbative QCD for three-jet configurations and corrected for non-perturbative effects, reducing the theoretical uncertainties in the central plateau from 6% at next-to-leading order (NLO) down to 2% for the NNLO predictions.

The ratio of data to theoretical prediction for the TEEC functions in the various H_T2 bins is shown in figure 1. The overall description of the shape at NNLO is found to be in agreement with the data, thus confirming our understanding of strong interactions over a large range of momentum transfers. Values of α_s are determined from individual fits of the theoretical predictions to data in each H_T2 bin. In addition, a global fit to the measured TEEC distributions results in α_s(m_Z) = 0.1175 ± 0.0001 (stat.) ± 0.0006 (syst.) ^+0.0034 _–0.0017 (theo.), where the theoretical uncertainty is dominated by the scale uncertainties that estimate the contribution of higher-order corrections. All extracted values of α_s agree with the recent world average. 

This result represents the first α_s determination with NNLO accuracy in three-jet production. Figure 2 compares the fitted values of α_s with previous determinations from ATLAS and other experiments as a function of the momentum scale. The measurements clearly exhibit asymptotic freedom, and a significant improvement in precision compared to previous measurements.

The post Strong coupling probed beyond the TeV scale appeared first on CERN Courier.

One of the fundamental questions in quantum chromodynamics is how hadrons are produced in high-energy collisions. More specifically: what determines the relative production rates of baryons, which contain three valence quarks, and mesons, which consist of a quark and an antiquark?

In electron–positron, electron–proton and proton–proton collisions, where a small number of particles is produced, the hadronisation process is expected to be universal: it does not depend on the type of colliding particles, but only on the parent quark or gluon. It has been found, however, that in proton–proton and proton–lead collisions at LHC energies, the baryon-to-meson ratios p/π, Λ/K_S⁰ and Λ_c/D⁰ are significantly larger than in e⁺e^– and ep collisions. This enhancement, at intermediate transverse momentum p_T (1–5 GeV), in small systems is qualitatively similar to that observed in heavy-ion collisions, e.g. between lead ions. However, in heavy-ion collisions this behaviour has been related to the interplay between hadronisation and the creation and expansion of a hot and dense quark–gluon plasma (QGP) via so-called quark recombination involving partons from within the QGP.

To shed light on hadron-production mechanisms in collisions at the LHC, ALICE has performed a novel study of the production of strange and multi-strange hadrons inside and outside energetic jets in proton–proton and proton–lead collisions. The method separates particles produced in association with a hard scattering process (within jets) from those produced in soft processes with low momentum transfer that dominate the underlying event.

The results show that the strange baryon-to-meson Λ/K_S⁰ ratio enhancement seen in the inclusive measurement is absent within the jet cone and restricted to soft-particle production processes outside the jet cone (figure 1, left). On the other hand, the multi- strange to single-strange hyperon yield ratio Ω^±/Λ (figure 1, right) shows a similar p_T dependence in the interval 2 < p_T < 5 GeV inside and outside the jet cone, while different behaviour is observed for the Ξ^±/Λ ratio (not shown). This suggests that the baryon production mechanism also depends on the strangeness content of the baryons; the production of hyperons in jets becomes similar to that in the underlying event for baryons with large strangeness content.

These measurements will help to improve the modelling of particle production at the LHC

These measurements provide new input to understand the relative contribution of soft and hard processes to multi-strange hadron production, and thus will help to improve the modelling of particle production at the LHC. To illustrate how, two calculations with different hadronisation models are shown in the figure: one model includes string formation beyond leading colour (CR-BLC, blue band), while the other includes colour ropes (red dashed line). While one of the models describes the Λ/K_S⁰ ratio inside jets, the other shows a better agreement with the Ω^±/Λ ratio.

ALICE has also investigated the multiplicity dependence of strange baryon-to-meson and baryon-to-baryon ratios in jets in proton–lead collisions and compared it with those in proton–proton collisions. Within the current experimental precision, no difference between the two collision systems, nor a dependence on the event multiplicity, is observed. These studies will further benefit from the increased precision that will be achieved with the substantially larger data samples from ongoing LHC runs.

The post Multi-strange production constrains hadronisation appeared first on CERN Courier.

The ALICE experiment at the LHC was conceived to study the properties of the quark–gluon plasma (QGP), the state of matter prevailing a few microseconds after the Big Bang. Collisions between large nuclei in the LHC produce matter at temperatures of about 3 × 10¹² K, sufficiently high to liberate quarks and gluons, and thus to study the deconfined QGP state in the laboratory. The heavy-ion programme at LHC Runs 1 and 2 has already enabled the ALICE collaboration to study the formation of the QGP, its collective expansion and its properties, using for example the interactions of heavy quarks and high-energy partons with the QGP. ALICE 3 builds on these discoveries to reach the next level of understanding. 

One of the most striking discoveries at the LHC is that J/ψ mesons not only “melt” in the QGP but can also be regenerated from charm quarks produced in independent hard scatterings. The LHC programme has also shown that the energy loss of partons propagating through the plasma depends on their mass. Furthermore, collective behaviour and enhanced strange-baryon production have been observed in selected proton–proton collisions in which large numbers of particles are produced, signalling that high densities may be reached in such collisions. 

During Long Shutdown 2, a major upgrade of the ALICE detector (ALICE 2) was completed on budget and in time for the start of Run 3 in 2022. Together with improvements in the LHC itself, the experiment will profit from a factor-50 higher Pb–Pb collision rate and also provide a better pointing resolution. This will bring qualitative improvements for the entire physics programme, in particular for the detection of heavy-flavour hadrons and thermal di-electron radiation. However, several important questions – for example concerning the mechanisms leading to thermal equilibrium and the formation of hadrons in the QGP – will remain open even after Runs 3 and 4. To address these, the collaboration is pursuing next-generation technologies to build a new detector with a significantly larger rapidity coverage and excellent pointing resolution and particle identification (see “Brand new” figure). A letter of intent for ALICE 3, to be installed in 2033/2034 (Long Shutdown 4) and operated during Runs 5 and 6 (starting in 2035), was submitted to the LHC experiments committee in 2021 and led to a positive evaluation by the extended review panel in March 2022. 

Behind the curtain of hadronisation

In heavy-ion collisions at the LHC, a large amount of energy is deposited in a small volume, forming a QGP. The plasma immediately starts expanding and cooling down, eventually reaching a temperature at which hadrons are formed. Although hadrons formed at the boundary of this phase transition carry information about the expansion of the plasma, they do not inform us directly about the temperature and other properties of the hot plasma phase of the collision before hadronisation takes place. Photons and di-lepton pairs, which are produced as thermal radiation in electromagnetic processes and do not participate in the strong interaction, allow us to look behind the curtain of hadronisation. However, measurements of photon and dilepton emission are challenging due to the large background from electromagnetic decays of light hadrons and weak decays of heavy-flavour hadrons. 

One of the goals of the current ALICE 2 upgrades is to enable the first measurements of the thermal emission of electron–positron pairs (from virtual photons), and thus to determine the average temperature of the system before the formation of hadrons, during Runs 3 and 4. To further understand the evolution of temperature with time, larger data samples and excellent background rejection are needed. The early-stage temperature is determined from the exponential slope of the mass distribution above the ρ resonance, i.e. pair masses larger than 1.2 GeV/c² (see “Taking the temperature” figure, upper panel). ALICE 3 would be able to explore the time dependence of the temperature before hadronisation using more differential measurements, e.g. of the azimuthal asymmetry of di-electron emission and of the slope of the mass spectrum as a function of transverse momentum. 

The di-electron mass spectrum also carries unique information about the mechanism of chiral symmetry breaking – a fundamental quantum-chromodynamics (QCD) effect that generates most of the hadron mass. At the phase transition to the QGP, chiral symmetry is restored and quarks and gluons are deconfined. One of the predicted signals of this transition is mixing between the ρ and a₁ vector-meson states, which gives the di-electron invariant mass spectrum a characteristic exponential shape in the mass range above the ρ meson peak (0.8–1.1 GeV/c²). Only the excellent electron identification and rejection of electrons from heavy-flavour decays possible with ALICE 3 can give physicists experimental access to this effect (see “Taking the temperature” figure, lower panel).

Another important goal of the ALICE physics programme is to understand how energetic quarks and gluons interact with the QGP and eventually thermalise and form a plasma that behaves as a fluid with very low internal friction. The thermalisation process and the properties of the QGP are governed by low-momentum interactions between quarks and gluons, which cannot be calculated using perturbative techniques. Experimental input is therefore important to understand these phenomena and to link them to fundamental QCD.

Heavy quarks  

The heavy charm and beauty quarks are of particular interest because their interactions with the plasma can be calculated using lattice-QCD techniques with good theoretical control. Heavy quarks and antiquarks are mostly produced as back-to-back pairs in hard scatterings in the early phase of the collision. Subsequent interactions between the quarks and the plasma change the angle between the quark and antiquark. In addition, the “drag” from the plasma leads to an asymmetry in the overall azimuthal distributions of heavy quarks (elliptic flow) with respect to the reaction plane. The size of these effects is a measure of the strength of the interactions with the plasma. Since quark flavour is conserved in interactions in the plasma, measurements of hadrons containing heavy quarks, such as the D meson and Λ_c baryon, are directly sensitive to the interactions between heavy quarks and the plasma. While the increase in statistics and the improved spatial resolution of ALICE 2 will already allow us to measure the production of charm baryons, measurements of azimuthal correlations of charm–hadron pairs are needed to directly address how they interact with the plasma. These will only become possible with the precision, statistics and acceptance of ALICE 3. 

Heavier beauty quarks are expected to take longer to thermalise and therefore lose less information through their interactions with the QGP. Therefore, systematic measurements of transverse-momentum distributions and azimuthal asymmetries of beauty mesons and baryons in heavy-ion collisions are essential to map out the interactions of heavy-flavour quarks with the QGP and to understand the mechanisms that drive the system towards thermal equilibrium.

To understand how hadrons emerge from the QGP, those containing multiple heavy quarks are of particular interest because they can only be formed from quarks that were produced in separate hard-scattering processes. If full thermal equilibrium is reached in Pb–Pb collisions, the production rates of such states are expected to be enhanced by up to three orders of magnitude with respect to pp collisions. This implies enormous sensitivity to the probability for combining independently produced quarks during hadronisation and to the degree of thermalisation. At ALICE 3, the precision with which multi-charm baryon yields can be measured is enhanced (see “Multi-charm production” figure). 

In addition to precision measurements of di-electrons and heavy-flavour hadrons, ALICE 3 will allow us to investigate many more aspects of the QGP. These include fluctuations of conserved quantum numbers, such as flavour and baryon number, which are sensitive to the nature of the deconfinement phase transition of QCD. ALICE 3 will also aim to answer questions in hadron physics, for example by searching for the existence of nuclei containing charm baryons (analogous to strange baryons in hypernuclei) and by studying the interaction potentials between unstable hadrons, which may elucidate the structure of exotic hadronic states that have recently been discovered in electron–positron collisions and in hadronic collisions at the LHC. In addition, ALICE 3 will use ultra-peripheral collisions to study the structure of resonances such as the ρ′ and to look for new fundamental particles, such as axion-like particles and dark photons. A dedicated detector system is foreseen to study very low-energy photon production, which can be used to test “soft theorems” that link the production of very soft photons in a collision to the hadronic final state.

Pushing the experimental limits 

To pursue this ambitious physics programme, ALICE 3 is designed to be a compact, large-acceptance tracking and particle-identification detector with excellent pointing resolution as well as high readout rates. The main tracking information is provided by an all-silicon tracker in a magnetic field provided by a superconducting magnet system, complemented by a dedicated vertex detector that will have to be retractable to provide the required aperture for the LHC at injection energy. To achieve the ultimate pointing resolution, the first hits must be detected as close as possible to the interaction point (5 mm at the highest energy) and the amount of material in front of it be kept to a minimum. The inner tracking layers will also enable so-called strangeness tracking – the direct detection of strange baryons before they decay – to improve the pointing resolution and suppress combinatorial background, for example in the measurement of multi-charm baryon decays.

ALICE 3 is a compact, large-acceptance tracking and particle-identification detector with excellent pointing resolution as well as high readout rates

First feasibility studies of the mechanical design and the integration with the LHC for the vertex tracker have been conducted and engineering models have been produced to demonstrate the concept and explore production techniques for the components (see “Close encounters” image). The detection layers are to be constructed from bent, wafer-scale pixel sensors. The development of the next generation of CMOS pixel sensors in 65 nm technology with higher radiation tolerance and improved spatial resolution has already started in the context of the ITS 3 project in ALICE, which will be an important milestone on the way to ALICE 3 (see “Next-gen tracking” image). The outer tracker, which has to cover the cylindrical volume to a radius of 80 cm over a total length of ±4 m, will also use CMOS pixel sensors. These will be integrated into larger modules for an effective instrumentation of about 60 m² while minimising the material used for mechanical support and services. The foreseen material budget for the tracker is 1% of a radiation length per layer for the outer tracker, and only 0.05% per layer for the vertex tracker.

For particle identification, five different detector systems are foreseen: a silicon-based time-of-flight system and a ring-imaging Cherenkov (RICH) detector that provide hadron and electron identification over a broad momentum range, a muon identifier starting from a transverse momentum of about 1.5 GeV/c, an electromagnetic calorimeter for photon detection and identification, and a forward tracker to reconstruct photons at very low momentum from their conversions to electron–positron pairs. For the time-of-flight system, the main R&D line aims at the integration of a gain layer in monolithic CMOS sensors to achieve the required time resolution of at least 20 ps (alternatively, low-gain avalanche diodes with external readout circuitry can be used). The calorimeter is based on a combination of lead-sampling and lead-tungstate segments, both of which would be read out by commercially available silicon photomultipliers (SiPMs). For the detection layers of the muon identifier, both resistive plate chambers and scintillating bars are being considered. Finally, for the RICH design, the R&D goal is to integrate the digital readout circuitry in SiPMs to enable efficient detection of photons in the visible range. 

ALICE 3 provides a roadmap for an exciting heavy-ion physics programme, along with the other three large LHC experiments, in Runs 5 and 6. An R&D programme for the coming years is being set up to establish the technologies and enable the preparation of technical design reports in 2026/2027. These developments not only constitute an important contribution to the full physics exploitation of the LHC, but are of strategic interest for future particle detectors and will benefit the particle and nuclear physics community at large.

The post ALICE 3: a heavy-ion detector for the 2030s appeared first on CERN Courier.

For almost 40 years, charmonium, a bound state of a heavy charm–anticharm pair (hence also called a hidden charm), has provided a unique probe to study the properties of the quark–gluon plasma (QGP), the state of matter composed by deconfined quarks and gluons present in the early instants of the universe and produced experimentally in ultrarelativistic heavy-ion collisions. Charmonia come in a rich variety of states. In a new analysis investigating how these different bound charmonium states are affected by the QGP, the ALICE collaboration has opened a novel way to study the strong interaction at extreme temperatures and densities. 

In the QGP, the production of charmonium is suppressed due to “colour screening” by the large number of quarks and gluons present. The screening, and thus the suppression, increases with the temperature of the QGP and is expected to affect different charmonium states to different degrees. The production of the ψ(2S) state, for example, which is 10 times more weakly bound and two times larger in size than the most tightly bound state, the J/ψ, is expected to be more suppressed. 

This hierarchical suppression is not the only fate of charmonia in the quark–gluon plasma. The large number of charm quarks and antiquarks in the plasma – up to about 100 in head-on lead–lead collisions – also gives rise to a mechanism, called recombination, that forms new charmonia and counters the suppression to a certain extent. This process is expected to depend on the type and momentum of the charmonia, with the more weakly bound charmonia being produced through recombination later in the evolution of the plasma and charmonia with the lowest (transverse) momentum having the highest recombination rate. 

Previous studies, using data first from the Super Proton Synchrotron and then from the LHC, have shown that the production of the ψ(2S) state is indeed more suppressed than that of the J/ψ, and ALICE has also previously provided evidence of the recombination mechanism in J/ψ production. But so far, no studies of ψ(2S) production at low transverse particle momentum had been precise enough to provide conclusive results in this momentum regime, preventing a complete picture of ψ(2S) production from being obtained.

The ALICE collaboration has now reported the first measurements of ψ(2S) production down to zero transverse momentum, based on lead–lead collision data from the LHC collected in 2015 and 2018. The results indicate that the ψ(2S) yield is largely suppressed with respect to a proton–proton baseline, almost a factor of two more suppressed than the J/ψ. The suppression, shown as a function of the collision centrality (N_part) in the figure, is quantified through the nuclear modification factor (R_AA), which compares the particle production in lead–lead collisions with respect to the expectations based on proton–proton collisions.  

Theoretical predictions based on a transport approach that includes suppression and recombination of charmonia in the QGP (TAMU) or on the Statistical Hadronisation Model (SHMc), which assumes charmonia to be formed only at hadronisation, describe the J/ψ data, while the ψ(2S) production is underestimated in central events by the SHMc. This observation represents one of the first indications that dynamical effects in the QGP, as taken into account in the transport models, are needed to reproduce the yields of the various charmonium states. It also shows that precision studies, including these and those of other charmonia, and foreseen for Run 3 of the LHC, may lead to a final understanding of the modification of the force binding these states in the extreme environment of the QGP. 

The post Hidden charm in the quark–gluon plasma appeared first on CERN Courier.

The LHC collaborations that study the top quark presented a wealth of recent results based on Run 2 data, many of which were shown for the first time, and even included a measurement with the very first data collected in Run 3. CMS and ATLAS presented new top-quark mass results, new measurements of top-quark production asymmetries, new cross-section measurements as well as searches for new production and decay modes, both within and beyond the Standard Model (SM). These included ttW and four top-quark production, and processes involving flavour-changing-neutral-current interactions that could produce sizable rates beyond the SM prediction.

Earlier this year, CMS released a preliminary mass measurement that profiles all uncertainties, including a finely split set of signal-modelling uncertainties based on variations of Monte Carlo generators. To account for the limited statistical power for some of these variations, this precision analysis implements a fully consistent treatment of the resulting fluctuations leading to a 380 MeV uncertainty. ATLAS presented a top-quark mass measurement of 172.63 ± 0.20 (stat) ± 0.67 (syst) ± 0.37 (recoil) GeV. The last uncertainty represents the ambiguity in assigning the recoil of gluon emissions in the top-quark decay chain that was neither considered in Run 1 analyses nor in the CMS measurement and requires further studies. The large difference in the modelling uncertainties assigned by both collaborations underlines the importance to overcome the limitations of Monte Carlo generators for these precision measurements.

Run 2 of the LHC opens up new production processes that could not be probed at the Tevatron or in Run 1. Recently, ATLAS announced the observation of the rare production process of a single top quark and a photon, thus completing the list of associated top-quark production processes with SM gauge bosons. CMS followed with a brand-new analysis of the four top-quark production process, the rarest process accessible by the LHC to date. Together with combined ATLAS analyses, there is now very strong evidence that this elusive process exists. While most results in the classical top-quark pair and single-top production modes agree very well with the SM predictions, slight excesses are seen in several rare production modes, such as ttW and four-top production. None of these excesses are statistically significant, but they form an interesting pattern that requires experimental results and theory predictions to be considered extra carefully, while keeping an eye open for more exotic explanations.

Theory ahead

Theory contributions at TOP 2022 revolved around two major themes: precision calculations and beyond-SM models. For the former, several groups presented new calculations that enable a more precise comparison of measurements with SM predictions. These calculations provide an integrated treatment of the top-quark and boson decays, including off-shell effects, which are small in the total cross section, but which can be significantly enhanced locally in some corners of phase space. Including these effects is therefore relevant for the highest-precision differential measurements at the LHC. For the second theme, the most popular approach is to expand around the SM with minimal model dependence using effective field theory. This is complemented by more focussed efforts in concrete new-physics scenarios, including composite Higgs (and top) models as well as leptoquarks. A dedicated theory mini-workshop discussed the interplay of top-quark measurements with results in flavour physics.

Perhaps the most exciting result, the first at Run 3, was presented by CMS. On 5 July, just two months before the conference, the LHC switched back on after a three-year shutdown and started to produce the first proton-proton collisions at a record centre-of-mass energy of 13.6 TeV. Stretching over the next few years, Run 3 will increase the size of available datasets involving top quarks by a factor of three to four. Both ATLAS and CMS made a tremendous effort to prepare the detectors, to collect and check the quality of the data, and to provide preliminary calibrations for leptons and jets. In a race against the clock, CMS isolated the top-quark pair production process in the data collected in July and August in time for the conference. Even at this very early stage, the data are understood well enough that a cross-section measurement with a total uncertainty below 8% was possible by making use of the top-quark events themselves to calibrate most of the relevant experimental uncertainties in situ.

With these first results showing that the LHC and the experiments are smoothly operating, TOP22 kicks off the Run 3 top-quark physics programme. We can look back on a very exciting edition of the TOP conference and look forward to meeting again in Michigan in 2023.

The post Back on TOP in Durham appeared first on CERN Courier.

The study of elastic hadron scattering is a cornerstone in understanding the non-perturbative properties of strong interactions. A key role is played by experiments at the LHC, where it is possible to precisely measure proton–proton (pp) interactions at a very high centre-of-mass energy. The goal is to detect the process pp → pp, in which the interacting protons remain intact and are scattered at angles of a few microradians with respect to the beamline. The importance of such measurements follows from their relation to the total hadronic pp cross section via the optical theorem, and to properties of proton interactions at asymptotically high energies via dispersion relations.

In ATLAS, elastic scattering is studied using a dedicated experimental setup – the ALFA detectors, which allow measurements of scattered-proton trajectories inside the beam pipe, just a few millimetres from the LHC beam. They are installed inside so-called Roman pots located at distances of 237 and 245 m on either side of the ATLAS interaction point.

Recently, ATLAS reported a measurement of elastic scattering at a centre-of-mass energy of 13 TeV. The data were collected with a special setting of the LHC magnets characterised by a high β* of 2500 m, which results in a large beam-spot size and a very small beam divergence. The latter allows precise measurements of small scattering angles. With these optics, the ALFA system detected events characterised by very small values of the Mandelstam t variable, which is proportional to the scattering angle squared. Measurements of small |t| values give access to the Coulomb–nuclear interference (CNI) kinematic region, where the contribution from electromagnetic and strong interactions are of similar magnitude. 

The ALFA detectors use scintillating- fibre technology to measure the position of the passing proton. The t value for each event is reconstructed from the measured positions using knowledge of the magnetic fields of the LHC magnets between the interaction point and the detectors. The selection of candidate events is based on the strong correlations between the elastically scattered protons, resulting from energy and momentum conservation. The analysis is heavily based on data-driven techniques, which are used for the alignment of the detectors, background estimation, evaluation of reconstruction efficiency and optics tuning.

Figure 1 presents the measured differential elastic cross section as a function of t. The shape of the distribution is sensitive to important physics parameters, such as the total cross section (σ_tot) and the ρ parameter, defined as the ratio of real to imaginary parts of the forward scattering amplitude. The smallest |t| values, and thus the smallest scattering angles, are dominated by the electromagnetic interaction between the protons. The CNI effects are strongest for |t| around 10^–3 GeV² and provide the sensitivity to the ρ parameter. For larger |t| values, the strong interaction dominates, and the spectrum depends on the value of σ_tot. The physics parameters are extracted from a fit to the t distribution.

The ρ parameter is related, through dispersion relations, to the energy dependence of σ_tot, with a certain sensitivity also to energies above those at the LHC. In addition, ρ is sensitive to possible differences between pp and pp scattering amplitudes at asymptotic energies. ATLAS measured ρ = 0.098 ± 0.011, in agreement with a previous TOTEM measurement. The result is in conflict with pre-LHC theoretical expectations (see the COMPETE line in figure 2), which assumed that no pp/pp  difference is present asymptotically and that σ_tot increases proportionally to the squared logarithm of the centre-of-mass energy, similarly to the evolution observed at accessible energies back then. This suggests that one of the above assumptions is incorrect: either the increase of σ_tot slows down above LHC energies, or protons and antiprotons interact differently at asymptotic energies. The second statement is often associated with the so-called odderon exchange. Both possibilities affect our understanding of the high-energy behaviour of strong interactions. 

ATLAS also measured the total pp hadronic cross section σ_tot = (104.7 ± 1.1) mb. This is the most precise measurement to date at this energy, due to a dedicated luminosity measurement that contributed less than 1 mb to the total systematic uncertainty. However, the long-standing tension between the ATLAS and TOTEM σ_tot measurements, with the latter being about 5% higher than ATLAS, persists.

ATLAS has collected more elastic scattering data in LHC Run 2, which are currently being analysed. New data taking is planned during Run 3, where a special run is foreseen at a centre-of-mass energy of 13.6 TeV.

The post Probing QCD beyond LHC energies appeared first on CERN Courier.

At the LHC, light nuclei and antinuclei are produced both in proton–proton and in heavy-ion collisions. Unstable nuclei, called hypernuclei, are also produced. First observed in cosmic rays in 1953, hypernuclei are formed by a mix of protons, neutrons and hyperons containing one or more strange quarks and undergo weak decays. Almost 70 years since their discovery, hypernuclei are still a source of fascination for nuclear physicists since their production is very rare and the measurement of their properties is extremely challenging.

The only hypernucleus observed so far at the LHC is the hypertriton (³_ΛH), composed of a Lambda baryon (Λ), a proton and a neutron. While, traditionally, hypernuclei are studied in low-energy nuclear experiments, the hundreds of hypertritons and antihypertritons produced in each lead–lead run at the LHC provide one of the largest data samples for their study. The hypertritons fly for a few centimetres in the experimental apparatus before decaying into a ³He nucleus and a charged pion, which are then identified by the detectors.

The ALICE collaboration recently completed a new analysis of the largest Run 2 data sample, achieving the most precise measurements to date of the hypertriton lifetime and its Λ-separation energy (the energy required to separate the Λ from the rest of the hypertriton). The lifetime, measured from the distribution of reconstructed two-body decay lengths, was found to be 253 ± 11 (stat.) ± 6 (syst.) ps, while the separation energy, obtained from the hypertriton invariant-mass distribution, was measured to be 72 ± 63 (stat.) ± 36 (syst.) keV.

These two quantities are fundamental to understand the structure of this hypernucleus and therefore the nature of the strong interaction. While the strong force binding neutrons and protons inside atomic nuclei is well understood, the characteristics of the strong force binding nucleons and hyperons are not precisely known.

The study of this interaction is not only interesting per se, but it is also an input for modelling of the dense core of neutron stars. Indeed, the creation of hyperons is energetically favoured compared to ordinary nucleonic matter in the inner core of neutron stars. Therefore, detailed knowledge of the interactions between nucleons and hyperons is required to understand these compact astrophysical objects.

The new ALICE measurements indicate that the interaction between the hyperon inside the hypertriton and the other two nucleons is extremely feeble (see figure 1). This is also confirmed by the lifetime of the hypertriton, which is compatible with the free Λ-baryon lifetime. Finally, since at the LHC matter and antimatter are produced in the same amount, the ALICE collaboration could compare the lifetimes of the antihypertriton and the hypertriton. Within the experimental uncertainty, the lifetimes were found to be compatible, as expected from CPT invariance.

During LHC Run 3, ALICE will extend its studies to heavier hypernuclei, putting tighter constraints on the interaction models among hyperons and nucleons. 

The post Hypertriton characterised with unprecedented precision appeared first on CERN Courier.

Understanding hadronic final states is key to a successful physics programme at the LHC. The quarks and gluons flying out from proton–proton collisions instantly hadronise into sprays of particles called jets. Each jet has a unique composition that makes their flavour identification and energy calibration challenging. While the performance of jet-classification schemes has been increased by the fast-paced evolution of machine-learning algorithms, another, more subtle, revolution is ongoing in terms of precision jet-energy corrections.

CMS physicists have taken advantage of the data collected during LHC Run 2 to observe jets in many different final states and systematically understand their differences in detail. The main differences originate from the varying fractions of gluons making up the jets and the different amounts of final-state radiation (FSR) in the events, causing an imbalance between the leading jet and its companions. The gluon uncertainty was constrained by splitting the Z+jet sample by flavour, using a combination of quark–gluon likelihood and b/c-quark tagging, while FSR was constrained by combining the missing-E_T projection fraction (MPF) and direct balance (DB) methods. The MPF and DB methods have been well established at the LHC since Run 1: while in the DB method the jet response is evaluated by comparing the reconstructed jet momentum directly to the momentum of the reference object, the MPF method considers the response of the whole hadronic activity in the event, recoiling versus the reference object. Figure 1 shows the agreement achieved with the Run 2 data after carefully accounting for these biases for samples with different jet-flavour compositions.

Precise jet-energy corrections are critical for some of the recent high-profile measurements by CMS, such as an intriguing double dijet excess at high mass, a recent exceptionally accurate top-quark mass measurement, and the most precise extraction of the strong coupling constant at hadron colliders using inclusive jets.

The expected increase of pileup in Run 3 and at the High-Luminosity LHC will pose additional challenges in the derivation of precise jet-energy corrections, but CMS physicists are well prepared: CMS will adopt the next-generation particle-flow algorithm (PUPPI, for PileUp Per Particle Id) as the default reconstruction algorithm to tackle pileup effects within jets at the single-particle level.

Jets can be used to address some of the most intriguing puzzles of the Standard Model (SM), in particular: is the SM vacuum metastable, or do some new particles and fields stabilise it? The top-quark mass and strong-coupling-constant measurements address the former question via their interplay with the Higgs-boson mass, while dijet-resonance searches tackle the latter. 

Underlying these studies are the jet-energy corrections and the awareness that each jet flavour is unique. 

The post Jet-energy corrections blaze a trail appeared first on CERN Courier.

Photon-induced reactions are regularly studied in ultra-peripheral nucleus–nucleus collisions (UPCs) at the LHC. In these collisions, the accelerated ions, which carry a strong electromagnetic field, pass by each other with an impact parameter (the distance between their centres) larger than the sum of their nuclear radii. Hadronic interactions between nuclei are therefore strongly suppressed. At LHC energies, the photo­production of charmonium (a bound state of charm and anti-charm quarks) in UPCs is sensitive to the gluon distributions in nuclei over a wide low Bjorken-x range. In particular, in coherent interactions, the photon emitted by one of the nuclei couples to the other nucleus as a whole, leaving it intact, while a J/ψ meson is emitted with a characteristic low transverse momentum (p_T) of about 60 MeV, which is roughly of the order of the inverse of the nuclear radius.

Surprisingly, in 2016 ALICE measured an unexpectedly large yield of J/ψ mesons at very low p_T in peripheral, not ultra-peripheral, PbPb collisions at a centre-of-mass energy of 2.76 TeV. The excess with respect to expectations from hadronic J/ψ-meson production was interpreted as the first indication of coherent photoproduction of J/ψ mesons in PbPb collisions with nuclear overlap. This effect comes with many theoretical challenges. For instance, how can the coherence condition survive in the photon–nucleus interaction if the latter is broken up during the hadronic collision? Do only the non-interacting spectator nucleons participate in the coherent process? Can the photoproduced J/ψ meson be affected by interactions with the formed and fast-expanding quark–gluon plasma (QGP) created in nucleus–nucleus collisions? Recent theoretical developments on the subject are based on calculations for UPCs in which the J/ψ meson photoproduction-cross section is computed as the product of an effective photon flux and an effective photonuclear cross section for the process γPb → J/ψPb, with both terms usually modified to account for the nuclear overlap.

The ALICE experiment has recently measured the coherently photoproduced J/ψ mesons in PbPb collisions at a centre-of-mass energy of 5.02 TeV, using the full Run 2 data sample. The measurement is performed at forward rapidity (2.5 < y < 4) in the dimuon decay channel. For the first time, a significant (> 5σ) coherently photoproduced J/ψ-meson signal is observed even in semi-central PbPb collisions. In figure 1, the coherently photoproduced J/ψ cross section is shown as a function of the mean number of nucleons participating in the hadronic interaction (<N_part>). In this representation, the most central head-on PbPb collisions correspond to large <N_part> values close to 400. The photoproduced J/ψ cross section does not exhibit a strong dependence on collision centrality (i.e. on the amount of nuclear overlap) within the current experimental precision. A UPC-like model (the red line in figure 1) reproduces the semi-central to central PbPb data if a modified photon flux and photonuclear cross section to account for the nuclear overlap are included.

To clarify the theory behind this experimental observation of coherent J/ψ photoproduction, the upcoming Run 3 data will be crucial in several aspects. ALICE expects to collect a much larger data sample, thereby measuring a statistically significant signal in most central collisions. At midrapidity, the larger data sample and the excellent momentum resolution of the detector will allow for p_T-differential cross-section measurements, which will shed light on the role of spectator nucleons for the coherence condition. By extending the coherently photo-produced J/ψ cross-section measurement towards most central PbPb collisions, ALICE will study the possible interaction of these charmonia with the QGP. Photoproduced J/ψ mesons could therefore turn out to be a completely new probe of the charmonium dissociation in the QGP.

The post J/ψ photoproduction in hadronic PbPb collisions appeared first on CERN Courier.

The top quark – the heaviest known elementary particle – differs from the other quarks by its much larger mass and a lifetime that is shorter than the time needed to form hadronic bound states. Within the Standard Model (SM), the top quark decays almost exclusively into a W boson and a b quark, and the dominant production mechanism in proton–proton (pp) collisions is top-quark pair (tt) production.

Measurements of tt production at various pp centre-of-mass energies at the LHC probe different values of Bjorken-x, the fraction of the proton’s longitudinal momentum carried by the parton participating in the initial interaction. In particular, the fraction of tt events produced through quark–antiquark annihilation increases from 11% at 13 TeV to 25% at 5.02 TeV. A measurement of the tt production cross-section thus places additional constraints on the proton’s parton distribution functions (PDFs), which describe the probabilities of finding quarks and gluons at particular x values.

In November 2017, the ATLAS experiment recorded a week of pp-collision data at a centre-of-mass energy of 5.02 TeV. Although the main motivation of this 5.02 TeV dataset is to provide a proton reference sample for the ATLAS heavy-ion physics programme, it also provides a unique opportunity to study top-quark production at a previously unexplored energy in ATLAS. The majority of the data was recorded with a mean number of two inelastic pp collisions per bunch crossing compared to roughly 35 collisions during the 13 TeV runs. Due to much lower pileup conditions, the ATLAS calorimeter cluster noise thresholds were adjusted accordingly, and a dedicated jet-energy scale calibration was performed.

Now, the ATLAS collaboration has released its measurement of the tt production cross-section at 5.02 TeV in two final states. Events in the dilepton channel were selected by requiring opposite-charge pairs of leptons, resulting in a small, high-purity sample. Events in the single-lepton final states were separated into subsamples with different signal-to-background ratios, and a multivariate technique was used to further separate signal from background events. The two measurements were combined, taking the correlated systematic uncertainties into account.

The measured cross section in the dilepton channel (65.7 ± 4.9 pb) corresponds to a relative uncertainty of 7.5%, of which 6.8% is statistical. The single-lepton measurement (68.2 ± 3.1 pb), on the other hand, has a 4.5% uncertainty that is primarily systematic. This measurement is slightly more precise than the single-lepton measurement at 13 TeV, despite the much smaller (almost a factor of 500!) integrated luminosity. The combination of the two measurements gives 67.5 ± 2.6 pb, corresponding to an uncertainty of just 3.9%.

The new ATLAS result is consistent with the SM prediction and with a measurement by the CMS collaboration, though with a total uncertainty reduced by almost a factor of two. It thus improves our understanding of the top-quark production at different centre-of-mass energies and allows an important test of the compatibility with predictions from different PDF sets (see figure 1). The result also provides a new measurement of high-x proton structure and shows a 5% reduction in the gluon PDF uncertainty in the region around x = 0.1, which is relevant for Higgs-boson production. Moreover, the measurement paves the way for the study of top-quark production in collisions involving heavy ions.

The post Low-pileup data pin down top-quark production appeared first on CERN Courier.

For the past 60 years, the second has been defined in terms of atomic transitions between two hyperfine states of caesium-133. Such transitions, which correspond to radiation in the microwave regime, enable state-of-the art atomic clocks to keep time at the level of one second in more than 300 million years. A newer breed of optical clocks developed since the 2000s exploit frequencies that are about 10⁵ times higher. While still under development, optical clocks based on aluminium ions are already reaching accuracies of about one second in 33 billion years, corresponding to a relative systematic frequency uncertainty below 1 × 10^–18. 

To further reduce these uncertainties, in 2003 Ekkehard Peik and Christian Tamm of Physikalisch-Technische Bundesanstalt in Germany proposed the use of a nuclear instead of atomic transition for time measurements. Due to the small nuclear moments (corresponding to the vastly different dimensions of atoms and nuclei), and thus the very weak coupling to perturbing electromagnetic fields, a “nuclear clock” is less vulnerable to external perturbations. In addition to enabling a more accurate timepiece, this offers the potential for nuclear clocks to be used as quantum sensors to test fundamental physics. 

Clockwork 

A clock typically consists of an oscillator and a frequency-counting device. In a nuclear clock (see “Nuclear clock schematic” figure), the oscillator is provided by the frequency of a transition between two nuclear states (in contrast to a transition between two states in the electronic shell in the case of an atomic clock). For the frequency-counting device, a narrow-band laser resonantly excites the nuclear-clock transition, while the corresponding oscillations of the laser light are counted using a frequency comb. This device (the invention of which was recognised by the 2005 Nobel Prize in Physics) is a laser source whose spectrum consists of a series of discrete, equally spaced frequency lines. After a certain number of oscillations, given by the frequency of the nuclear transition, one second has elapsed. 

The need for direct laser excitation strongly constrains applicable nuclear-clock transitions: their energy has to be low enough to be accessible with existing laser technology, while simultaneously exhibiting a narrow linewidth. As the linewidth is determined by the lifetime of the excited nuclear state, the latter has to be long enough to allow for highly stable clock operation. So far, only the metastable (isomeric) first excited state of ²²⁹Th, denoted ^229mTh, qualifies as a candidate for a nuclear clock, due to its exceptionally low excitation energy. 

The existence of the isomeric state was conjectured in 1976 from gamma-ray spectroscopy of ²²⁹Th, and its excitation energy has only recently been determined to be 8.19 ± 0.12 eV (corresponding to a vacuum-ultraviolet wavelength of 151.4 ± 2.2 nm). Not only is it the lowest nuclear excitation among the roughly 184,000 excited states of the 3300 or so known nuclides, its expected lifetime is of the order of 1000 s, resulting in an extremely narrow relative linewidth (ΔE/E ~ 10^–20) for its ground-state transition (see “Unique transition” figure). Besides high resilience against external perturbations, this represents another attractive property for a thorium nuclear clock. 

Networks of ultra-precise synchronised nuclear clocks could enable a search for ultra light dark matter

Achieving optical control of the nuclear transition via a direct laser excitation would open a broad range of applications. A nuclear clock’s sensitivity to the gravitational redshift, which causes a clock’s relative frequency to change depending on its absolute height, could enable more accurate global positioning systems and high-sensitivity detections of fluctuations of Earth’s gravitational potential induced by seismic or tectonic activities. Furthermore, while the few-eV thorium transition emerges from a fortunate near-degeneracy of the two lowest nuclear-energy levels in ²²⁹Th, the Coulomb and strong-force contributions to these energies differ at the MeV level. This makes the nuclear-level structure of ²²⁹Th uniquely sensitive to variations of fundamental constants and ultralight dark matter. Many theories predict variations of the fine structure constant, for example, but on tiny yearly rates. The high sensitivity provided by the thorium isomer could allow such variations to be identified. Moreover, networks of ultra-precise synchronised clocks could enable a search for (ultra light) dark-matter signals. 

Two different approaches have been proposed to realise a nuclear clock: one based on trapped ions and another using doped solid-state crystals. The first approach starts from individually trapped Th ions, which promises an unprecedented suppression of systematic clock-frequency shift and leads to an expected relative clock accuracy of about 1 × 10^–19. The other approach relies on embedding ²²⁹Th atoms in a vacuum–ultraviolet (VUV) transparent crystal such as CaF₂. This has the advantage of a large concentration (> 10¹⁵/cm³) of Th nuclei in the crystal, leading to a considerably higher signal-to-noise ratio and thus a greater clock stability. 

Precise characterisation 

A precise characterisation of the thorium isomer’s properties is a prerequisite for any kind of nuclear clock. In 2016 the present authors and colleagues made the
first direct identification of ^229mTh by detecting electrons emitted from its dominant decay mode: internal-conversion (IC), whereby a nuclear excited state decays by the direct emission of one of its atomic electrons (see “Isomeric signal” figure). This brought the long-term objective of a nuclear clock into the focus of international research. 

Currently, experimental access to ^229mTh is possible only via radioactive decays of heavier isotopes or by X-ray pumping from higher-lying rotational nuclear levels, as shown by Takahiko Masuda and co-workers in 2019. The former, based on the alpha decay of ²³³U (2% branching ratio), is the most commonly used approach. Very recently, however, a promising new experiment exploiting β^– decay from ²²⁹Ac was performed at CERN’s ISOLDE facility led by a team at KU Leuven. Here, ²²⁹Ac is online-produced and mass-separated before being implanted into a large-bandgap VUV-transparent crystal. In both population schemes, either photons or conversion electrons emitted during the isomeric decay are detected. 

In the IC-based approach, a positively charged ^229mTh ion beam is generated from alpha-decay daughter products recoiling off a ²³³U source placed inside a buffer-gas stopping cell. The decay products are thermalised, guided by electrical fields towards an exit nozzle, extracted into a longitudinally 15-fold segmented radiofrequency quadrupole (RFQ) that acts as an ion guide, phase-space cooler and optionally a beam buncher, followed by a quadrupole mass separator for beam purification. In charged thorium isomers, the otherwise dominant IC decay branch is energetically forbidden, leading to a prolongation of the lifetime by up to nine orders of magnitude. 

Operating the segmented RFQ as a linear Paul trap to generate sharp ion pulses enables the half-life of the thorium isomer to be determined. In work performed by the present authors in 2017, pulsed ions from the RFQ were collected and neutralised on a metal surface, triggering their IC decay. Since the long ionic lifetime was inaccessible due to the limited ion-storage time imposed by the trap’s vacuum conditions, the drastically reduced lifetime of neutral isomers was targeted. Time-resolved detection of the low-energy conversion electrons determined the lifetime to be 7 ± 1 μs. 

Excitation energy

Recently, considerable progress has been made in determining the ^229mTh excitation energy – a milestone en route to a nuclear clock. In general, experimental approaches to determine the excitation energy fall into three categories: indirect measurements via gamma-ray spectroscopy of energetically low-lying rotational transitions in ²²⁹Th; direct spectroscopy of fluorescence photons emitted in radiative decays; and via electrons emitted in the IC decay of neutral ^229mTh. The first approach led to the conjecture of the isomer’s existence and finally, in 2007, to the long-accepted value of 7.6 ± 0.5 eV. The second approach tries to measure the energy of photons emitted directly in the ground-state decay of the thorium isomer. 

The first direct measurement of the thorium isomer’s excitation energy was reported by the present authors and co-workers in 2019. Using a compact magnetic-bottle spectrometer equipped with a repulsive electrostatic potential, followed by a microchannel-plate detector, the kinetic energy of the IC electrons emitted after an in-flight neutralisation of Th ions emitted from a ²³³U source could be determined. The experiment provided a value for the excitation energy of the nuclear-clock transition of 8.28 ± 0.17 eV. At around the same time in Japan, Masuda and co-workers used synchrotron radiation to achieve the first population of the isomer via resonant X-ray pumping into the second excited nuclear state of ²²⁹Th at 29.19 keV, which decays predominantly into ^229mTh. By combining their measurement with earlier published gamma-spectroscopic data, the team could constrain the isomeric excitation energy to the range 2.5–8.9 eV. More recently, led by teams at Heidelberg and Vienna, the excited isomers were implanted into the absorber of a custom-built cryogenic magnetic micro-calorimeter and the isomeric energy was measured by detecting the temperature-induced change of the magnetisation using SQUIDs. This produced a value of 8.10 ± 0.17 eV for the clock-transition energy, resulting in a world-average of 8.19 ± 0.12 eV. 

Besides precise knowledge of the excitation energy, another prerequisite for a nuclear clock is the possibility to monitor the nuclear excitation on short timescales. Peik and Tamm proposed a method to do this in 2003 based on the “double resonance” principle, which requires knowledge of the hyperfine structure of the thorium isomer. Therefore, in 2018, two different laser beams were collinearly superimposed on the ²²⁹Th ion beam, initiating a two-step excitation in the atomic shell of ²²⁹Th. By varying both laser frequencies, resonant excitations of hyperfine components both of the ²²⁹Th ground state and the ^229mTh isomer could be identified and thus the hyperfine splitting signature of both states could be established by detecting their de-excitation (see “Hyperfine splitting” figure). The eventual observation of the ^229mTh hyperfine structure in 2018 not only will in the future allow a non-destructive verification of the nuclear excitation, but enabled the isomer’s magnetic dipole and electrical quadrupole moments, and the mean-square charge radius, to be determined. 

Roadmap towards a nuclear clock

So far, the identification and characterisation of the thorium isomer has largely been driven by nuclear physics, where techniques such as gamma spectroscopy, conversion-electron spectroscopy and radioactive decays offer a description in units of electron volts. Now the challenge is to refine our knowledge of the isomeric excitation energy with laser-spectroscopic precision to enable optical control of the nuclear-clock transition. This requires bridging a gap of about 12 orders of magnitude in the precision of the ^229mTh excitation energy, from around 0.1 eV to the sub-kHz regime. In a first step, existing broad-band laser technology can be used to localise the nuclear resonance with an accuracy of about 1 GHz. In a second step, using VUV frequency-comb spectroscopy presently under development, it is envisaged to improve the accuracy into the (sub-)kHz range. 

Another practical challenge when designing a high-precision ion-trap-based nuclear clock is the generation of thermally decoupled, ultra-cold ²²⁹Th ions via laser cooling. ²²⁹Th³⁺ is particularly suited due to its electronic level structure, with only one valence electron. Due to the high chemical reactivity of thorium, a cryogenic Paul trap is the ideal environment for laser cooling, since almost all residual gas atoms will freeze out at 4 K, increasing the trapping time into the region of a few hours. This will form the basis for direct laser excitation of ^229mTh and will also enable a measurement of the not yet experimentally determined isomeric lifetime of ²²⁹Th ions. For the alternative development of a compact solid-state nuclear clock it will be necessary to suppress the ^229mTh decay via internal conversion in a large band-gap, VUV transparent crystal and to detect the γ decay of the excited nuclear state. Proof-of-principle studies of this approach are currently ongoing at ISOLDE. 

Laser-spectroscopy activities on the thorium isomer are also ongoing in the US, for example at JILA, NIST and UCLA

Many of the recent breakthroughs in understanding the ²²⁹Th clock transition emerged from the European Union project “nuClock”, which terminated in 2019. A subsequent project, ThoriumNuclearClock (ThNC), aims to demonstrate at least one nuclear clock by 2026. Laser-spectroscopy activities on the thorium isomer are also ongoing in the US, for example at JILA, NIST and UCLA. 

In view of the large progress in recent years and ongoing worldwide efforts both experimentally and theoretically, the road is paved towards the first nuclear clock. It will complement highly precise optical atomic clocks, while in some areas, in the long run, nuclear clocks might even have the potential to replace them. Moreover, and beyond its superb timekeeping capabilities, a nuclear clock is a unique type of quantum sensor allowing for fundamental physics tests, from the variation of fundamental constants to searches for dark matter.

The post From atomic to nuclear clocks appeared first on CERN Courier.

Despite two decades of extensive studies, the production of antinuclei in heavy-ion collisions is not yet fully understood. Antinuclei production is usually modelled by two conceptually different theoretical models, the statistical hadronisation model (SHM) and coalescence models. In the SHM, deuteron antinuclei are produced from a locally thermally equilibrated source, while antinuclei are formed from the binding of constituent nucleons, which are close in momentum and position phase space in the coalescence model. Both models predict very similar production yields of, for example, deuteron antinuclei, bound states of an antiproton and an antineutron. This calls for new experimental observables that discern different production models.

Measuring higher moments of the multiplicity distribution of antinuclei as well as the correlation with antinucleons produced in the collision have been recently proposed as sensitive variables to antinucleosynthesis processes in heavy-ion collisions. The first measurement of the variance to mean ratio of the multiplicity distribution of antideuterons is compared to the predictions of the SHM and coalescence models (figure 1). The coalescence model fails to describe the observed ratio of the variance and mean of the multiplicity distribution of antideuterons. The measurements are consistent with the statistical baseline, a Poissonian distribution, as well as with the SHM in the presence of baryon number conservation. However, this observable proves insensitive to the size of the correlation volume used in the SHM to conserve the baryon number.

The Pearson correlation coefficient between the number of produced antideuterons and antiprotons constrains the latter effectively. The small negative correlation reflects that there are less protons observed in events with at least one deuteron than in an average event (figure 1). The coalescence model does not reproduce the measurement, whereas it is possible to fit the measurement to extract the correlation volume out of the SHM. The obtained correlation volume is 1.6 times the volume of the fireball per unit of rapidity, which is smaller compared to those describing proton yields and a similar measurement of net-proton number fluctuations. These findings point to a later formation of the correlation among protons and deuterons compared to that among antiprotons and protons.

Overall, these results present a severe challenge to the current understanding of antinuclei production in heavy-ion collisions at the LHC energies. With the LHC Run 3 data it will be possible to extend these measurements to heavier antinuclei and to higher order correlation coefficients and moments of the antinuclei multiplicity distribution that are even more sensitive to details of the nucleosynthesis process in heavy-ion collisions.

The post Antinucleosynthesis beyond the average appeared first on CERN Courier.

More than 30 years after it was predicted, a phenomenon in quantum chromodynamics (QCD) called the dead-cone effect has been directly observed by the ALICE collaboration. The result, reported in Nature on 18 May, not only confirms a fundamental feature of the theory of the strong force, but enables a direct experimental observation of the non-zero mass of the charm quark in the partonic phase.

In QCD, the dead-cone effect predicts a suppression of gluon bremsstrahlung from a quark within a cone centred on the quark’s flight direction. This cone has an angular size m_q/E, where m_q is the mass of the quark and E is its energy. The effect arises due to the conservation of angular momentum during the gluon emission and is significant for low-energy heavy-flavour quarks. 

The dead cone has been indirectly observed at particle colliders. A direct observation from the parton shower’s radiation pattern has remained challenging, however, because it relies on the determination of the emission angle of the gluon, as well as the emitting heavy-flavour quark’s energy, at each emission vertex in the parton shower (see “Showering” figure). This requires a dynamic reconstruction of the cascading quarks and gluons in the shower from experimentally accessible hadrons, which had not been possible until now. In addition, the dead-cone region can be obscured and filled by other sources such as the decay products of heavy-flavour hadrons, which must be removed during the measurement.

To observe the dead-cone effect directly, ALICE used jets tagged with a reconstructed D⁰-meson in a 25 nb^–1 sample of pp collisions at a centre-of-mass-energy of 13 TeV collected between 2016 and 2018. The D⁰-mesons were reconstructed with transverse momenta between 2 and 36 GeV/c through their decay into a kaon and pion pair. Jet-finding was then performed on the events with the “anti-k_T” algorithm, and jets with the reconstructed D⁰-meson amongst their constituents were tagged. The team used recursive jet-clustering techniques to reconstruct the gluon emissions from the radiating charm quark by following the branch containing the D⁰-meson at each de-clustering step, which is equivalent to following the emitting charm quark through the shower. A similar procedure was carried out on a flavour-untagged sample of jets, which contain primarily gluon and light-quark emissions and form a baseline where the dead-cone effect is absent.

Comparisons between the gluon emissions from charm quarks and from light quarks and gluons directly reveal the dead-cone effect through a suppression of gluon emissions from the charm quark at small angles, compared to the emissions from light quarks and gluons. Since QCD predicts a mass-dependence of the dead cones, the result also directly exposes the mass of the charm quark, which is otherwise inaccessible due to confinement. ALICE’s successful technique to directly observe a parton shower’s dead cone may therefore offer a way to measure quark masses.

The upgraded ALICE detector in LHC Run 3 will enable an extension of the measurement to jets tagged with a B⁺ meson. This will allow the reconstruction of gluon emissions from beauty quarks which, due to their larger mass, are expected to have a larger dead cone than charm quarks. Comparisons between the angular distribution of gluon emissions from beauty quarks and those from charm quarks will isolate mass-dependent effects in the shower and remove the contribution from effects pertaining to the differences between quark and gluon fragmentation, bringing deeper insights into the intriguing workings of the strong force.

The post Dead-cone effect exposed by ALICE appeared first on CERN Courier.

The bound states of a heavy quark and its antiquark, called quarkonia, have long been regarded as ideal probes to study the quark–gluon plasma (QGP) formed in high-energy heavy-ion collisions. The golden signature is the suppression of their production yield in lead–lead (PbPb) collisions with respect to extrapolations from proton–proton (pp) collisions, caused by modifications of the binding potential in the QGP. The suppression of the different quarkonium states is expected to depend on their binding energies. Quarkonia can also be produced by recombination processes. The ϒ states (bound states of b quarks and antiquarks) are much less affected by recombination effects than charmonium states, given the very small probability that b quarks are produced. A comparison of their suppression patterns is particularly informative because of the different binding energies of the ϒ(1S), ϒ(2S) and ϒ(3S) states.

The suppression of quarkonium production is quantified via the nuclear modification factor R_AA, defined as the ratio between the yield in nucleus–nucleus (AA) collisions and the yield extrapolated from pp data. Previous measurements of R_AA for the ϒ mesons by experiments at RHIC and the LHC revealed a significant suppression of the ϒ(1S) state and a larger suppression for the ϒ(2S) state. However, these experiments could only set upper limits for the ϒ(3S) state due to its very low production yield. The CMS experiment recently changed this situation by presenting the first observation of the ϒ(3S) meson in heavy-ion collisions. The ϒ mesons are detected using their decay to two muons. The analysis used the large PbPb data sample collected in 2018 and extracted the ϒ(3S) signals from the large background of muon pairs by using a boosted decision tree algorithm.

The new R_AA results are shown together with the previously published ϒ(1S) values as a function of the average number of nucleons participating in the PbPb collisions, <N_part> (figure 1). Collisions with larger <N_part> show a bigger overlap between the two nuclei, producing a larger and hotter QGP. As previously observed, the degree of suppression increases from peripheral to central collisions, i.e. as N_part increases, indicating a more substantial dissociation effect at higher QGP temperatures. The new ϒ(3S) suppression measurement completes the picture of suppression patterns for five different quarkonium states, which was started 35 years ago at the CERN SPS with the J/ψ and ψ(2S) results of NA38. The stage is set for a deeper understanding of deconfinement in the QGP.

The post Upsilon suppression in heavy-ion collisions appeared first on CERN Courier.

Keeping with the tradition of Moriond, several new experimental results were presented by major experimental collaborations, with participants enjoying ample opportunities to debate cases where measurements and theoretical predictions do not agree. Held 10 years after the Higgs discovery, the conference started with a review of how the Higgs boson came of age – from early exploration to a precision era. An exciting mix of new precision results and interesting observations in Higgs physics were presented, including the first measurement of the Higgs-charm coupling as well as studies of off-shell Higgs production and di-Higgs production by the ATLAS and CMS collaborations.

The first observation of tqγ production by ATLAS as well as many measurements in top-quark physics, including a mass measurement based on single top quarks by CMS, were discussed. Many recent studies of Z and W bosons and their interactions were reported, including a new CMS result that resolved an earlier mild LEP tension in the decay rates of W bosons to leptons, and the observation of triple-W production at the LHC by ATLAS. The LHCb collaboration presented its first measurement of the W mass, while CMS discussed the first observation of WW and triple-J/ψ production in double-parton scattering.

Several sessions were devoted to flavour measurements and anomalies, including possible lepton-flavour universality violations in B-meson decays. LHCb presented the most precise value of the CKM matrix angle γ measured in a single experiment, as well as the most precise measurement of the charm-mixing parameter y_CP. New results on lepton-flavour universality attracted a lot of attention. Among them are LHCb’s measurement of the ratio of Br(B⁺ → K⁺μ⁺μ^–) to Br(B⁺ → K⁺e⁺e^–), which is 3.1σ away from the SM, new LHCb limits on rare B⁰ decays, and the CMS measurement of the Drell–Yan forward–backward asymmetry difference between di-muons and di-electrons. The status of selected Standard Model (SM) calculations was described with the conclusion that the predictions are robust and therefore possible deficiencies of the SM a very unlikely source of the flavour anomalies. A number of talks demonstrated that there are many ways to accommodate the flavour anomalies into a consistent physics picture, which predicts subtle signals at the LHC that could have easily evaded detection so far.

Several speakers emphasised the importance of new creative analysis concepts

Continuing the topic of searches for new physics, several speakers emphasised the importance of new creative analysis concepts, including searching for anomalous energy losses, non-pointing tracks, delayed photons, displaced jets, displaced collimated leptons and tagging missing mass with forward detectors. Among the results of many interesting searches presented at Moriond, a 3σ excess in the number of highly ionising particles reported by the ATLAS collaboration caused some excitement and discussion, indicating that further studies (and statistics!) are very much needed.

Several talks presented theoretical predictions at high orders of perturbative QCD for basic SM processes at the LHC and future lepton colliders, such as the Drell–Yan and jet-production processes. These tour de force computations, representing cutting-edge applications of quantum field theory to collider physics, force us to think about how such advances in the theory of hard hadron collisions can be used to search for physics beyond the SM. Several talks addressed this issue by considering specific physics examples pointing towards new, exciting opportunities during LHC Run 3.

Emphasising the need for a refined knowledge of the fundamental input parameters used to describe hadron collisions, four new extractions of the strong coupling constant were reported, based on HERA, CDF, LEP and CMS data. The role of precision deep-inelastic scattering (HERA) and W/Z (ATLAS/CMS) data in constraining parton distribution functions was clearly elucidated.

An element of nonperturbative QCD that keeps theorists on their toes is hadronic spectroscopy

Turning towards the non-perturbative sector of QCD, a measurement of Λ_c production down to zero transverse momentum allowed the ALICE collaboration to extract the total charm cross-section in pp collisions. Interestingly, the fraction of Λ_c is significantly above the e⁺e^– baseline. Jet substructure measurements presented by ALICE and CMS allow a detailed comparison to Monte Carlo event generators. Furthermore, the first direct observation of the dead-cone effect, a suppression of forward gluon radiation in case of a massive emitter, was presented by the ALICE collaboration using charm-tagged jets.

An element of non-perturbative QCD that keeps theorists on their toes is hadronic spectroscopy. This trend continued at Moriond where the discoveries of several new states were presented, including the same-sign doubly charmed T⁺_cc (c–c–u–d) (LHCb) and the Z^–_cs (c–c–s–u) (BES III). The exploration of the χ_c1, earlier known as X(3872), with the hope of revealing its molecular or tetraquark nature, continues in pp as well as in PbPb collisions.

The best constraint of the charm diffusion coefficient in the quark–gluon plasma (ALICE), jet quenching studies with Z-hadron correlations (CMS) and surprising results on ridge structures in γp and γPb collisions (ATLAS) were presented during a dedicated heavy-ion session. Interestingly, by studying the abundant nuclei produced in heavy-ion collisions, the ALICE collaboration ruled out simple coalescence models for antideuteron production in PbPb collisions.

Finally, the current status of the muon anomalous magnetic moment was reviewed. The experimental value presented last year by the Fermilab g-2 collaboration shows a 1.5–4.2σ discrepancy with the SM prediction, depending on the theoretical baseline. An interesting comparison between continuum and lattice computations of the hadronic vacuum polarisation contributions was presented, and a new lattice result on hadronic light-by-light scattering was described, indicating that this “troublemaking” contribution is being brought under theoretical control.

Exciting experimental results and developments in the theory of QCD and high-energy interactions that, perhaps, remained somewhat hidden during the pandemic years, were on full display at Moriond, making the 56th edition of this conference a resounding success.

The post Tour de QCD and beyond appeared first on CERN Courier.

Ultra-relativistic collisions between heavy nuclei probe the high-temperature and high-density limit of the phase diagram of nuclear matter. These collisions create a new state of matter, known as the quark–gluon plasma (QGP), in which quarks and gluons are no longer confined in hadrons but instead behave quasi-freely over a relatively large volume. By creating and studying this novel state of matter, which last existed in the microseconds after the Big Bang, we gain a deeper understanding of the strong nuclear force and quantum chromodynamics (QCD).

Nearly 50 years ago, the first relativistic heavy-ion collision experiments were performed at the Bevatron at Berkeley, reaching energies of 1 to 2 GeV. Since then, heavier ions were collided at higher energies at Brookhaven’s AGS, CERN’s SPS and Brookhaven’s RHIC facilities. Since 2010, heavy-ion physics has entered the TeV regime with lead–lead (PbPb) collisions at 2.76 and 5.02 TeV at the LHC. While the ALICE detector is designed specifically to focus on such collisions, all four large LHC experiments have active heavy-ion physics programmes and are contributing to our understanding of extreme QCD matter.

In a heavy-ion collision, the initial energy deposited by the colliding nuclei undergoes a fast equilibration, within roughly 10^–24 s, to form the QGP. The resulting deconfined and thermalised medium expands and cools over the next few 10^–24 s, before the quarks and gluons recombine to form a hadron gas. It is the goal of heavy-ion experiments at the LHC to use the detected final-state hadrons to reconstruct the properties and dynamical behaviour of the system throughout its evolution. So far, the LHC experiments have  delivered a series of results that are sensitive to various aspects of the heavy-ion collision system, with Run 3 set to push our understanding much further. 

Properties and dynamics

The initial energy-density distribution and subsequent expansion of the heavy-ion collision system is largely determined by the geometrical overlap of the colliding nuclei. Collisions can range from head-on “central” collisions, where the nuclear overlap is large, to glancing “peripheral” collisions where the overlap region is smaller and roughly almond-shaped. Since the interaction region in non-central events is not rotationally symmetric, anisotropic pressure gradients build up. These preferentially boost particles along the minor axis of the ellipsoidal overlap region, resulting in an observable anisotropy in the distribution of final-state hadrons. The distribution of the particles in the azimuthal angle can be described well by a Fourier cosine series, where the largest term is the second harmonic, characterised by the parameter v₂, due to the ellipsoidal shape of the nuclear overlap region. Fluctuations in the positions of the individual constituent nucleons lead to significant higher-order terms. It was discovered that these Fourier coefficients, v_n, are best described by models where the QGP dynamics obeys hydrodynamic equations, and thus behaves as a liquid exhibiting what we call “collective flow”.

Remarkably, in order to fit hydrodynamic models to experimental data it is necessary for the medium’s viscosity to be very low, corresponding to a shear-viscosity to entropy-density ratio of the order η/s ∼ 0.1. With a shear viscosity that is orders of magnitude smaller than other materials, the QGP is known as the “perfect” liquid. Measurements of the higher order harmonics, as well as their event-by-event fluctuations and correlations, provide even greater sensitivity to medium properties and the initial-state dynamics. Precision measurements of the v_n harmonics, charged-particle density, mean transverse momentum p_T, and mean-p_T fluctuations by ALICE have been used to extract the shear and bulk viscosity of the system as a function of temperature (see “Flow coefficients” figure).

While the QGP created in heavy-ion collisions is too small and short-lived to be examined with conventional probes, its properties can be investigated using the products of hard (high momentum-transfer, q²) scatterings that occur in the early stages of the collision and then propagate through the medium as it evolves. The production rates of these internally generated hard probes can be calculated in perturbative QCD and thus are considered calibrated probes of the QGP medium. The high-momentum quarks and gluons produced in these hard scatterings traverse the medium and fragment into collimated jets of hadrons. While these jets appear as a small signal on top of a large, fluctuating background, advances in re-clustering algorithms as well as the higher production rates of jets at the LHC have made it possible to study jets with high precision across a wide range of energies. 

The increase in the LHC luminosity will allow us to perform measurements that were previously inaccessible

Compared to jets in proton–proton (pp) collisions, jets in nucleus–nucleus (AA) collisions appear significantly suppressed or “quenched” due to their interactions with the medium (see “Jet quenching” figure). This is in contrast to electroweak probes, which interact only minimally with the coloured QGP medium. When the presence of hard scattering is identified by a high-p_T jet, photon or Z boson, the recoiling jet measured in the opposite direction is often reconstructed with a significantly lower energy, indicating that some of its energy has been transferred and absorbed by the medium. Recent, detailed jet-structure studies show that jets in heavy-ion collisions are softer (they fragment into lower-p_T hadrons) and broader than their counterparts in pp collisions, due to their interactions with the surrounding coloured QGP medium.

Another class of hard probes are heavy-flavour hadrons, since even heavy quarks (charm and beauty) with low p_T are produced in high-q² processes. Similar to jets, which mainly come from the fragmentation of light quarks and gluons, heavy hadrons are also suppressed in heavy-ion collisions relative to pp collisions. Recent precision measurements at the LHC of the yield of D mesons (containing charm quarks) as well as non-prompt D and J/ψ mesons (from the decays of hadrons containing beauty quarks), compared to the yields in pp collisions, demonstrate a mass-dependent suppression. This observation is consistent with the “dead cone” effect, which predicts that quarks with larger masses will be less significantly suppressed than those with smaller masses. The suppression of quarkonia (quark–antiquark bound states) depends on the binding energy, with loosely bound states such as the Υ(3S) and ψ(2S) more likely to become dissociated in the hot and dense medium than the tightly bound Υ(1S) and J/ψ(1S) states. However, it was discovered at the LHC that final-state J/ψ are actually less suppressed than in lower energy AA collisions at RHIC. This was attributed to the larger number of charm quarks being produced at LHC energies, which enhances the probability that charm and anti-charm quarks can recombine to form J/ψ states within the QGP. These dual effects of suppression and recombination are considered a signature of the production of a deconfined, thermalised medium in heavy-ion collisions.

Freeze out

As the QGP expands and cools, it undergoes a phase transition into a hadron gas in which quarks and gluons become confined into hadrons. At chemical freeze-out, inelastic collisions cease and the thermochemical properties of the system become fixed. Comparing ALICE measurements of the inclusive yields of multiple hadron species with a model of statistical hadronisation shows excellent agreement over nine orders of magnitude in mass, from pions to anti-⁴He nuclei (see “Statistical production” figure). This indicates that the bulk chemistry of the QGP freeze-out can be described by purely statistical particle production from a system in thermal equilibrium with a common temperature (155 MeV) and volume (~5000 fm³).

One of the first surprising results to come from the LHC was the discovery of azimuthal correlations between particles over large distances in pseudorapidity in small collision systems, pp and pPb. These long-range correlations are observed in heavy-ion collisions, where they are traditionally attributed to anisotropic flow (parameterised by v_n coefficients). However, the presence of collective behaviour in small systems, where a QGP was not expected to be formed, raised many questions about our understanding of both large and small nuclear collisions.

A second surprising observation was made in the measurement of the ratios of strange and multistrange hadrons (e.g. K⁰_S, Λ, Ξ and Ω) with respect to pions, as a function of the number of particles produced in the collision (multiplicity). The enhancement of strangeness production in AA compared to pp collisions was historically predicted as a signature of the formation of a QGP, although it is now understood as being due to the suppression of strangeness in small systems. However, measurements by ALICE showed a smooth increase in the strangeness enhancement with multiplicity across all collision systems: pp, pPb, XeXe and PbPb – opening further questions about the presence of a thermalised medium in both small and large systems.

In contrast, the suppression of hard probes, which has long been viewed as a complementary effect to anisotropic flow, has not been observed in pp or pPb collisions within current experimental uncertainties. In order to gain a more complete understanding of QCD from the soft to the hard scales, and from small to large systems, we must expand our experimental programmes.

To Run 3 and beyond

All four large experiments at the LHC have undergone significant upgrades during Long Shutdown 2 to extend their reach and allow the collection of heavy-ion data at higher luminosities. The increase in luminosity by a factor of 10 in Runs 3 and 4 at the LHC will allow us to make precision measurements of soft and hard probes of the QGP. Rare probes such as heavy-flavour hadrons will become accessible with high statistical precision, and we will be able to explore the charm and beauty sector at a level commensurate with that of the strangeness studies in Runs 1 and 2. Jet measurements will become significantly more precise as we further explore the medium-induced modification of well-calibrated probes such as γ– and Z-tagged jets. 

Our understanding of collective behaviour and the medium evolution will be enhanced by studies of the correlations and fluctuations of flow coefficients, which provide additional and complementary information above and beyond what we learn from v_n alone. Measurements that were severely statistically-limited in Runs 1 and 2, such as those of virtual photons produced as thermal radiation, will be performed with unprecedented precision in Runs 3 and 4. The higher order fluctuations of identified particles, which are expected to be sensitive to critical behaviour around the phase transition, will also come within reach in Runs 3 and 4 and make it possible to map out the phase diagram of QCD matter in great detail.

Furthermore, studies of small systems will continue to shed light on the development of QGP-like signals from pp to AA collisions. In particular, oxygen nuclei will be collided at the LHC, which will allow us to investigate collective effects in collisions with a geometry similar to PbPb collisions but with multiplicities of the order of those in pp and pPb collisions. High-precision and multi-differential jet measurements in pp, pPb and OO collisions will finally allow us to resolve open questions about the relationship between jet quenching and collective behaviour, and whether such effects are observed across all nuclear collision systems. Through these experimental measurements, we will make major progress in our understanding of nuclear matter from small to large collision systems, towards our ultimate goal of a unified description of QCD phenomenology from the microscopic level to the emergent bulk properties of the QGP.

While the heavy-ion physics programme in Runs 3 and 4 will provide deep insights into the rich field of QCD phenomenology, open questions will remain that can only be addressed with further advancements in detector performance and with the significant increase in heavy-ion luminosity anticipated in Run 5 (expected in 2035–2038). This extension of the LHC heavy-ion programme through the 2030s has been supported by the 2020 update of the European strategy for particle physics, and the LHC-experiment collaborations are exploring the potential for novel measurements in light- and heavy-ion collision systems based on their planned detector upgrades. In particular, ALICE is proposing to build a new dedicated heavy-ion experiment, ALICE 3, based on a large-acceptance ultra-light (low material budget) silicon tracking system surrounded by multiple layers of particle identification technology. The increase in the LHC luminosity coupled with state-of-the-art detector upgrades will allow us to dramatically extend our experimental reach and perform measurements that were previously inaccessible. The goals of the future heavy-ion programme at the LHC – from measuring electromagnetic radiation from the QGP and exotic heavy-flavour hadrons to beyond-the-Standard-Model searches for axions – will provide unprecedented insight into the fundamental constituents and forces of nature. 

The post Heavy-ion physics: past, present and future appeared first on CERN Courier.

The primary goal of the ultrarelativistic heavy-ion collision programme at the LHC is to study the properties of the quark–gluon plasma (QGP), a state of strongly interacting matter in which quarks and gluons are deconfined over large distances compared to the typical size of a hadron. The rapid expansion of the QGP under large pressure gradients is imprinted in the momentum distributions of final-state particles. The azimuthal-anisotropy flow coefficients v_n and the mean transverse momentum 〈p_T〉 of particles, which are described by hydrodynamic models, have been extensively measured by experiments at the LHC and  at the RHIC collider. These observables are also used as experimental inputs to global Bayesian analyses that provide information on both the initial stages of the heavy-ion collision, before QGP formation, and on key transport coefficients of the QGP itself, such as the shear and bulk viscosities. However, due to the limited constraints on the initial conditions, uncertainties remain in the QGP’s transport coefficients.

The ALICE collaboration recently reported correlations between v_n and 〈p_T〉 in terms of the modified Pearson coefficient ρ. The measurements were performed in lead–lead (PbPb) and xenon–xenon (XeXe) collisions at centre-of-mass energies per nucleon–nucleon collision of 5.02 and 5.44 TeV, respectively. As the correlations between v_n and 〈p_T〉 are predicted to be mainly driven by the shape and size of the initial profile of the energy distribution in the transverse plane, these studies provide a new approach to characterise the initial state. 

The measurements show a positive correlation between v_n and 〈p_T〉 in both PbPb and XeXe collisions (figure 1). These measurements are compared to hydrodynamic calculations using the initial-state models IP-Glasma (based on the colour-glass-condensate effective theory with gluon saturation) and Trento, a parameterised model with nucleons as the relevant degrees of freedom. The centrality dependence of ρ is better described by IP-Glasma than by Trento. In particular, the positive measured values of ρ suggest an effective nucleon width of the order of 0.3–0.5 fm, which is significantly smaller than what has been extracted in all Bayesian analy­ses using Trento initial conditions. The Pearson correlation measurements can now be included in Bayesian analyses to better constrain the initial state in nuclear collisions, thus impacting  the resulting QGP parameters. As a bonus, the measurements in XeXe collisions are sensitive to the quadrupole deformation parameter β₂ of the ¹²⁹Xe nucleus, potentially opening a new window for studying nuclear structure with ultrarelativistic heavy-ion collisions.

The post Accessing the precursor stage of QGP formation appeared first on CERN Courier.

Understanding the mechanisms of hadron formation represents one of the most interesting open questions in particle physics. Hadronisation is a non-perturbative process that is not calculable in quantum chromodynamics and is typically described with phenomenological models, such as the Lund string model. Ultrarelativistic nuclear collisions, where a high-density plasma of deconfined quarks and gluons, the quark–gluon plasma (QGP), is created, provide an ideal setup to test the limits of this description. In these conditions, hadrons may be formed via a combination of deconfined quarks close in phase space. This process can lead, for example, to increased production of baryons with respect to mesons in momentum ranges up to 10 GeV/c. The ALICE and CMS experiments at the LHC, and PHENIX and STAR at RHIC, have indeed observed substantial modifications of the event hadro-chemistry in heavy-ion collisions compared to proton–proton and e⁺e^– collisions. In particular, the total abundances of light and strange hadrons were found to follow, quite remarkably, the “thermal’’ expectations for a deconfined medium close to equilibrium. 

Measurements of heavy-flavour hadron production play a unique role in such studies. Heavy quarks are mostly produced in hard scatterings at the early stages of the collisions, well before the QGP is formed. Furthermore, their thermal production is negligible since their masses are larger than the typical QGP temperature. Due to the much better theoretical control on their production and propagation in the medium, heavy quarks provide unique constraints on the QGP properties and the nature of hadronisation mechanisms, compared to light quarks. Heavy-flavour measurements in heavy-ion collisions also test whether the transverse momenta (p_T) integrated yields of charm hadrons are consistent with the hypothesis of statistical models, in which charm quarks are expected to reach an almost complete thermalisation in the QGP, despite being initially very far from equilibrium.

ALICE has recently made an improvement towards a quantitative understanding of hadron formation from a QGP

The ALICE experiment has recently made an improvement towards a quantitative understanding of hadron formation from a QGP by performing the first measurement of the charm baryon-to-meson ratio Λ⁺_c/D⁰ in central (head-on) Pb–Pb collisions at √s_NN = 5.02 TeV. By exploiting its unique tracking and particle-identification capabilities, and using machine-learning techniques, ALICE has measured the ratio down to very low p_T (less than 1 GeV/c), where hadronisation mechanisms via a combination of quarks are expected to dominate (figure 1, left). The measured production ratio of Λ⁺_c/D⁰ in central Pb–Pb collisions is found to be larger than in pp collisions at p_T of 4–8 GeV/c (figure 1, right). On the other hand, the p_T-integrated ratio was found to be compatible with the result of pp collisions within one standard deviation. 

A comparison with theoretical calculations confirms the discrimination power of this measurement. The experimental data are well described by transport models that include mechanisms of the combination of quarks from the deconfined medium (TAMU and Catania). Given the current uncertainties, a conclusive answer on the agreement with statistical models (SHMc) cannot yet be reached. This motivates future high-precision and more differential measurements with the upgraded ALICE detector during the upcoming LHC Run-3 Pb–Pb runs. Thanks to the increased rate-capabilities of the new readout systems of the time projection chamber and the new inner tracking system, ALICE will increase its acquisition rate by up to a factor of about 50 in Pb–Pb collisions and will benefit from a much higher tracking resolution (by a factor 3–6 for low-p_T tracks). High-accuracy measurements performed in Runs 3 and 4 will therefore provide significant discrimination power on theoretical calculations and strong constraints on the mechanisms underlying the hadronisation of charm quarks from the QGP.

The post Charm baryons constrain hadronisation appeared first on CERN Courier.

The precise determination of the Z-boson parameters at e⁺e^– colliders was crucial for the establishment of the electroweak theory of the Standard Model. Today, the Z boson has become an essential object of experimental study at the LHC. In particular, measurements of the Z boson’s production and decay properties in high-energy proton–proton collisions provide insights into the parton distribution functions (PDFs) of the proton and are an implicit test of quantum chromodynamics (QCD). 

Recently, using a sample of Z → μ⁺μ^– events, the LHCb collaboration reported the most precise measurement to date of the Z-boson production cross section in the forward region at a centre-of-mass energy of 13 TeV (see figure 1). The collaboration also reported the first measurements of the angular coefficients in Z → μ⁺μ^– decays in the forward region, which encode key information about the QCD mechanisms underlying the Z-boson production mechanism. In addition to improving knowledge of the proton PDFs, these two analyses contribute to the study of spin-momentum correlations in the proton, complementing ATLAS and CMS measurements in the central region.

In addition to the up and down valence quarks, a proton comprises a sea of quark–antiquark pairs primarily produced via gluon splitting. Given their similar masses, one would expect that the nucleon sea is flavour-symmetric for up and down quarks. However, in the early 1990s, the New Muon Collaboration at CERN found that this symmetry is violated. Later, the ratio of down antiquarks to up antiquarks in the proton was directly measured by the NA51 experiment at CERN and the NuSea/E866 experiment at Fermilab, revealing a significant asymmetry in the sea-quark PDF distributions. Recently, the SeaQuest/E906 experiment at Fermilab reported a new result on this ratio, showing different trends in the larger Bjorken-× range (× > 0.2) compared to the previous results and raising the tension with the NuSea measurement. 

With a detector instrumented in the forward region, LHCb is ideally placed to study decays of highly boosted Z bosons produced by interactions between one parton with large-× and another with small-×. Considering that both the NuSea and SeaQuest results have large contributions from nuclear effects, the current LHCb measurement of the Z production cross section based on a data sample of 5.1 fb^–1 provides important complementary constraints in the large-× region.

The measurement of the angular coefficient “A₂” in Z → μ⁺μ^– decays is sensitive to the transverse-momentum-dependent (TMD) PDFs, as it is proportional to the convolution of the two so-called Boer–Mulders functions of the two initial partons. A measurement of A₂ can thus provide stringent constraints on the nonperturbative partonic spin-momentum correlation within unpolarised protons. By comparing the measured A₂ in different dimuon mass ranges, the LHCb measurement provides an important input for the determination of the proton TMD PDFs, which are crucial to properly describe the production of electroweak bosons at the LHC. Together with the production cross section, these results from LHCb reinforce the importance of a forward detector to complement other measurements at the LHC.

The post Precision Z-boson production measurements appeared first on CERN Courier.

The ALICE collaboration has reported a new measurement of the production of D_s⁺ mesons, which contain a charm and an anti-strange quark, in Pb–Pb collisions collected in 2018 at a centre- of-mass energy per nucleon pair of 5.02 TeV. The large data sample and the use of machine-learning techniques for the selection of particle candidates led to increased precision on this important quantity. 

D-meson measurements probe the interaction between charm quarks and the quark–gluon plasma (QGP) formed in ultra-relativistic heavy-ion collisions. Charm quarks are produced in the early stages of the nucleus–nucleus collision and thus experience the whole system evolution, losing part of their energy via scattering processes and gluon radiation. The presence of the QGP medium also affects the charm-quark hadronisation and, in addition to the fragmentation mechanism, a competing process based on charm–quark recombination with light quarks of the medium might occur. Given that strange quark–antiquark pairs are abundantly produced in the QGP, the recombination mechanism could enhance the yield of D_s⁺ mesons in Pb–Pb collisions with respect to that of D⁰ mesons, which do not contain strange quarks. 

ALICE investigated this possibility using the ratio of the yields of D_s⁺ and D⁰ mesons. The figure displays the D_s⁺/D⁰ yield ratio in central (0–10%) Pb–Pb collisions divided by the ratio in pp collisions, showing that the values of the ratio in the 2 < p_T< 8 GeV/c interval are higher in central Pb–Pb collisions by about 2.3σ. The measured D_s⁺ /D⁰ double ratio also hints at a peak for p_T ≃ 5–6 GeV/c. Its origin could be related to the different D-meson masses and to the collective radial expansion of the system with a common flow-velocity profile. In addition, the hadronisation via fragmentation becomes dominant at high transverse momenta, and consequently, the values of the D_s⁺/D⁰ ratio become similar between Pb–Pb and pp collisions.

The measurement was compared with theoretical calculations based on charm–quark transport in a hydrodynamically expanding QGP (LGR, TAMU, Catania and PHSD), which implement the strangeness enhancement and the hadronisation of charm quarks via recombination in addition to the fragmentation in the vacuum. The Catania and PHSD models predict a ratio almost flat in p_T, while TAMU and LGR describe the peak at p_T ≃  3–5 GeV/c. 

Complementary information was obtained by comparing the elliptic flow coefficient v₂ of D_s⁺ and non-strange D mesons (D⁰, D⁺ and D^*+) in semi-central (30–50%) Pb–Pb collisions. The D_s⁺– meson v₂ is positive in the 2 < p_T < 8 GeV/c interval with a significance of 6.4σ, and is compatible within uncertainties with that of non-strange D mesons. These features of the data are described by model calculations that include recombination of charm and strange quarks.

The freshly-completed upgrade of the detectors and the harvest of Pb–Pb collision data expected in Run 3 will allow the ALICE collaboration to further improve the measurements, deepening our understanding of the heavy-quark interaction and hadronisation in the QGP.

The post Charm-strange mesons probe hadronisation appeared first on CERN Courier.

A new measurement by the ALICE collaboration has demonstrated for the first time that jets become narrower after “quenching” in quark–gluon plasma (QGP). RHIC and LHC data show that the QGP behaves like a strongly-coupled liquid with very low viscosity, but it is an open question how this arises from the asymptotic limit of weakly-coupled quarks and gluons at short lengths. The new results provide quantitative new insights into the hot and dense medium created in heavy-ion collisions and how it modifies the substructure of jets and dissipates part of their energy.

An important property of the QGP is its ability to “resolve” nearby partons as effectively independent colour charges above the medium’s characteristic resolution scale – a parameter that is very poorly predicted by theory, but thought to be in the vicinity of a femtometre or less. In recent years, jet quenching has been proposed to determine this scale. Jets originate from a single quark or gluon that showers into more partons, either by radiating a gluon or splitting into a quark–antiquark pair. When a jet moves through the medium, each individual splitting results in two distinct colour charges that, depending on their angular separation and the medium’s resolution length, can interact as one coherent object or two independent charges. At the LHC, we can put our understanding of this resolution scale to test using special measurements of the angular structure of jets. This allows us to test whether wider jets are more likely to be resolved.

ALICE “groomed” jets using track clustering

To identify the relevant two-prong splittings, ALICE “groomed” jets using track clustering. The algorithm reclusters and unwinds the jet shower to find the first parton splitting satisfying a grooming condition (figure 1). The excellent tracking resolution in ALICE allows for very precise measurements of jet substructure even at small angular distance scales. The angular width of the jet was found to be significantly modified in Pb–Pb compared to pp collisions (figure 2). In particular, wider splittings are suppressed in Pb–Pb compared to pp collisions, demonstrating that the interaction of jets with the QGP filters out wide jets.

This measurement is the first of its kind to be fully corrected for large background effects, allowing direct quantitative comparisons with theoretical calculations of jet quenching. Most theoretical models describe the general narrowing trend seen in the data, despite the different implementations of jet-medium interactions. The data is consistent with models implementing an incoherent interaction in which the medium resolves the splittings (Pablos, L_res = 0). Interestingly, however, another calculation demonstrates this narrowing effect with a fully coherent interaction, in which the jet splittings are not resolved, but by modifying the initial quark and gluon fractions (Yuan, quark). While the precision of the data currently precludes a precise extraction of the medium’s resolving power within a given model, the measurement places quantitative constraints on medium properties, and demonstrates for the first time a direct modification to the angular structure of jets in heavy-ion collisions. This opens the door to increasingly precise measurements with the high-precision data anticipated in LHC Run 3.

The post Quark–gluon plasma narrows jets appeared first on CERN Courier.

The international nuclear-physics community will be front-and-centre as a unique research facility called the Electron–Ion Collider (EIC) moves from concept to reality through the 2020s – the latest progression in the line of large-scale accelerator programmes designed to probe the fundamental forces and particles that underpin the structure of matter. 

Decades of research in particle and nuclear physics have shown that protons and neutrons, once thought to be elementary, have a rich, dynamically complex internal structure of quarks, anti-quarks and gluons, the understanding of which is fundamental to the nature of matter as we experience it. By colliding high-energy beams of electrons with high-energy beams of protons and heavy ions, the EIC is designed to explore this hidden subatomic landscape with the resolving power to image its behaviour directly. Put another way: the EIC will provide the world’s most powerful microscope for studying the “glue” that binds the building blocks of matter.

Luminous performance

When the EIC comes online in the early 2030s, the facility will perform precision “nuclear femtography” by zeroing in on the substructure of quarks and gluons in a manner comparable to the seminal studies of the proton using electron–proton collisions at DESY’s HERA accelerator in Germany between 1992 and 2007 (see “Nuclear femtography to delve deep into nuclear matter” panel). However, the EIC will produce a luminosity (collision rate) 100 times greater than the highest achieved by HERA and, for the first time in such a collider, will provide spin-polarised beams of both protons and electrons, as well as high-energy collisions of electrons with heavy ions. All of which will require unprecedented performance in terms of the power, intensity and spatial precision of the colliding beams, with the EIC expected to provide not only transformational advances in nuclear science, but also transferable technology innovations to shape the next generation of particle accelerators and detectors.

The US Department of Energy (DOE) formally initiated the EIC project in December 2019 with the approval of a “mission need”. That was followed in June of this year with the next “critical decision” to proceed with funding for engineering and design prior to construction (with the estimated cost of the build about $2 billion). The new facility will be sited at Brookhaven National Laboratory (BNL) in Long Island, New York, utilising components and infrastructure from BNL’s Relativistic Heavy Ion Collider (RHIC), including the polarised proton and ion-beam capability and the 3.8 km underground tunnel. Construction will be carried out as a partnership between BNL and Thomas Jefferson National Accelerator Facility (JLab) in Newport News, Virginia, home of the Continuous Electron Beam Accelerator Facility (CEBAF), which has pioneered many of the enabling technologies needed for the EIC’s new electron rings. 

Beyond the BNL–JLab partnership, the EIC is very much a global research endeavour. While the facility is not scheduled to become operational until early in the next decade, an international community of scientists is already hard at work within the EIC User Group. Formed in 2016, the group now has around 1300 members – representing 265 universities and laboratories from 35 countries – engaged collectively on detector R&D, design and simulation as well as initial planning for the EIC’s experimental programme. 

A cutting-edge accelerator facility

Being the latest addition to the line of particle colliders, the EIC represents a fundamental link in the chain of continuous R&D, knowledge transfer and innovation underpinning all manner of accelerator-related technologies and applications – from advanced particle therapy systems for the treatment of cancer to ion implantation in semiconductor manufacturing. 

The images “The EIC in outline” and “Going underground” show the planned layout of the EIC, where the primary beams circulate inside the existing RHIC tunnel to enable the collisions of high-energy (5–18 GeV) electrons (and possibly positrons) with high-energy ion beams of up to 275 GeV/nucleon. One thing is certain: the operating parameters of the EIC, with luminosities of up to 10³⁴ cm^–2 s^–1 and up to 85% beam polarisation, will push the design of the facility beyond the limits set by previous accelerator projects in a number of core technology areas.

For starters, the EIC will require significant advances in the field of superconducting radiofrequency (SRF) systems operating under high current conditions, including control of higher-order modes, beam RF stability and crab cavities. A major challenge is the achievement of strong cooling of intense proton and light-ion beams to manage emittance growth owing to intrabeam scattering. Such a capability will require unprecedented control of low-energy electron-beam quality with the help of ultrasensitive and precise photon detection technologies – innovations that will likely yield transferable benefits for other areas of research reliant on electron-beam technology (e.g. free-electron lasers). 

The EIC design for strong cooling of the ion beams specifies a superconducting energy-recovery linac with a virtual beam power of 15 MW, an order-of-magnitude increase versus existing machines. With this environmentally friendly new technology, the rapidly cycling beam of low-energy electrons (150 MeV) is accelerated within the linac and passes through a cooling channel where it co-propagates with the ions. The cooling electron beam is then returned to the linac, timed to see the decelerating phase of the RF field, and the beam power is thus recovered for the next accelerating cycle – i.e. beam power is literally recycled after each cooling pass.

The EIC will also require complex operating schemes. A case in point: fresh, highly polarised electron bunches will need to be frequently injected into the electron storage ring without disturbing the collision operation of previously injected bunches. Further complexity comes in maximising the luminosity and polarisation over a large range of centre-of-mass energies and for the entire spectrum of ion beams. With a control system that can monitor hundreds of beam parameters in real-time, and with hundreds of points where the guiding magnetic fields can be tuned on the fly, there is a vast array of “knobs-to-be-turned” to optimise overall performance. Inevitably, this is a facility that will benefit from the use of artificial intelligence and machine-learning technologies to maximise its scientific output. 

At the same time, the EIC and CERN’s High-Luminosity LHC user communities are working in tandem to realise more capable technologies for particle detection as well as innovative electronics for large-scale data read-out and processing. Exploiting advances in chip technology, with feature sizes as small as 65 nm, multipixel silicon sensors are in the works for charged-particle tracking, offering single-point spatial resolution better than 5 µm, very low mass and on-chip, individual-pixel readout. These R&D efforts open the way to compact arrays of thin solid-state detectors with broad angular coverage to replace large-volume gaseous detectors. 

Coupled with leading-edge computing capabilities, such detectors will allow experiments to stream data continuously, rather than selecting small samples of collisions for readout. Taken together, these innovations will yield no shortage of downstream commercial opportunities, feeding into next-generation medical imaging systems, for example, as well as enhancing industrial R&D capacity at synchrotron light-source facilities.

The BNL–JLab partnership

As the lead project partners, BNL and JLab have a deep and long-standing interest in the EIC programme and its wider scientific mission. In 2019, BNL and JLab each submitted their own preconceptual designs to DOE for a future high-energy and high-luminosity polarised EIC based around existing accelerator infrastructure and facilities. In January 2020, DOE subsequently selected BNL as the preferred site for the EIC, after which the two labs immediately committed to a full partnership between their respective teams (and other collaborators) in the construction and operation of the facility. 

Nuclear femtography to delve deep into nuclear matter

Nuclear matter is inherently complex because the interactions and structures therein are inextricably mixed up: its constituent quarks are bound by gluons that also bind themselves. Consequently, the observed properties of nucleons and nuclei, such as their mass and spin, emerge from a dynamical system governed by quantum chromodynamics (QCD). The quark masses, generated via the Higgs mechanism, only account for a tiny fraction of the mass of a proton, leaving fundamental questions about the role of gluons in the structure of nucleons and nuclei still unanswered. 

The underlying nonlinear dynamics of the gluon’s self-interaction is key to understanding QCD and fundamental features of the strong interactions such as dynamical chiral symmetry-breaking and confinement. Yet despite the central role of gluons, and the many successes in our understanding of QCD, the properties and dynamics of gluons remain largely unexplored. 

If that’s the back-story, the future is there to be written by the EIC, a unique machine that will enable physicists to shed light on the many open questions in modern nuclear physics. 

Back to basics

At the fundamental level, the way in which a nucleon or nucleus reveals itself in an experiment depends on the kinematic regime being probed. A dynamic structure of quarks and gluons is revealed when probing nucleons and nuclei at higher energies, or with higher resolutions. Here, the nucleon transforms from a few-body system, with its structure dominated by three valance quarks, to a regime where it is increasingly dominated by gluons generated through gluon radiation, as discovered at the former HERA electron–proton collider at DESY. Eventually, the gluon density becomes so large that the gluon radiation is balanced by gluon recombination, leading to nonlinear features of the strong interaction.

The LHC and RHIC have shown that neutrons and protons bound inside nuclei already exhibit the collective behaviour that reveals QCD substructure under extreme conditions, as initially seen with high-energy heavy-ion collisions. This has triggered widespread interest in the study of the strong force in the context of condensed-matter physics, and the understanding that the formation and evolution of the extreme phase of QCD matter is dominated by the properties of gluons at high density.

The subnuclear genetic code

The EIC will enable researchers to go far beyond the present one-dimensional picture of nuclei and nucleons, where the composite nucleon appears as a bunch of fast-moving (anti-)quarks and gluons whose transverse momenta or spatial extent are not resolved. Specifically, by correlating the information of the quark and gluon longitudinal momentum component with their transverse momentum and spatial distribution inside the nucleon, the EIC will enable nuclear femtography. 

Such femtographic images will provide, for the first time, insight into the QCD dynamics inside hadrons, such as the interplay between sea quarks and gluons. The ultimate goal is to experimentally reconstruct and constrain the so-called Wigner functions – the quantities that encode the complete tomographic information and constitute a QCD “genetic map” of nucleons and nuclei.

 • Adapted from “Electron–ion collider on the horizon” by Elke-Caroline Aschenauer, BNL, and Rolf Ent, JLab.

The construction project is led by a joint BNL–JLab management team that integrates the scientific, engineering and management capabilities of JLab into the BNL design effort. JLab, for its part, leads on the design and construction of SRF and cryogenics systems, the energy-recovery linac and several of the electron injector and storage-ring subsystems within the EIC accelerator complex. 

More broadly, BNL and JLab are gearing up to work with US and international partners to meet the technical challenges of the EIC in a cost-effective, environmentally responsible manner. The goal: to deliver a leading-edge research facility that will build upon the current CEBAF and RHIC user base to ensure engagement – at scale – from the US and international nuclear-physics communities. 

As such, the labs are jointly hosting the EIC experiments in the spirit of a DOE user facility for fundamental research, while the BNL–JLab management team coordinates the engagement of other US and international laboratories into a multi-institutional partnership for EIC construction. Work is also under way with prospective partners to define appropriate governance and operating structures to enhance the engagement of the user community with the EIC experimental programme. 

With international collaboration hard-wired into the EIC’s working model, the EIC User Group has been in the vanguard of a global effort to develop the science goals for the facility – as well as the experimental programme to realise those goals. Most importantly, the group has carried out intensive studies over the past two years to document the measurements required to deliver EIC’s physics objectives and the resulting detector requirements. This work also included an exposition of evolving detector concepts and a detailed compendium of candidate technologies for the EIC experimental programme.

Cornerstone collaborations 

The resulting Yellow Report, released in March 2021, provides the basis for the ongoing discussion of the most effective implementation of detectors, including the potential for complementary detectors in the two possible collision points as a means of maximising the scientific output of the EIC facility (see “Detectors deconstructed”). Operationally, the report also provides the cornerstone on which EIC detector proposals are currently being developed by three international “proto-collaborations”, with significant components of the detector instrumentation being sourced from non-US partners. 

The EIC represents a fundamental link in the chain of continuous R&D and knowledge transfer

Along every coordinate, it’s clear that the EIC project profits enormously from its synergies with accelerator and detector R&D efforts worldwide. To reinforce those benefits, a three-day international workshop was held in October 2020, focusing on EIC partnership opportunities across R&D and construction of accelerator components. This first Accelerator Partnership Workshop, hosted by the Cockcroft Institute in the UK, attracted more than 250 online participants from 26 countries for a broad overview of EIC and related accelerator-technology projects. A follow-up workshop, scheduled for October 2021 and hosted by the TRIUMF Laboratory in Canada, will focus primarily on areas where advanced “scope of work” discussions are already under way between the EIC project and potential partners.

Nurturing talent 

While discussion and collaboration between the BNL and JLab communities were prioritised from the start of the EIC planning process, a related goal is to get early-career scientists engaged in the EIC physics programme. To this end, two centres were created independently: the Center for Frontiers in Nuclear Science (CFNS) at Stony Brook University, New York, and the Electron-Ion Collider Center (EIC²) at JLab.

The CFNS, established jointly by BNL and Stony Brook University in 2017, was funded by a generous donation from the Simons Foundation (a not-for-profit organisation that supports basic science) and a grant from the State of New York. As a focal point for EIC scientific discourse, the CFNS mentors early-career researchers seeking long-term opportunities in nuclear science while simultaneously supporting the formation of the EIC’s experimental collaborations. 

Core CFNS activities include EIC science workshops, short ad-hoc meetings (proposed and organised by members of the EIC User Group), alongside a robust postdoctoral fellow programme to guide young scientists in EIC-related theory and experimental disciplines. An annual summer school series on high-energy QCD also kicked off in 2019, with most of the presentations and resources from the wide-ranging CFNS events programme available online to participants around the world. 

In a separate development, the CFNS recently initiated a dedicated programme for under-represented minorities (URMs). The Edward Bouchet Initiative provides a broad portfolio of support to URM students at BNL, including grants to pursue masters or doctoral degrees at Stony Brook on EIC-related research. 

Meanwhile, the EIC² was established at JLab with funding from the State of Virginia to involve outstanding JLab students and postdocs in EIC physics. Recognising that there are many complementary overlaps between JLab’s current physics programme and the physics of the future EIC, the EIC² provides financial support to three PhD students and three postdocs each year to expand their current research to include the physics that will become possible once the new collider comes online. 

Beyond their primary research projects, this year’s cohort of six EIC² fellows worked together to organise and establish the first EIC User Group Early Career workshop. The event, designed specifically to highlight the research of young scientists, was attended by more than 100 delegates and is expected to become an annual part of the EIC User Group meeting.

The future, it seems, is bright, with CFNS and EIC² playing their part in ensuring that a diverse cadre of next-generation scientists and research leaders is in place to maximise the impact of EIC science over the decades to come.

The post Partnership yields big wins for the EIC appeared first on CERN Courier.

Most processes resulting from proton–proton collisions at the LHC are affected by the strong force – a difficult-to-model part of the Standard Model involving non-perturbative effects. This can be problematic when measuring rare processes not mediated by strong interactions, such as those involving the Higgs boson, and when searching for new particles or interactions. To ensure such processes are not obscured, precise knowledge of the more dom­inant strong-interaction effects, including those caused by the initial-state partons, is a prerequisite to LHC physics analyses.

The electromagnetic production of a photon pair is the dominant background to the H → γγ decay channel – a process that is instrumental to the study of the Higgs boson. Despite its electromagnetic nature, diphoton production is affected by surprisingly large strong-interaction effects. Thanks to precise ATLAS measurements of diphoton processes using the full Run-2 dataset, the collaboration is able to probe these effects and scrutinise state-of-the-art theoretical calculations.

Measurements studying strong interactions typically employ final states that include jets produced from the showering and hadronisation of quarks and gluons. However, the latest ATLAS analysis instead uses photons, which can be very precisely measured by the detector. Although photons do not carry a colour charge, they interact with quarks as the latter carry electric charge. As a result, strong-interaction effects on the quarks can alter the characteristics of the measured photons. The conservation of momentum allows us to quantify this effect: the LHC’s proton beams collide head-on, so the net momentum transverse to the beam axis must be zero for the final-state particles. Any signs to the contrary indicate additional activity in the event with equivalent but opposite transverse momentum, usually arising from quarks and gluons radiated from the initial-state partons. Therefore, by measuring the transverse momentum of photon pairs, and related observables, the strong interaction may be indirectly probed.

A surprising role of the strong interaction in electro-magnetic diphoton production is revealed

Comparing the measured values to predictions reveals the surprising role of the strong interaction in electromagnetic diphoton production. In a simple picture without the strong interaction, the momentum of each photon should perfectly balance in the transverse plane. However, this simplistic expectation does not match the measurements (see figure 1). Measuring the differential cross-section as a function of the transverse momentum of the photon pair, ATLAS finds that most of the measured photon pairs (black points) have low but non-zero transverse momenta, with a peak at approximately 10 GeV, followed by a smoothly falling distribution towards higher values.

Extending calculations to encompass next-to-next-to-leading order corrections in the strong-interaction coupling constant (purple line), the impact of the strong interaction becomes manifest. The measured values at high transverse momenta are well described by these predictions, including the bump observed at 70 GeV, which is another manifestation of higher-order strong-interaction effects. Monte Carlo event generators like Sherpa (red line), which combine similar calculations with approximate simulations of arbitrarily many-quark and gluon emissions – especially relevant at low energies – properly describe the entire measured distribution.

The results of this analysis, which also include measurements of other distributions such as angular variables between the two photons, don’t just viscerally probe the strong interaction – they also provide a benchmark for this important background process.

The post Strongly unbalanced photon pairs appeared first on CERN Courier.

The COMPASS experiment at CERN has reported the first direct evidence for a long-hypothesised interplay between hadron decays which can masquerade as a resonance. The analysis, which was published last week in Physical Review Letters, suggests that the “a₁(1420)” signal observed by the collaboration in 2015 is not a new exotic hadron after all, but the first sighting of a so-called triangle singularity.

“Triangle singularities are a mechanism for generating a bump in the decay spectrum that does not correspond to a resonance,” explains analyst Mikhail Mikhasenko of the ORIGINS Cluster in Munich. “One gets a peak that has all features of a new hadron, but whose true nature is a virtual loop with known particles.” 

“This is a prime example of an aphorism which is commonly attributed to Dick Dalitz,” agrees fellow analyst Bernhard Ketzer, of the University of Bonn: “Not every bump is a resonance, and not every resonance is a bump!”

Triangle singularities take their name from the triangle in a Feynman diagram when a secondary decay product fuses with a primary decay product. If the particle masses line up such that the process can proceed as a cascade of on-mass-shell hadron decays, the matrix element is enhanced by a so-called logarithmic singularity which can easily be mistaken for a resonance. But the effect is usually rather small, requiring a record 50 million π^–p→π^–π⁺π^–p events, and painstaking work by the COMPASS collaboration to make certain that the a₁(1420) signal, which makes up less than 1% of the three-pion sample, wasn’t an artefact of the analysis procedure.

Hadron experiments are reaching the precision needed to see one of the most peculiar multi-body features of QCD

Mikhail Mikhasenko

“The correspondence of this small signal with a triangle singularity is noteworthy because it shows that hadron experiments are finally reaching the precision and statistics needed to see one of the most peculiar features of the multi-body non-perturbative regime of quantum chromodynamics,” says Mikhasenko.

Triangle singularities were dreamt up independently by Lev Landau and Richard Cutkosky in 1959. After five decades of calculations and speculations, physicists at the Institute for High-Energy Physics in Beijing in 2012 used a triangle singularity to explain why intermediate f₀(980) mesons in J/ψ meson decays at the BESIII experiment at the Beijing Electron–Positron Collider II were unusually long-lived. In 2019, the LHCb collaboration ruled out triangle singularities as the origin of the pentaquark states they discovered that year. The new COMPASS analysis is the first time that a “bump” in a decay spectrum has been convincingly explained as more likely due to a triangle singularity than a resonance.

COMPASS collides a secondary beam of charged pions from CERN’s Super Proton Synchrotron with a hydrogen target in the laboratory’s North Area. In this analysis, gluons emitted by protons in the target excite the incident pions, producing the final state of three charged pions which is observed by the COMPASS spectrometer. Intermediate resonances display a variety of angular momentum, spin and parity configurations. In 2015, the collaboration observed a small but unmistakable “P-wave” (L=1) component of the f₀(980)π system with a peak at 1420 MeV and J^PC=1⁺⁺. Dubbed a₁(1420), the apparent resonance was suspected to be exotic, as it was narrower, and hence more stable, than the ground-state meson with the same quantum numbers, a₁(1260). It was also surprisingly light, with a mass just above the K*K threshold of 1.39 GeV. A tempting interpretation was that a₁(1420) might be a dsūs̄ tetraquark, and thus the first exotic hadronic state with no charm quarks, and a charged cousin of the famous exotic X(3872) at the D*D threshold to boot, explains Mikhasenko.

According to the new COMPASS analysis, however, the bump at 1420 MeV can more simply be explained by a triangle singularity, whereby an a₁(1260) decays to a K*K pair, and the kaon from the resulting K*→Kπ decay annihilates with the initial anti-kaon to create a light unflavoured f₀(980) meson which decays to a pair of charged pions. Crucially, the mass of f₀(980) is just above the KK threshold, and the roughly 300 MeV width of the conventional a₁(1260) meson is wide enough for the particle to be said to decay to K*K on-mass-shell.

A new resonance is not required. That is phenomenologically significant.

Ian Aitchison

“The COMPASS collaboration have obviously done a very thorough job, being in possession of a complete partial-wave analysis,” says Ian Aitchison, emeritus professor at the University of Oxford, who in 1964 was among the first to propose that triangle graphs with an unstable internal line (in this case the K*) could lead to observable effects. This enables the whole process to occur nearly on-shell for all particles, which in turn means that the singularities of the amplitude will be near the physical region, and hence observable, explains Aitchison. “This is not unambiguous evidence for the observation of a triangle singularity, but the paper shows pretty convincingly that it is sufficient to explain the data, and that a new resonance is not required. That is phenomenologically significant.”

The collaboration now plans further studies of this new phenomenon, including its interference with the direct decay of the a₁(1260). Meanwhile, observation by Belle II of the a₁(1420) phenomenon in decays of the tau meson to three pions should confirm our understanding and provide an even cleaner signal, says Mikhasenko.

The post COMPASS points to triangle singularity appeared first on CERN Courier.

The conference surveyed topics relating to multi-loop and multi-leg calculations in quantum chromodynamics (QCD) and electroweak processes. In scattering processes, loops are closed particle lines and legs represent external particles. Both present computational challenges. Recent progress on many inclusive processes has been reported at three- or four-loop order, including for deep-inelastic scattering, jets at colliders, the Drell–Yan process, top-quark and Higgs-boson production, and aspects of bottom-quark physics. Much improved descriptions of scaling violations of parton densities, heavy-quark effects at colliders, power corrections, mixed QCD and electroweak corrections, and high-order QED corrections for e⁺e^– colliders have also recently been obtained. These will be important for many processes at the LHC, and pave the way to physics at facilities such as the proposed Future Circular Collider (FCC).

Quantum field theory provides a very elegant way to solve Einsteinian gravity

Weighty considerations

Although merging black holes can have millions of solar masses, the physics describing them remains classical, and quantum gravity happened, if at all, shortly after the Big Bang. Nevertheless, quantum field theory provides an elegant way to solve Einsteinian gravity. At this year’s Loop Summit, perturbative approaches to gravity were discussed that use field-theoretic methods at the level of the 5th and 6th post-Newtonian approximations, where the nth post-Newtonian order corresponds to a classical n-loop calculation between black-hole world lines. These calculations allow predictions of the binding energy and periastron advance of spiralling-in pairs of black holes, and relate them to gravitational-wave effects. In these calculations, the classical loops all link to world lines in classical graviton networks within the framework of an effective-field-theory representation of Einsteinian gravity.

Other talks discussed important progress on advanced analytic computation technologies and new mathematical methods such as computational improvements in massive Dirac-algebra, new ways to calculate loop integrals analytically, new ways to deal consistently with polarised processes, the efficient reduction of highly connected systems of integrals, the solution of gigantic systems of differential equations, and numerical methods based on loop-tree duality. All these methods will decrease the theory errors for many processes due to be measured in the high-luminosity phase of the LHC, and beyond.

Half of the meeting was devoted to developing new ideas in subgroups. In-person discussions are invaluable for highly technical discussions such as these — there is still no substitute for gathering around the blackboard informally and jotting down equations and diagrams. The next Loop Summit in this triennial series will take place in summer 2024.

The post Loop Summit convenes in Como appeared first on CERN Courier.

All the exotic hadrons that have been observed so far decay rapidly via the strong interaction. The ccūd̄ tetraquark (T_cc⁺ ) just discovered by the LHCb collaboration is no exception. However, it is the longest-lived state yet, and reinforces expectations that its beautiful cousin, bbūd , will be stable with respect to the strong interaction when its peak emerges in future data.

“We have discovered a ccūd tetraquark with a mass just below the D^*+D⁰ threshold which, according to most models, indicates that it is a bound state,” says LHCb analyst Ivan Polyakov (Syracuse University). “It still decays to D mesons via the strong interaction, but much less intensively than other exotic hadrons.”

Most of the exotic hadronic states discovered in the past 20 years or so are cc̄qq̄ tetraquarks or cc̄qqq pentaquarks, where q represents an up, down or strange quark. A year ago LHCb also discovered a hidden-double-charm cc̄cc̄ tetraquark, X(6900), and two open-charm csūd tetraquarks, X₀(2900) and X₁(2900). The new ccūd state, presented today at the European Physical Society conference on high-energy physics to have been observed with a significance substantially in excess of five standard deviations, is the first exotic hadronic state with so-called double open heavy flavour — in this case, two charm quarks unaccompanied by antiparticles of the same flavour.

Astoundingly, its observation by LHCb reveals that it is a mere 270 keV below the threshold

Prime Candidate

Tetraquark states with two heavy quarks and two light antiquarks have been the prime candidates for stable exotic hadronic states since the 1980s. LHCb’s discovery, four years ago, of the Ξ_cc⁺⁺ (ccu) baryon allowed QCD phenomenologists to firmly predict the existence of a stable bbūd tetraquark, however the stability of a potential ccūd state remained unclear. Predictions of the mass of the ccūd state varied substantially, from 250 MeV below to 200 MeV above the D^*+D⁰ mass threshold, say the team. Astoundingly, its observation by LHCb reveals that it is a mere 273 ± 61 keV below the threshold — a bound state, then, but with the threshold for strong decays to D^*+D⁰ lying within the observed resonance’s narrow width of 410 ± 165 keV, prescribed by the uncertainty principle. The T_cc⁺ tetraquark can therefore decay via the strong interaction, but strikingly slowly. By contrast, most exotic hadronic states have widths from tens to several hundreds of MeV.

“Such closeness to the threshold is not very common in heavy-hadron spectroscopy,” says analyst Vanya Belyaev (Kurchatov Institute/ITEP). “Until now, the only similar closeness was observed for the enigmatic χ_c1(3872) state, whose mass coincides with the D^*0D⁰ threshold with a precision of about 120 keV.” As it is wider, however, it is not yet known whether the χ_c1(3872) is below or above threshold.

I am fascinated by the idea that a strong coupling to a decay channel might attract the bare mass of the hadron

Mikhail Mikhasenko

“The surprising proximity of T_cc⁺ and χ_c1(3872) to the D^*D thresholds must have deep reasoning,” adds analyst Mikhail Mikhasenko (ORIGINS, Munich). “I am fascinated by the idea that, roughly speaking, a strong coupling to a decay channel might attract the bare mass of the hadron. Tremendous progress in lattice QCD over the past 10 years gives us hope that we will discover the answer soon.”

The cause of this attraction, says Mikhasenko, could be linked to a “quantum admixture” of two models that vie to explain the structure of the new tetraquark: it could be a D^*+ and a D⁰ meson, bound by the exchange of colourneutral objects such as light mesons, or a colour-charged cc “diquark” tightly bound via gluon exchange to up and down antiquarks (see “Jumbled together” figure). Diquarks are a frequently employed mathematical construct in low-energy quantum chromodynamics (QCD): if two heavy quarks are sufficiently close together, QCD becomes perturbative, and they may be shown to attract each other and exhibit effective anticolour charge. For example, a red-green cc diquark would have a wavefunction similar to an anti-blue anti-quark, and could pair up with a blue quark to form a baryon — or, hypothetically, a blue anti-diquark, to form a colour-neutral tetraquark.

“The question is if the D and D^* are more or less separated, jumbled together to such a degree that all quarks are intertwined in a compact object, or something in between,” says Polyakov. “The first scenario resembles a relatively large ~4 fm deuteron, whereas the second can be compared to a relatively compact ~2 fm alpha particle.”

The new T_cc⁺ tetraquark is an enticing target for further study. Its narrow decay into a D⁰D⁰π⁺ final state — the virtual D^*+ decays promptly into D⁰π⁺ — includes no particles that are difficult to detect, leading to a better precision on its mass than for existing measurements of charmed baryons. This, in turn, can provide a stringent test for existing theoretical models and could potentially probe previously unreachable QCD effects, says the team. And, if detected, its beautiful cousin would be an even bigger boon. “Observing a tightly bound exotic hadron that would be stable with respect to the strong interaction would be a cornerstone in understanding QCD at the scale of hadrons,” says Polyakov. “The bbūd , which is believed to satisfy this requirement, is produced rarely and is out of reach of the current luminosity of the LHC. However, it may become accessible in LHC Run 3 or at the High-Luminosity LHC.” In the meantime, there is no shortage of work in hadron spectroscopy, jokes Belyaev. “We definitely have more peaks than researchers!”

The post New tetraquark a whisker away from stability appeared first on CERN Courier.

The assumption that charm-to-hadron fragmentation is universal is not valid

The large data samples collected during Run 2 of the LHC at √s = 5.02 TeV allowed the ALICE collaboration measure the vast majority of charm quarks produced in the pp collisions by reconstructing the decays of the ground-state charm hadrons, measuring all the charm-meson species and the most abundant charm baryons (Λ_c⁺, and Ξ_c^0,+) down to very low transverse momenta. The result was presented today at the European Physical Society conference on high-energy physics (EPS-HEP 2021).

Charm fragmentation fractions, f(c → H_c), represent the probability for a charm quark to hadronise into a given charm hadron. These have now been measured for the first time at the LHC in pp collisions at midrapidity, and, in the case of the Ξ_c⁰ , for the first time in any collision system (figure 1). The measured f(c → H_c) are observed to be different from those measured in e⁺e^– and ep collisions – evidence that the assumption that charm-to-hadron fragmentation is universal is not valid.

Charm quarks were found to hadronise into baryons almost 40% of the time – four times more often than at colliders with electron beams. Several models have been proposed to explain this “baryon enhancement”. The explanations feature various different assumptions, such as including hadronisation via coalescence, considering a set of as-yet-unobserved higher-mass charm-baryon states, and including string formation beyond the leading-colour approximation.

The cc̄ production cross section per unit of rapidity at midrapidity (dσ^cc̄/dy|_|y|<0.5) was calculated by summing the cross sections of all measured ground-state charm hadrons (D⁰, D⁺, D_s⁺ , Λ_c⁺ , and Ξ_c⁰). The contribution of the Ξ_c⁰ was multiplied by a factor of two, in order to account for the contribution of the Ξ_c⁺. The resulting cc̄ cross section per unit of rapidity at midrapidity is dσ^cc̄/dy|_|y|<0.5 = 1165 ± 44(stat) ⁺¹³⁴ _–101 (syst) μb. This measurement was obtained for the first time in hadronic collisions at the LHC including the charm-baryon states. The cc̄  cross section measured at the LHC lies at the upper edge of the theoretical pQCD calculations.

The measurements described above not only provide constraints to pQCD calculations, but also act as important references for investigating the interaction of charm quarks with the medium created in heavy-ion collisions. These measurements could be extended to include rarer baryons and studied as a function of the event multiplicity in pp and heavy-ion systems in future LHC runs.

The post Charm breaks fragmentation universality appeared first on CERN Courier.

New results on the production of strangeness in heavy-ion collisions were presented for a variety of collision energies and systems. In an experimental highlight, the ALICE collaboration reported that the number of strange baryons depends more on the final-state multiplicity than the initial-state energy. On the theory side, it was shown that several models can explain the suppression of strange particles at low multiplicities. ALICE also presented new measurements of the charm cross section and fragmentation functions in proton–proton (pp) collisions. When compared to e⁺e^– collisions, these results suggest that the universality of parton-to-hadron fragmentation may be broken. 

Moving on to heavy flavours, the ATLAS collaboration presented results for the suppression of heavy-flavour production compared to pp collisions and the angular anisotropy of heavy mesons in heavy-ion collisions. These measurements are crucial for constraining models of in-medium energy loss. Interestingly, while charm seems to follow the flow of the quark–gluon plasma, beauty does not seem to flow. Better statistics are needed to constrain theoretical models. On the theory side, extremely interesting new calculations using open quantum systems coupled with potential non-relativistic QCD calculations were used to compute both the suppression and anisotropic flow of bottomonium states.

Hints of extrema

Another important goal of the field is to determine experimentally whether a critical point exists in the phase diagram of strongly interacting matter, and, if so, where it is located. The STAR experiment at the Relativistic Heavy Ion Collider (RHIC) presented results on higher order cumulants of net-proton fluctuations over a range of collision energies. Extrema as a function of beam energy are expected to indicate critical behaviour. New data from the Beam Energy Scan II programme at RHIC is expected to provide much-needed statistics to confirm hints of extrema in the data. On the theory side, new lattice QCD calculations of second-order net-baryon cumulants were presented, as well as new expansion schemes to extend the lattice-QCD equation of state to larger net baryon chemical potentials that are not computable directly, because of the fermion-sign problem. Another study included the lattice-QCD equation of state and susceptibilities in a hydrodynamic calculation to allow for a more direct comparison to experimental measurements of net-proton fluctuations. Significant differences between net-proton and net-baryon fluctuations were quantified. 

The study of the quark–gluon plasma’s vorticity via the measurement of the polarisation of hyperons was also a major topic. Theoretical calculations obtain the opposite sign to the data for the angular differential measurement. Attempts to solve this discrepancy presented at SQM 2021 featured shear-dependent terms and a stronger “memory” of the strange-quark spin.

Various new applications of machine learning and artificial intelligence were also discussed, for example, for determining the order of the phase transition and constraining the neutron-star equation of state. 

Overall, there were 41 plenary and 96 parallel talks at SQM 2021, poignantly including presentations in memory of Jean Cleymans, Jean Letessier, Dick Majka and Jack Sandweiss, who all made exceptional impacts on the field.

The next SQM conference will be held from 13 to 18 June 2022 in Busan, South Korea.

The post Experiment and theory trade blows at SQM 2021 appeared first on CERN Courier.

The LHCP format traditionally has plenary sessions in the mornings and late afternoons, with parallel sessions in the middle of the day. This “shape” was kept for the online meeting, with a shorter day to improve the practicality of joining from distant time zones. This resulted in a dense format with seven-fold parallel sessions, allowing all parts of the LHC programme, both experimental and theoretical, to be explored in detail. The overall vitality of the programme is illustrated by the raw statistics: a grand total of 238 talks and 122 posters were presented.

Last year saw a strong focus on the couplings to the second generation

Nine years on from the discovery of the 125 GeV Higgs boson, measurements have progressed to a new level of precision with the full Run-2 data. Both ATLAS and CMS presented new results on Higgs production, helping constrain the dynamics of the production mechanisms via differential and “simplified template” cross-section measurements. While the couplings of the Higgs to third-generation fermions are now established, last year saw a strong focus on the couplings to the second generation. After first evidence for Higgs decays to muons was reported from CMS and ATLAS results earlier in the year, ATLAS presented a new search with the full Run-2 data for Higgs decays to charm quarks using powerful new charm-tagging techniques. Both CMS and ATLAS showed updated searches for Higgs-pair production, with ATLAS being able to exclude a production rate more than 4.1 times the Standard Model (SM) prediction at 95% confidence. This is a process that should be observable with High-Luminosity LHC statistics, if it is as predicted in the SM. A host of searches were also reported, some using the Higgs as a tool to probe for new physics.

Puzzling hints

The most puzzling hints from the LHC Run 1 seem to strengthen in Run 2. LHCb presented analyses relating to the “flavour anomalies” found most notably in b→sµ⁺µ⁻ decays, updated to the full data statistics, in multiple channels. While no result yet passes a 5σ difference from SM expectations, the significances continue to creep upwards. Searches by ATLAS and CMS for potential new particles or effects at high masses that could indicate an associated new-physics mechanism continue to draw a blank, however. This remains a dilemma to be studied with more precision and data in Run 3. Other results in the flavour sector from LHCb included a new measurement of the lifetime of the Ω_c, four times longer than previous measurements (CERN Courier July/August 2021 p17) and the first observation of a mass difference between the mixed D⁰–D⁰ meson mass eigenstates (CERN Courier July/August 2021 p8).

A wealth of results was presented from heavy-ion collisions. Measurements with heavy quarks were prominent here as well. ALICE reported various studies of the differences in heavy-flavour hadron production in proton–proton and heavy-ion collisions, for example using D mesons. CMS reported the first observation of B_c meson production in heavy-ion collisions, and also first evidence for top-quark pair production in lead–lead collisions. ATLAS used heavy-flavour decays to muons to compare suppression of b- and c-hadron production in lead–lead and proton–proton collisions. Beyond the ions, ALICE also showed intriguing new results demonstrating that the relative rates of different types of c-hadron production differ in proton–proton collisions compared to earlier experiments using e⁺e⁻ and ep collisions at LEP and HERA.

Looking forward, the experiments reported on their preparations for the coming LHC Run 3, including substantial upgrades. While some work has been slowed by the pandemic, recommissioning of the detectors has begun in preparation for physics data taking in spring 2022, with the brighter beams expected from the upgraded CERN accelerator chain. One constant to rely on, however, is that LHCP will continue to showcase the fantastic panoply of physics at the LHC.

The post LHCP sees a host of new results appeared first on CERN Courier.

Which charmed baryon decays first? The LHCb collaboration recently challenged the received wisdom of fixed-target experiments by almost quadrupling the measured lifetime of the doubly strange Ω_c⁰. Now, a follow-up measurement by the collaboration confirms the revised hierarchy, offering valuable input to theoretical models of the decays.

The situation changed dramatically in 2018

Ground-state baryons containing a charm quark (c), such as Λ_c⁺ (udc), Ξ_c⁺ (usc), Ξ_c⁰ (dsc) and Ω_c⁰ (ssc), decay via the weak interaction. The ordering of their lifetimes has long been thought to be τ(Ξ_c⁺) > τ(Λ_c⁺) > τ(Ξ_c⁰) > τ(Ω_c⁰), based on measurements from fixed-target experiments nearly 20 years ago. However, the situation changed dramatically in 2018 when LHCb joined the game using a sample of Ω_c⁰ baryons obtained from bottom- baryon semileptonic decays. That LHCb study measured the Ω_c⁰ lifetime to be nearly four times larger than previously measured, transforming the hierarchy into τ(Ξ_c⁺) > τ(Ω_c⁰) > τ(Λ_c⁺) > τ(Ξ_c⁰). One year later, LHCb significantly improved the precisions of the lifetimes of the other three charmed baryons using the same method, also finding the lifetime of the Ξ_c⁰ baryon to be larger than the world-average value by about 3σ (figure 1).

Theoretically challenging

The corresponding theoretical calculations are challenging. In the charm sector, an effective theory of heavy-quark expansion is taken to calculate lifetimes of charmed baryons through an expansion in powers of 1/m_c, where m_c is the constituent charm–quark mass. Calculations up to order 1/m_c³ imply a lifetime hierarchy consistent with the original fixed-target measurements, though only qualitatively. Attempts at higher-order calculations up to order 1/m_c⁴, however, cannot accommodate the old hierarchy, but can explain the new one if a suppression factor to the constructive Pauli-interference and semileptonic terms is written in. The origin of the suppression factor is still unknown, but probably due to even higher order effects. An independent measurement was therefore needed to confirm the experimental situation.

The charmed-baryon lifetime puzzle has now been resolved by a new measurement from LHCb using a much larger sample of Ω_c⁰ and Ξ_c⁰ baryons produced directly in proton–proton collisions. Both particles are detected in the final state pK^–K^–π⁺. The measurement is made relative to the lifetime distribution of the charmed meson D⁰ via D⁰ → K⁺K^–π⁺π^– decays, in order to control systematic uncertainties. Taking advantage of the performance and detailed understanding of the LHCb detector, the lifetimes of the Ω_c⁰ and Ξ_c⁰ baryons are found to be τ(Ω_c⁰) = 276.5 ± 13.4 (stat) ± 4.5 (syst) fs and τ(Ξ_c⁰) = 148.0 ± 2.3 (stat) ± 2.2 (syst) fs, respectively, where the precision of the Ω_c⁰ lifetime is improved by a factor of two compared to the semileptonic measurement. The new results are consistent with the previous LHCb measurements, and hence establish the new lifetime hierarchy. Combining this measurement with the previous LHCb results gives τ(Ω_c⁰) = 274.5 ± 12.4 fs and τ(Ξ_c⁰) = 152.0 ± 2.0 fs, the most precise charm-baryon lifetimes to date. The newly confirmed lifetime hierarchy will help improve our knowledge of QCD dynamics in charm hadrons, and provides a crucial input to calibrate theoretical calculations.

The post New charmed-baryon lifetime hierarchy cast in stone appeared first on CERN Courier.

The production of different types of hadrons provides insights into one of the most fundamental transitions in nature – the “hadronisation” of highly energetic partons into hadrons with confined colour charge. To understand how this transition takes place we have to rely on measurements, and measurement-driven modelling. This is because the strong interaction processes that govern hadronisation are characterised by a scale given by the typical size of hadrons – about 1 fm – and cannot be calculated with perturbative techniques. The ALICE collaboration has recently performed a novel study of hadronisation by comparing the production of strange neutral baryons and mesons inside and outside of charged-particle jets.

One of the ways to contrast baryon and meson production is to analyse the ratio of their momentum distributions. This has been done in most of the collision systems, but the comparison is particularly interesting in heavy-ion collisions, where a large baryon-to-meson enhancement is often referred to as the “baryon anomaly”. A characteristic maximum at intermediate transverse momenta (1–5 GeV) is found in all systems, but in Pb–Pb collisions the ratio is strongly increased, to the extent that it exceeds unity, implying the production of more baryons than mesons. The rise of the ratio has been associated with either hadron formation from the recombination of two or three quarks, or the migration of the heavier baryons to higher momenta by the strong all-particle “radial” flow associated with the production and expansion of a quark–gluon plasma. 

A recent result adds an extra twist to the study of strange baryons and mesons

The ALICE collaboration has studied baryon-to-meson ratios extensively. A recent result adds an extra twist to the study of strange baryons and mesons by studying the ratios in two parts of the events separately – inside jets and in the event portion perpendicular to a jet cone. This allows physicists to look “under the peak” to reveal more about its origin. The latest study focuses on the neutral and weakly decaying Λ baryon and K⁰_S meson – particles often known collectively as V⁰ due to their decay particles forming a “V” within a detector. The ALICE detector can reconstruct these decaying particles reliably even at high momenta via invariant-mass analysis using the charged-particle tracks seen in the detectors.

The particles associated with the jets show the typical ratio known from the high momentum tail of the inclusive baryon-to-meson distribution – essentially no enhancement – and similar values were found in both pp and p–Pb collisions, consistent with simulations of hard pp collisions using PYTHIA 8 (see figure 1). By contrast, the particles found away from jets do indeed show a baryon-to-meson enhancement that qualitatively resembles the observations in Pb–Pb collisions. The new study clarifies that the high rise of the ratio is associated with the soft part of the events (regions where no jet with more than p_T = 10 GeV is produced) and brings the first quantitative guidance for modelling the baryon-to-meson enhancement with an additional important constraint – the absence of the jet. Moreover, finding that the “within-jet” ratio is similar in pp and p–Pb collisions, while the “out-of-jet” ratio shows larger values in p–Pb than in pp collisions, gives even more to ponder about the possible origin of the effect in relation to an expanding strongly interacting system. Future measurements involving multi-strange baryons may shed further light on this question. 

The post Hadron formation differs outside of jets appeared first on CERN Courier.

Neutral pion (π⁰) and eta-meson (η) production cross sections at midrapidity have recently been measured up to unprecedentedly high transverse momenta (p_T) in proton–proton (pp) and proton–lead (p–Pb) collisions at √s_NN = 8 and 8.16 TeV, respectively. The mesons were reconstructed in the two-photon decay channel for p_T from 0.5 and 1 GeV up to 200 and 50 GeV for π⁰ and η mesons, respectively. The high momentum reach for the π⁰ measurement was achieved by identifying two-photon showers reconstructed as a single energy deposit in the ALICE electromagnetic calorimeter.

In pp collisions, measurements of identified hadron spectra are used to constrain perturbative predictions from quantum chromodynamics (QCD). At large momentum transfer (Q²), one relies in these perturbative approximations of QCD (pQCD) on the factorisation of computable short-range parton scattering processes such as quark–quark, quark–gluon and gluon–gluon scatterings from long-range properties of QCD that need experimental input. These properties are modelled by parton distribution functions (PDFs), which describe the fractional-momentum (x) distributions of quarks and gluons within the proton, and fragmentation functions, which describe the fractional-momentum distribution of quarks or gluons for hadrons of a certain species.

In p–Pb collisions, nuclear effects are expected to significantly affect particle production, in particular at small parton fractional momentum x, compared to pp collisions. Modification at low p_T (~1 GeV), usually attributed to nuclear shadowing (CERN Courier March/April 2021 p19), can be parameterised by nuclear parton distribution functions (nPDFs). However, since high parton densities are reached at the LHC, the Colour Glass Condensate (CGC) framework is also applicable at low p_T (x values as small as ~5 × 10^–4), which predicts strong particle suppression due to saturation of the parton phase space in nuclei. Above momenta of about 10 GeV/c, measurements in p–Pb collisions can also be sensitive to the energy loss of the outgoing partons in nuclear matter.

The nuclear modification factor (R_pPb), shown in the lower panel of the figure, was measured as the ratio of the cross sections in p–Pb and pp collisions normalised by the atomic mass number. Below 10 GeV, R_pPb is found to be smaller than unity, while above 10 GeV it is consistent with unity. The measurement is described by calculations over the full transverse momentum range and provides further constraints to the nPDF parameterisations for lower than about 5 GeV. The direct comparison of the neutral pion cross section in pp collisions at 8 TeV, with pQCD calculations shown in the upper panel of the figure, reveals differences in the low to intermediate p_T range, which, however, cancel in R_pPb, since similar differences are also present for the p–Pb cross section. Future high-precision measurements are ongoing using the large dataset from pp collisions at 13 TeV, providing further constraints to pQCD calculations.

The post Light neutral mesons probed to high p_T appeared first on CERN Courier.

Heavy quarks are powerful probes of properties of the QGP

The experimental quest for the QGP started in the 1980s using fixed-target collisions at the Alternating Gradient Synchrotron at Brookhaven National Laboratory (BNL) and the Super Proton Synchrotron at CERN. This side of the millennium, collider experiments have provided a big jump in energy, first at the Relativistic Heavy Ion Collider (RHIC) at BNL, and now at the LHC. Both facilities allow a thorough investigation of the QGP at different points on the still-mysterious phase diagram of quantum chromodynamics.

Among the most striking features of the QGP formed at the LHC is the development of “collective” phenomena, as spatial anisotropies are transformed by pressure gradients into momentum anisotropies. The ALICE experiment is designed to study the collective behaviour of the torrent of particles created in the hadronisation of QGP droplets. Following detailed studies of the “flow” of the abundant light hadrons that are produced, ALICE has recently demonstrated, alongside certain competitive measurements by CMS and ATLAS, the flow of heavy-flavour (HF) hadrons – particles that probe the entire lifetime of a droplet of QGP.

A perfect fluid

The QGP created in lead–ion collisions at the LHC is made up of thousands of quarks and gluons – far too many quantum fields to keep track of in a simulation. In the early 2000s, however, measurements at RHIC revealed that the QGP has a simplifying property: it is a near perfect fluid, with a very low viscosity, as indicated by observations of the highest collective flows allowable in viscous hydrodynamic simulations. More precisely, its shear viscosity-to-entropy ratio – the generalisation of the non-relativistic kinematic viscosity – appears to be only a little above the conjectured quantum limit of 1/4π derived using holographic gravity (AdS/CFT) duality. As the QGP is a near-perfect fluid, its expansion can be modelled using a few local quantities such as energy density, velocity and temperature.

In noncentral heavy-ion collisions, the overlap region between the two incoming nuclei has an almond shape, which naturally imprints a spatial anisotropy to the initial state of the system: the QGP is less elongated along the symmetry plane that connects the centres of the colliding nuclei. As the system evolves, interactions push the QGP more strongly along the shorter symmetry-plane axis than along the longer one (see “Noncentral collision” figure). This is called elliptic flow.

Density fluctuations in the initial state may also lead to other anisotropic flows in the velocity field of the QGP. Triangular flow, for example, pushes the system along three axes. In general, this collective motion is decomposed as 1 + 2 ∑ v_n cos(n(ϕ–Ψ_n)), where v_n are harmonic coefficients, ϕ is the azimuthal angle of the final-state particles in transverse-momentum (p_T) space, and Ψ_n are the orientation of the symmetry planes. v₁, which is expected to be negligible at mid-rapidity, is “directed flow” towards a single maximum, while v₂ and v₃ signal elliptic and triangular flows. The LHC’s impressive luminosity has allowed ALICE to measure significant values for the flow of light-flavour hadrons up to v₉ (see “Light-flavour flow” figure).

The importance of being heavy

The bulk of the QGP is composed of thermally produced gluons and light quarks. By contrast, thermal HF production is negligible as the typical temperature of the system created in heavy-ion collisions is a few hundred MeV – significantly below the mass of a charm or beauty quark–antiquark pair. HF quarks are instead created in quark–antiquark pairs in early hard-scattering processes on shorter timescales than the QGP formation time, and experience the whole evolution of the system. 

Heavy quarks are therefore powerful probes of properties of the QGP. As they traverse the medium, they interact with its constituents, gaining or losing energy depending on their momenta. High-momentum HF quarks lose energy via both elastic (collisional) and inelastic (gluon radiation) processes. Low-momentum HF quarks are swept along with the flow of the medium, partially thermalising with it via multiple interactions. The thermalisation time is inversely proportional to the particle’s mass, and so a higher degree of thermalisation is expected for charm than for beauty. Subsequent hadronisation brings additional complexity: as colour-charged quarks arrange themselves in colour-neutral hadrons, extra contributions to their flow arise from the influence of the surrounding medium when they coalesce with nearby light quarks.

In the past two years, the ALICE collaboration has measured the elliptic and triangular flow coefficients of HF hadrons with open and hidden charm and beauty. The results are currently unique in both scope and transverse-momentum coverage, and depend on the simultaneous reconstruction of thousands of particles in the ALICE detectors (see “ALICE in action” panel). In each case, these HF flows should be compared to the flow of the abundant light-particle species such as charged pions. Within the hydrodynamic description, particles originating from the thermally expanding medium at relatively low transverse momenta typically exhibit flow coefficients that increase with transverse momentum. Faster particles also interact with the medium, but might not reach thermal equilibrium. For these particles, an azimuthal anisotropy develops due to the shorter length of medium they traverse along the symmetry plane, but it is not as large, and anisotropy coefficients are expected to fall with increasing transverse momentum. When thermal equilibrium is achieved, it imprints the same velocity field to all particles: the result is a mass hierarchy wherein heavier particles exhibit lower flow coefficients for a given transverse momentum.

ALICE in action

The geometrical overlap between the two colliding nuclei varies from head-on collisions that produce a huge number of particles, sending several thousand hadrons flying to ALICE’s detectors (“0% centrality”, as a percentile of the hadronic cross section) to peripheral collisions where the two nuclei barely overlap (“100% centrality”). Since the initial geometry is not directly experimentally accessible, centrality is estimated using either the total particle multiplicity or the energy deposited in the detectors. 

Among the cloud of particles are a handful of open and hidden heavy-flavour hadrons that are reconstructed from their decay products using tracking, particle-identification and decay-vertex reconstruction. Charm mesons are reconstructed through hadronic decay channels using the central barrel detectors. Open beauty hadrons are also reconstructed in the central barrel using their semileptonic decay to an electron as a proxy. Compelling evidence of heavy-quark energy loss in a deconfined strongly interacting matter is provided by the suppression of high-p_T open heavy-flavour hadron yields in central nucleus–nucleus collisions relative to proton–proton collisions (after scaling by the average number of binary nucleon–nucleon collisions). 

A small fraction of the initially created heavy-quark pairs will bind together to form charmonium (c–c) or bottomonium (b-b) states that are reconstructed in the forward muon spectrometer using their decay channel to two muons. Charmonium states were among the first proposed probes of the deconfinement of the QGP. The potential between the heavy quark and antiquark pair is partially screened by the high density of colour charges in the QGP, leading to a suppression of the production of charmonium states. Interestingly, however, ALICE observes less suppression of the J/ψ in lead–lead collisions than is seen at the lower collision energies of RHIC, despite the increased density of colour charges at higher collision energies. This effect may be understood as due to J/ψ regeneration as the copiously produced charm quarks and antiquarks recombine. By contrast, bottomonia are not expected to have a large regeneration contribution due to the larger mass and thus lower production cross section of the beauty quark. 

D mesons are the lightest and most abundant hadrons formed from a heavy quark, and are key to understan­ding the dynamics of charm quarks in the collision. A substantial anisotropy is observed for D mesons in non-central collisions (see “Elliptic flow” figure). As expected, the measured p_T dependence is similar to that for light particles, suggesting that D mesons are strongly affected by the surrounding medium, participating in the collective motion of the QGP and reaching a high degree of thermalisation. J/ψ mesons, which do not contain light-flavour quarks, also exhibit significant positive elliptic flow with a similar p_T shape. Open beauty hadrons, whose mass is dominated by the b quark, are also seen to flow, and in the low to intermediate p_T region, below 4 GeV, an apparent mass hierarchy is seen: the lighter the particle, the greater the elliptic flow, as expected in a hydrodynamical description of QGP evolution. Above 6 GeV, the elliptic flows of the three particles converge, perhaps as a result of energy loss as energetic partons move through the QGP. In contrast to the other particles, ϒ mesons do not show any significant elliptic flow. This is not surprising as the transverse momentum of peak elliptic flow is expected to scale with the mass of the particle according to the hydrodynamic description of the evolution of the QGP – for ϒ mesons that should be beyond 10 GeV, where the uncertainties are currently large.

Theoretical descriptions of elliptic flow are also making progress. Models of HF flow need to include a realistic hydrodynamic expansion of the QGP, the interaction of the heavy quarks with the medium via collisional and radiative processes, and the hadronisation of heavy quarks via both fragmentation and coalescence. For example, the “TAMU” model describes the measurements of the D mesons and electrons from beauty-hadron decays reasonably well, but shows some tension with the measurement of J/ψ at intermediate and high transverse momenta, perhaps indicating that a mechanism related to parton energy loss is not included. 

Triangular flow

Triangular flow is observed for D and J/ψ mesons in central collisions, demonstrating that energy-density fluctuations in the initial state have a measurable effect on the heavy quark sector (see “Triangular flow” figure). These measurements of a triangular flow of open- and hidden- charm mesons pose new challenges to models describing HF interactions in the QGP: models now need to account not only for the properties of the medium and the transport of the HF quarks through it, but also for fluctuations in the initial conditions of the heavy-ion collisions.

In the coming years, measurements of HF flow will continue to strongly constrain models of the QGP. It is now clear that charm quarks take part in the collective motion of the medium and partially thermalise. More data is needed to make firm conclusions about open and hidden beauty hadrons. All four LHC experiments will study how heavy quarks diffuse in a colour-deconfined and hydrodynamically expanding medium with the greater luminosities set to be delivered in LHC Run 3 and Run 4. Currently ongoing upgrades to ALICE will extend its unique advantages in track reconstruction at low momenta, and upgrades to LHCb will allow this asymmetric experiment to study non-central collisions in Run 3. In the next long shutdown of the LHC, upgrades to CMS and ATLAS will then extend their already impressive flow measurements to be competitive with ALICE in the crucial low transverse momentum domain, inching us closer to understanding both the early universe and the phase diagram of quantum chromodynamics. 

The post Going with the flow appeared first on CERN Courier.

The electroweak session of the Rencontres de Moriond convened more than 200 participants virtually from 22 to 27 March in a new format, with pre-recorded plenary talks and group-chat channels that went online in advance of live discussion sessions. The following week, the QCD and high-energy interactions session took place with a more conventional virtual organisation.

The highlight of both conferences was the new LHCb result on R_K based on the full Run 1 and Run 2 data, and corresponding to an integrated luminosity of 9 fb^–1, which led to the claim of the first evidence for lepton-flavour-universality (LFU) violation from a single measurement. R_K is the ratio of the branching fractions for the decays B⁺ → K⁺ μ⁺ μ^– and B⁺ → K⁺ e⁺ e^–. LHCb measured this ratio to be 3.1σ below unity, despite the fact that the two branching fractions are expected to be equal by virtue of the well-established property of lepton universality (see New data strengthens R_K flavour anomaly). Coupled with previously reported anomalies of angular variables and the R_K*, R_D and R_D* branching-fraction ratios by several experiments, it further reinforces the indications that LFU may be violated in the B sector. Global fits and possible theoretical interpretations with new particles were also discussed. 

Important contributions

Results from Belle II and BES III were reported. Some of the highlights were a first measurement of the B⁺ → K⁺ νν decay and the most stringent limits to date for masses of axions between 0.2 and 1 GeV from Belle II, based on the first data they collected, and searches for LFU violation in the charm sector from BES III that for the moment give negative results. Belle II is expected to give important contributions to the LFU studies soon and to accumulate an integrated luminosity of 50 ab^–1 10 years from now.

ATLAS and CMS presented tens of new results each on Standard Model (SM) measurements and searches for new phenomena in the two conferences. Highlights included the CMS measurement of the W leptonic and hadronic branching fraction with an accuracy larger than that measured at LEP for the branching fractions to the electron and muon, and the updated ATLAS evidence of the four-top-production process at 4.7σ (with 2.6σ expected). ATLAS and CMS have not yet found any indications of new physics but continue to perform many searches, expanding the scope to as-yet unexplored areas, and many improved limits on new-physics scenarios were reported for the first time at both conference sessions.

Several results and prospects of electroweak precision measurements were presented and discussed, including a new measurement of the fine structure constant with a precision of 80 parts per trillion, and a measurement at PSI of the null electric dipole moment of the neutron with an uncertainty of 1.1 × 10^–26 e∙cm. Theoretical predictions of (g–2)_μ were discussed, including the recent lattice calculation from the Budapest–Marseille–Wuppertal group of the hadronic–vacuum–polarisation contribution, which, if used in comparison with the experimental measurement, would bring the tension with the (g–2)_μ prediction to within 2σ.

In the neutrino session, the most relevant recent new results of last year were discussed. KATRIN reported updated upper limits on the neutrino mass, obtained from the direct measurement of the endpoint of the electron spectrum of the tritium β decay, while T2K showed the most recent results concerning CP violation in the neutrino sector, obtained from the simultaneous measurement of the ν_μ and ν_μ disappearance, and ν_e and ν_e  appearance. The measurement disfavours at 90% CL the CP-conserving values 0 and π of the CP-violating parameter of the neutrino mixing matrix, δ_CP, and all values between 0 and π.

The quest for dark matter is in full swing and is expanding on all fronts. XENON1T updated delegates on an intriguing small excess in the low-energy part of the electron-recoil spectrum, from 1 to 7 keV, which could be interpreted as originating from new particles but that is also consistent with an increased background from tritium contamination. Upcoming new data from the upgraded XENONnT detector are expected to be able to disentangle the different possibilities, should the excess be confirmed. The Axion Dark Matter eXperiment (ADMX) is by far the most sensitive experiment to detect axions in the explored range around 2 μeV. ADMX showed near-future prospects and the plans for upgrading the detector to scan a much wider mass range, up to 20 μeV, in the next few years. The search for dark matter also continues at accelerators, where it could be directly produced or be detected in the decays of SM particles such as the Higgs boson.

The quest for dark matter is in full swing and is expanding on all fronts

ATLAS and CMS also presented new results at the Moriond QCD and high-energy-interactions conference. Highlights of the new results are: the ATLAS full Run-2 search for double-Higgs-boson production in the bbγγ channel, which yielded the tightest constraints to date on the Higgs-boson self-coupling, and the measurement of the top-quark mass by CMS in the single-top-production channel that for the first time reached an accuracy of less than 1 GeV, now becoming relevant to future top-mass combinations. Several recent heavy-ion results were also presented by the LHC experiments, and by STAR and PHENIX at RHIC, in the dedicated heavy-ion session. One highlight was a result from ALICE on the measurement of the Λ_c⁺ transverse-momentum spectrum and the Λ_c⁺ /D⁰ ratio in pp and p–Pb collisions, showing discrepancies with perturbative QCD predictions.

The above is only a snapshot of the many interesting results presented at this year’s Rencontres de Moriond, representing the hard work and dedication of countless physicists, many at the early-career stage. As ever, the SM stands strong, though intriguing results provoked lively debate during many virtual discussions.

The post Anomalies intrigue at Moriond appeared first on CERN Courier.

A fermion’s spin tends to twist to align with a magnetic field – an effect that becomes dramatically macroscopic when electron spins twist together in a ferromagnet. Microscopically, the tiny magnetic moment of a fermion interacts with the external magnetic field through absorption of photons that comprise the field. Quantifying this picture, the Dirac equation predicts fermion magnetic moments to be precisely two in units of Bohr magnetons, e/2m. But virtual lines and loops add an additional 0.1% or so to this value, giving rise to an “anomalous” contribution known as “g–2” to the particle’s magnetic moment, caused by quantum fluctuations. Calculated to tenth order in quantum electrodynamics (QED), and verified experimentally to about two parts in 10¹⁰, the electron’s magnetic moment is one of the most precisely known numbers in the physical sciences. While also measured precisely, the magnetic moment of the muon, however, is in tension with the Standard Model.

Tricky comparison

The anomalous magnetic moment of the muon was first measured at CERN in 1959, and prior to 2021, was most recently measured by the E821 experiment at Brookhaven National Laboratory (BNL) 16 years ago. The comparison between theory and data is much trickier than for electrons. Being short-lived, muons are less suited to experiments with Penning traps, whereby stable charged particles are confined using static electric and magnetic fields, and the trapped particles are then cooled to allow precise measurements of their properties. Instead, experiments infer how quickly muon spins precess in a storage ring – a situation similar to the wobbling of a spinning top, where information on the muon’s advancing spin is encoded in the direction of the electron that is emitted when it decays. Theoretical calculations are also more challenging, as hadronic contributions are no longer so heavily suppressed when they emerge as virtual particles from the more massive muon.

All told, our knowledge of the anomalous magnetic moment of the muon is currently three orders of magnitude less precise than for electrons. And while everything tallies up, more or less, for the electron, BNL’s longstanding measurement of the magnetic moment of the muon is 3.7σ greater than the Standard Model prediction (see panel “Rising to the moment”). The possibility that the discrepancy could be due to virtual contributions from as-yet-undiscovered particles demands ever more precise theoretical calculations. This need is now more pressing than ever, given the increased precision of the experimental value expected in the next few years from the Muon g–2 collaboration at Fermilab in the US and other experiments such as the Muon g–2/EDM collaboration at J-PARC in Japan. Hotly anticipated results from the first data run at Fermilab’s E989 experiment were released on 7 April. The new result is completely consistent with the BNL value but with a slightly smaller error, leading to a slightly larger discrepancy of 4.2σ with the Standard Model when the measurements are combined (see Fermilab strengthens muon g-2 anomaly).

Hadronic vacuum polarisation

The value of the muon anomaly, a_μ, is an important test of the Standard Model because currently it is known very precisely – to roughly 0.5 parts per million (ppm) – in both experiment and theory. QED dominates the value of a_μ, but due to the non-perturbative nature of QCD it is strong interactions that contribute most to the error. The theoretical uncertainty on the anomalous magnetic moment of the muon is currently dominated by so-called hadronic vacuum polarisation (HVP) diagrams. In HVP, a virtual photon briefly explodes into a “hadronic blob”, before being reabsorbed, while the magnetic-field photon is simultaneously absorbed by the muon. While of order α² in QED, it is all orders in QCD, making for very difficult calculations.

Rising to the moment

In the Standard Model, the magnetic moment of the muon is computed order-by-order in powers of a for QED (each virtual photon represents a factor of α), and to all orders in as for QCD.

At the lowest order in QED, the Dirac term (pictured left) accounts for precisely two Bohr magnetons and arises purely from the muon (μ) and the real external photon (γ) representing the magnetic field.

 

At higher orders in QED, virtual Standard Model particles, depicted by lines forming loops, contribute to a fractional increase of aμ with respect to that value: the so-called anomalous magnetic moment of the muon. It is defined to be a_μ = (g–2)/2, where g is the gyromagnetic ratio of the muon – the number of Bohr magnetons, e/2m, which make up the muon’s magnetic moment. According to the Dirac equation, g = 2, but radiative corrections increase its value.

The biggest contribution is from the Schwinger term (pictured left, O(α)) and higher-order QED diagrams.

 

a_μ^QED = (116 584 718.931 ± 0.104) × 10^–11

Electroweak lines (pictured left) also make a well-defined contribution. These diagrams are suppressed by the heavy masses of the Higgs, W and Z bosons.

a_μ^EW = (153.6 ± 1.0) × 10–11

The biggest QCD contribution is due to hadronic vacuum polarisation (HVP) diagrams. These are computed from leading order (pictured left, O(α²)), with one “hadronic blob” at all orders in as (shaded) up to next-to-next-to-leading order (NNLO, O(α⁴), with three hadronic blobs) in the HVP.

 

 

Hadronic light-by-light scattering (HLbL, pictured left at O(α³) and all orders in α_s (shaded)), makes a smaller contribution but with a larger fractional uncertainty.

 

 

 

Neglecting lattice–QCD calculations for the HVP in favour of those based on e⁺e^– data and phenomenology, the total anomalous magnetic moment is given by

a_μ^SM = a_μ^QED + a_μ^EW + a_μ^HVP + a_μ^HLbL = (116 591 810 ± 43) × 10^–11.

This is somewhat below the combined value from the E821 experiment at BNL in 2004 and the E989 experiment at Fermilab in 2021.

a_μ^exp = (116 592 061 ± 41) × 10^–11

The discrepancy has roughly 4.2σ significance:

a_μ^exp– a_μ^SM = (251 ± 59) × 10^–11.

Historically, and into the present, HVP is calculated using a dispersion relation and experimental data for the cross section for e⁺e^– → hadrons. This idea was born of necessity almost 60 years ago, before QCD was even on the scene, let alone calculable. The key realisation is that the imaginary part of the vacuum polarisation is directly related to the hadronic cross section via the optical theorem of wave-scattering theory; a dispersion relation then relates the imaginary part to the real part. The cross section is determined over a relatively wide range of energies, in both exclusive and inclusive channels. The dominant contribution – about three quarters – comes from the e⁺e^– → π⁺π^– channel, which peaks at the rho meson mass, 775 MeV. Though the integral converges rapidly with increasing energy, data are needed over a relatively broad region to obtain the necessary precision. Above the τ mass, QCD perturbation theory hones the calculation.

Several groups have computed the HVP contribution in this way, and recently a consensus value has been produced as part of the worldwide Muon g–2 Theory Initiative. The error stands at about 0.58% and is the dominant part of the theory error. It is worth noting that a significant part of the error arises from a tension between the most precise measurements, by the BaBar and KLOE experiments, around the rho–meson peak. New measurements, including those from experiments at Novosibirsk, Russia and Japan’s Belle II experiment, may help resolve the inconsistency in the current data and reduce the error by a factor of two or so. 

The alternative approach, of calculating the HVP contribution from first principles using lattice QCD, is not yet at the same level of precision, but is getting there. Consistency between the two approaches will be crucial for any claim of new physics.

Lattice QCD

Kenneth Wilson formulated lattice gauge theory in 1974 as a means to rid quantum field theories of their notorious infinities – a process known as regulating the theory – while maintaining exact gauge invariance, but without using perturbation theory. Lattice QCD calculations involve the very large dimensional integration of path integrals in QCD. Because of confinement, a perturbative treatment including physical hadronic states is not possible, so the complete integral, regulated properly in a discrete, finite volume, is done numerically by Monte Carlo integration.

Lattice QCD has made significant improvements over the last several years, both in methodology and invested computing time. Recently developed methods (which rely on low-lying eigenmodes of the Dirac operator to speed up calculations) have been especially important for muon–anomaly calculations. By allowing state-of-the-art calculations using physical masses, they remove a significant systematic: the so-called chiral extrapolation for the light quarks. The remaining systematic errors arise from the finite volume and non-zero lattice spacing employed in the simulations. These are handled by doing multiple simulations and extrapolating to the infinite-volume and zero-lattice-spacing limits. 

The HVP contribution can readily be computed using lattice QCD in Euclidean space with space-like four-momenta in the photon loop, thus yielding the real part of the HVP directly. The dispersive result is currently more precise (see “Off the mark” figure”), but further improvements will depend on consistent new e⁺e^– scattering datasets.

Rapid progress in the last few years has resulted in first lattice results with sub-percent uncertainty, closing in on the precision of the dispersive approach. Since these lattice calculations are very involved and still maturing, it will be crucial to monitor the emerging picture once several precise results with different systematic approaches are available. It will be particularly important to aim for statistics-dominated errors to make it more straightforward to quantitatively interpret the resulting agreement with the no-new-physics scenario or the dispersive results. In the shorter term, it will also be crucial to cross-check between different lattice and dispersive results using additional observables, for example based on the vector–vector correlators.

With improved lattice calculations in the pipeline from a number of groups, the tension between lattice QCD and phenomenological calculations may well be resolved before the Fermilab and J-PARC experiments announce their final results. Interestingly, there is a new lattice result with sub-percent precision (BMW 2020) that is in agreement both with the no-new-physics point within 1.3σ, and with the dispersive-data-driven result within 2.1σ. Barring a significant re-evaluation of the phenomenological calculation, however, HVP does not appear to be the source of the discrepancy with experiments. 

The next most likely Standard Model process to explain the muon anomaly is hadronic light-by-light scattering. Though it occurs less frequently since it includes an extra virtual photon compared to the HVP contribution, it is much less well known, with comparable uncertainties to HVP.

Hadronic light-by-light scattering

In hadronic light-by-light scattering (HLbL), the magnetic field interacts not with the muon, but with a hadronic “blob”, which is connected to the muon by three virtual photons. (The interaction of the four photons via the hadronic blob gives HLbL its name.) A miscalculation of the HLbL contribution has often been proposed as the source of the apparently anomalous measurement of the muon anomaly by BNL’s E821 collaboration.

Since the so-called Glasgow consensus (the fruit of a 2009 workshop) first established a value more than 10 years ago, significant progress has been made on the analytic computation of the HLbL scattering contribution. In particular, a dispersive analysis of the most important hadronic channels has been carried out, including the leading pion–pole, sub-leading pion loop and rescattering diagrams including heavier pseudoscalars. These calculations are analogous in spirit to the dispersive HVP calculations, but are more complicated, and the experimental measurements are more difficult because form factors with one or two virtual photons are required. 

The project to calculate the HLbL contribution using lattice QCD began more than 10 years ago, and many improvements to the method have been made to reduce both statistical and systematic errors since then. Last year we published, with colleagues Norman Christ, Taku Izubuchi and Masashi Hayakawa, the first ever lattice–QCD calculation of the HLbL contribution with all errors controlled, finding a_μ^{HLbL, lattice} = (78.7 ± 30.6 (stat) ± 17.7 (sys)) × 10^–11. The calculation was not easy: it took four years and a billion core-hours on the Mira supercomputer at Argonne National Laboratory’s Large Computing Facility. 

Our lattice HLbL calculations are quite consistent with the analytic and data-driven result, which is approximately a factor of two more precise. Combining the results leads to a_μ^HLbL = (90 ± 17) × 10^–11, which means the very difficult HLbL contribution cannot explain the Standard Model discrepancy with experiment. To make such a strong conclusion, however, it is necessary to have consistent results from at least two completely different methods of calculating this challenging non-perturbative quantity. 

New physics?

If current theory calculations of the muon anomaly hold up, and the new experiments reduce its uncertainty by the hoped-for factor of four, then a new-physics explanation will become impossible to ignore. The idea would be to add particles and interactions that have not yet been observed but may soon be discovered at the LHC or in future experiments. New particles would be expected to contribute to the anomaly through Feynman diagrams similar to the Standard Model topographies (see “Rising to the moment” panel).

Calculations of the anomalous magnetic moment of the muon are not finished

The most commonly considered new-physics explanation is supersymmetry, but the increasingly stringent lower limits placed on the masses of super-partners by the LHC experiments make it increasingly difficult to explain the muon anomaly. Other theories could do the job too. One popular idea that could also explain persistent anomalies in the b-quark sector is heavy scalar leptoquarks, which mediate a new interaction allowing leptons and quarks to change into each other. Another option involves scenarios whereby the Standard Model Higgs boson is accompanied by a heavier Higgs-like boson.

The calculations of the anomalous magnetic moment of the muon are not finished. As a systematically improvable method, we expect more precise lattice determinations of the hadronic contributions in the near future. Increasingly powerful algorithms and hardware resources will further improve precision on the lattice side, and new experimental measurements and analysis methods will do the same for dispersive studies of the HVP and HLbL contributions.

To confidently discover new physics requires that these two independent approaches to the Standard Model value agree. With the first new results on the experimental value of the muon anomaly in almost two decades showing perfect agreement with the old value, we anxiously await more precise measurements in the near future. Our hope is that the clash of theory and experiment will be the beginning of an exciting new chapter of particle physics, heralding new discoveries at current and future particle colliders. 

The post An anomalous moment for the muon appeared first on CERN Courier.

This result probes the deepest features of quantum chromodynamics

Simone Giani

“This result probes the deepest features of quantum chromodynamics, notably that gluons interact between themselves and that an odd number of gluons are able to be ‘colourless’, thus shielding the strong interaction,” says TOTEM spokesperson Simone Giani of CERN. “A notable feature of this work is that the results are produced by joining the LHC and Tevatron data at different energies.”

States comprising two, three or more gluons are usually called “glueballs”, and are peculiar objects made only of the carriers of the strong force. The advent of quantum chromodynamics (QCD) led theorists to predict the existence of the odderon in 1973. Proving its existence has been a major experimental challenge, however, requiring detailed measurements of protons as they glance off one another in high-energy collisions.

While most high-energy collisions cause protons to break into their constituent quarks and gluons, roughly 25% are elastic collisions where the protons remain intact but emerge on slightly different paths (deviating by around a millimetre over a distance of 200 m at the LHC). TOTEM measures these small deviations in proton–proton (pp) scattering using two detectors located 220 m on either side of the CMS experiment, while DØ employed a similar setup at the Tevatron proton–antiproton (pp̄) collider.

Pomerons and odderons

At low energies, differences in pp vs pp̄ scattering are due to the exchange of different virtual mesons. At multi-TeV energies, on the other hand, proton interactions are expected to be mediated purely by gluons. In particular, elastic scattering at low-momentum transfer and high energies has long been explained by the exchange of a pomeron – a colour-neutral virtual glueball made up of an even number of gluons.

However, in 2018 TOTEM reported measurements at high energies that could not easily be explained by this traditional picture. Instead, a further QCD object seemed to be at play, supporting models in which a three-gluon compound, or one containing higher odd numbers of gluons, was being exchanged. The discrepancy came to light via measurements of a parameter called ρ, which represents the ratio of the real and imaginary parts of the forward elastic-scattering amplitude when there is minimal gluon exchange between the colliding protons and thus almost no deviation in their trajectories. The results were sufficient to claim evidence for the odderon, although not yet its definitive observation.

The new work is based on a model-independent analysis of data at medium-range momenta transfer. The TOTEM and DØ teams compared LHC pp data (recorded at collision energies of 2.76, 7, 8 and 13 TeV and extrapolated to 1.96 TeV), with Tevatron pp̄ data measured at 1.96 TeV. The odderon would be expected to contribute with different signs to pp and pp̄ scattering. Supporting this picture, the two data sets disagree at the 3.4σ level, providing evidence for the t-channel exchange of a colourless, C-odd gluonic compound.

“When combined with the ρ and total cross-section result at 13 TeV, the significance is in the range 5.2-5.7σ and thus constitutes the first experimental observation of the odderon,” said Christophe Royon of University of Kansas, who presented the results on behalf of DØ and TOTEM last week. “This is a major discovery by CERN/Fermilab.”

In addition to the new TOTEM-DØ model-independent study, several theoretical papers based on data from the ISR, SPS, Tevatron and LHC, and model-dependent inputs, provide additional evidence supporting the conclusion that the odderon exists.

The post Odderon discovered appeared first on CERN Courier.

The intensity of the electromagnetic field, and the corresponding photon flux, is proportional to the square of the electric charge. This type of interaction is therefore greatly enhanced in the collisions of lead ions (Z = 82). Ultra-peripheral collisions (UPCs), in which the impact parameter is larger than the sum of the radii of two Pb nuclei, are a particularly useful way to study photonuclear collisions. Here, purely hadronic interactions are suppressed, due to the short range of the strong force, and photonuclear interactions dominate. The photoproduction of vector mesons in these reactions has a clean experimental signature: the decay products of the vector meson are the only signals in an otherwise empty detector.

Nuclear shadowing was first observed by the European Muon Collaboration at CERN in 1982

Coherent heavy-vector–meson photoproduction, wherein the photon interacts consistently with all the nucleons in a nucleus, is of particular interest because of its connection with gluon distribution functions (PDFs) in protons and nuclei. At low Bjorken-x values, gluon PDFs are significantly suppressed in the nucleus relative to free proton PDFs – a phenomenon known as nuclear shadowing that was first observed by the European Muon Collaboration at CERN in 1982 by comparing the structure functions of iron and deuterium in the deep inelastic scattering of muons.

Heavy-vector–meson photoproduction measurements provide a powerful tool to study poorly known gluon-shadowing effects at low x. The scale of the four-momentum transfer of the interaction corresponds to the perturbative regime of QCD in the case of heavy charmonium states. The gluon shadowing factor – the ratio of the nuclear PDF to the proton PDF – can be evaluated by measuring the nuclear suppression factor, defined to be the square root of the ratio of the coherent vector–meson photonuclear production cross section on nuclei to the photonuclear cross-section in the impulse approximation that is based on the exclusive photoproduction measurements with a proton target.

Ultra-peripheral collisions

The ALICE collaboration recently submitted for publication the measurement of the coherent photoproduction of J/ψ and ψ′ at midrapidity |y| < 0.8 in Pb–Pb UPCs at 5.02 TeV. The J/ψ is reconstructed using the dilepton (ℓ⁺ℓ^–) and proton–antiproton decay channels, while for the ψ′, the dilepton and the ℓ⁺ℓ^– π⁺π^– decay channels are studied. These data complement the ALICE measurement of the coherent J/ψ cross-section at forward rapidity, –4 < y < –2.5, providing stringent constraints on nuclear gluon shadowing.

The nuclear gluon shadowing factor of about 0.65 at Bjorken-x between 0.3 × 10^–3 and 1.4 × 10^–3 is estimated from the comparison of the measured coherent J/ψ cross-section with the impulse approximation at midrapidity, which implies moderate nuclear shadowing. The measured rapidity dependence of the coherent cross-section is not completely reproduced by models in the full rapidity range. The leading twist approximation of the Glauber–Gribov shadowing (LTA-GKZ) and the energy-dependent hot-spot model (GG-HS (CCK)) gives the best overall description of the rapidity dependence but shows tension with data at semi-forward rapidities 2.5 < |y| < 3.5 (figure 1). The data might be better explained with a model where shadowing has a smaller effect at Bjorken x ~ 10^–2 or x ~ 5 ∙ 10^–5, corresponding to this rapidity range.

The ratio of the ψ′ to J/ψ cross-sections at midrapidity is consistent with the ratio of photoproduction cross sections measured by the H1 and LHCb collaborations, with the leading twist approximation predictions for Pb–Pb UPCs as well as with the ALICE measurement at forward rapidities. This leads to the conclusion that shadowing effects are similar for 2S (ψ′) and 1S (J/ψ) states.

In LHC Run 3 and 4, ALICE expects to collect a 10-times-larger data sample than in Run 2, taking data in a continuous mode, and thus with higher efficiency. UPC physics will profit from this by large integrated luminosity as well as lower systematic uncertainty connected to the measurement and will be able to provide the shadowing factor differentially in wide Bjorken-x intervals.

The post ALICE shines light inside lead nuclei appeared first on CERN Courier.

The new exotic states were observed in an almost pure sample of 24 thousand B⁺→J/ψφK⁺ decays, which, as a three-body decay, may be visualised using a Dalitz plot (see “Mountain ridges” figure). Horizontal and vertical bands indicate the temporary production of tetraquark resonances which subsequently decay to a J/ψ meson and a K⁺ meson or a J/ψ meson and a φ meson, respectively. The most prominent vertical bands correspond to the cc̄ss̄ tetraquarks X(4140), X(4274), X(4500) and X(4700) which were first observed in June 2016. The collaboration has now resolved two new horizontal bands corresponding to the cc̄us̄ states Z_cs(4000)⁺ and Z_cs(4220)⁺, and two additional vertical bands corresponding to the cc̄ss̄ states X(4685) and X(4630).

These states may have very different inner structures

Liming Zhang

The results have already triggered theoretical head scratching. In November, the BESIII collaboration at the Beijing Electron–Positron Collider II reported the discovery of the first candidate for a charged hidden-charm tetraquark with strangeness, tentatively dubbed Z_cs(3985)^– (CERN Courier January/February 2021 p12). It is unclear whether the new Z_cs(4000)⁺ tetraquark can be identified with this state, say physicists. Though their masses are consistent, the width of the BESIII particle is ten times smaller. “These states may have very different inner structures,” says lead analyst Liming Zhang of the LHCb collaboration. “The one seen by BESIII is a narrow and longer-lived particle, and is easier to understand with a nuclear-like hadronic molecular picture, where two hadrons interact via a residual strong force. The one we observed is much broader, which would make it more natural to interpret as a compact multiquark candidate.”

59 hadrons

The new observations take the tally of new hadronic states discovered at the LHC – which includes several pentaquarks as well as rare and excited mesons and baryons – to 59 (see “Diagram of discovery” figure). Though quantum chromodynamics naturally allows the existence of states beyond conventional two- and three-quark mesons and baryons, the detailed mechanisms responsible for binding multi-quark states are still largely mysterious. Tetraquarks, for example, could be tightly bound pairs of diquarks or loosely bound meson-meson molecules – or even both, depending on the production process.

Who would have guessed we’d find so many exotic hadrons?

Patrick Koppenburg

“Who would have guessed we’d find so many exotic hadrons?” says former LHCb physics coordinator Patrick Koppenburg, who put the plot together. “I hope that they bring us to a better modelling of the strong interaction, which is very much needed to understand, for instance, the anomalies we see in B-meson decays.”

The post LHCb observes four new tetraquarks appeared first on CERN Courier.

The XXXII international workshop of the Logunov Institute for High-Energy Physics of the NRC Kurchatov Institute in Protvino, near Moscow, brought more than 300 physicists together online from 9 to 13 November to discuss “hot problems in hot and cold quark matter”. The focus of the workshop was chiral theories and lattice simulations, which allow estimates beyond perturbation theory for studying the strongly coupled quark–gluon plasma (sQGP) – the hot and/or dense plasma of quarks and gluons that is created in heavy-ion collisions, and which may exist inside neutron stars.

Participants considered the QCD phase diagram (pictured) as a function of temperature, magnetic field (B), baryon and isospin chemical potentials (μ_B and μ_I), and varying quark masses. The crossover line (yellow strip), which marks a transition between hadronic matter and sQGP, has long attracted great interest. Vladimir Skokov (Brookhaven) employed recent progress in the Lee–Yang approach to phase transitions to derive from first principles that μ_B > 400 MeV at the critical end point (a possible termination of the first-order phase-transition boundary). Discussions of the phase diagram also included a decrease in the pseudocritical temperature with B, the possibility of a first-order phase transition at μ_B = 0 as B tends to infinity, the existence and location of a superconducting phase, the disagreement between measured and predicted collective flows of direct photons in heavy-ion collisions, and the diamagnetic and paramagnetic natures of the pion gas and deconfined matter, respectively. Evgeny Zabrodin (Oslo) explained that the rotating fireballs of strongly interacting matter that are produced in heavy-ion collisions are not only superfluids but also supervortical liquids.

Gravitational-wave astrophysics

Impressive work was also shared at the intersection of heavy-ion collisions and gravitational-wave astrophysics on the subject of the equation of state (EoS) of neutron-star cores. The EoS is the relationship between pressure and density, and can indicate whether hadronic or quark matter is inside. Theoretical bounds on the EoS come from chiral effective theories, perturbative QCD, and the bound on the speed of sound c_s < 1/3. The quantities that can be extracted from experimental data are the mass–radius relation and the relationship between the tidal deformabilities of merging neutron stars and the peak frequency of the emitted gravitational waves. Several speakers observed that tidal deformabilities, which are measured in the inspiral phase, and the peak gravitational- wave frequency, which is measured in the post-merger phase, may together reveal the state of a neutron-star interior. Mergers observed since 2017 may already be able to shed light on the existence of a deconfined phase inside these ultra-compact objects.

Mariana Araújo offered a solution to the longstanding quarkonium polarisation puzzle

The Protvino workshop also revealed the enduring importance of studying heavy-quark physics. Since heavy quarks can be considered as approximately statically coloured sources, studies of quarkonia production are a step towards understanding hadron formation and the confinement mechanism. Peter Petreczky (Brookhaven) concluded from a lattice study of Bethe–Salpeter amplitudes that the potential model fails to describe bottomonium in terms of screened potential at high temperatures, with further investigations clearly needed in this field. Carlos Lourenço (CERN) showed that the lowering of quarkonia binding energies in the sQGP leads to nontrivial measured suppression patterns. Eric Braaten (Ohio) showed that the decrease with multiplicity of the ratio of the prompt production rates of X(3872) and Ψ(2S) in proton–proton collisions can be explained by the scattering of co-moving pions off X(3872) if it is a weakly bound charm-meson molecule. With equally impressively scrupulousness, Mariana Araújo (Innsbruck) offered a solution to the longstanding “quarkonium polarisation puzzle” by making use of a model-independent fitting procedure and taking into account correlations between cross sections and polarisations.

The next “hot problems” workshop will be held in November.

The post Quark-matter fireballs hashed out in Protvino appeared first on CERN Courier.

The ability to accelerate charged particles using the “wakefields” of plasma density waves offers the promise of high-energy particle accelerators that are more compact than those based on radio-frequency cavities. Proposed in 1979, the idea is to create a wave inside a plasma upon which electrons can “surf” and gain energy over short distances. Although highly complex, wakefield acceleration (WFA) driven by laser pulses or electron beams has been successfully used to accelerate electron beams to tens of GeV within distances of less than a metre, and the AWAKE experiment at CERN is attempting to achieve higher energy gains by using protons as drive beams. Recent studies suggest that WFA may also occur naturally, potentially offering an explanation for some of the highest energy cosmic rays ever observed.

So-called Fermi acceleration, first conceived by the eponymous Italian in 1949, is considered to be the main mechanism responsible for high-energy cosmic rays. In this process, charged particles are accelerated due to relativistic shockwaves occurring within jets emitted by black-hole binaries, active galactic nuclei or gamma-ray bursts, to name just a few sources. As a charged particle travels within the jet it gets accelerated each time it passes through the shock wave, allowing it to gain energy until the magnetic field in the environment can no longer contain it. This process predicts the observed power-law spectrum of cosmic rays quite well, at least up to energies of around 10¹⁹ eV. Beyond this energy, however, Fermi acceleration becomes less efficient as the particles start to lose energy due to collisions and/or synchrotron radiation. The existence of ultra-high-energy cosmic rays (UHECRs), which have been observed up to energies of 10²¹ eV, indicates that a different acceleration mechanism could be at play in that energy domain. Thanks to its very high efficiency, WFA could provide such a mechanism.

Although there are clearly no laser beams in astrophysical objects, plasma fields that can support waves can be found in many astrophysical settings. For example, in theories developed by Toshiki Tajima of the University of California at Irvine (UCI), one of the inventors of WFA technology, waves could be produced by instabilities in the accretion disks around compact objects such as black holes. These accretion disks can periodically transition from a highly magnetised to a little magnetised state, emitting electromagnetic waves that can propagate into the disk’s jets in the form of Alfven waves. As these waves continue to propagate along the jets they transform back into electromagnetic waves that can accelerate electrons on the front of the plasma’s “bow wake” and protons on the back of it.

Clear predictions

The energies that are theoretically achievable in cosmic-ray WFA depend on the mass of the compact object, as do the periodicities with which such waves can be produced. This allows clear predictions to be made for a range of different objects, which can be tested against observational data.

Groups based at UCI and at RIKEN in Japan recently tested these predictions on a range of astrophysical objects, spanning from 1 to 10⁹ solar masses. Although not conclusive, these first comparisons between theory and observations indicate several interesting features that require further investigation. For example, WFA models predict periodic emission of both UHECRs – the protons on the back of the bow wake – in coincidence with electromagnetic radiation produced by the electrons from the front of the bow wake. Due to interactions with the intergalactic medium, UHECRs are also expected to produce secondary particles, including neutrinos. WFA could thereby also explain periodic outbursts of neutrinos in coincidence with gamma-rays from, for example, blazars, for which evidence was recently found by the IceCube experiment in collaboration with a range of electromagnetic instruments. Additionally, WFA could explain the non-uniformity of the UHECR sky such as that recently reported by the Pierre Auger Observatory (see CERN Courier December 2017 p15), as it allows for cosmic rays with energies up to 10²⁴ eV to be produced within objects that lie within the location of the observed hot-spot.

In concert with future space-based UHECR detectors such as JEM-EUSO and POEMMA, further analysis of existing data should definitively answer the question of whether WFA does indeed occur in space. The clear predictions relating to periodicity, and the coincident emission of neutrinos, gamma-rays and other electromagnetic radiation, make it an ideal subject to study within the multi-messenger frameworks that are currently being set up.

The post Cosmic plasma-wakefield acceleration appeared first on CERN Courier.

Charm and beauty quarks are excellent probes of the hot and dense state of deconfined quarks and gluons (quark–gluon plasma, QGP) which is created in high-energy heavy-ion collisions. These heavy quarks are produced in hard-scattering processes at the early stages of the collisions, and interact with the constituents of the newly created QGP through both elastic and inelastic processes. These quarks, which can be studied through their decays into leptons, lose energy while propagating through the QGP medium. Consequently, different production yields are observed at large momenta in nucleus–nucleus collisions compared to proton–proton collisions. This effect can be quantified using the nuclear modification factor, R_AA, which is the ratio of nucleus–nucleus and proton–proton particle yields, scaled by the average number of binary nucleon–nucleon collisions. Comparing measurements in different collision systems sheds light on heavy-quark energy-loss mechanisms, and provides high-precision tomography of the QGP.

The results show that collision geometry plays an important role in heavy-quark energy loss

A new analysis by the ALICE collaboration compares the production of leptons from heavy-flavour hadron decays in Pb–Pb and Xe–Xe collisions at √s_NN = 5.02 and 5.44 TeV, respectively. The measurements use the muon and electron decay channels at forward rapidity and mid-rapidity. The results show that collision geometry plays an important role in heavy-quark energy loss.

Remarkable agreement

A remarkable agreement is observed between the muon yields in head-on Xe–Xe collisions and slightly offset Pb–Pb collisions (figure 1, left). Given the larger size of the lead nucleus, these collision centrality classes – 0–10% and 10–20%, respectively – give rise to similar charged-particle multiplicities, and thus suggest the creation of similar QGP densities and sizes in the colliding systems.

In both cases, the production of muons from heavy-flavour hadron decays is suppressed up to a factor of about 2.5 for 5 GeV < p_T < 6 GeV. This suppression is successfully reproduced by the MC@sHQ+EPOS2 model, which considers both elastic and inelastic energy-loss processes of the heavy quarks in the QGP, but is underestimated by the PHSD model, which only includes elastic processes. The analysis also saw ALICE’s first sensitivity down to p_T = 0.2 GeV using a lower magnetic field (0.2 T) in the solenoid magnet (figure 1, right). The suppression pattern for muons and electrons from heavy-flavour hadron decays is similar at both forward and mid-rapidity, indicating that heavy quarks strongly interact with the medium over a wide rapidity interval. The suppression is smaller in these “glancing” semi-central collisions than in the previously discussed head-on collisions. This is compatible with the hypothesis that the in-medium energy loss depends on the energy density and on the size of the system created in the collision.

The precision of the measurements brings new insights into the nature of parton energy loss and new constraints to the modelling of its dependence on the size of the QGP medium in transport-model calculations. Further constraints will be set by future higher precision measurements during Run 3, when ALICE will measure leptons from charm and beauty decays separately, at both central and forward rapidity. A short run with the much smaller oxygen–oxygen system may also be scheduled and contribute to a deeper understanding of the dependence of system size on in-medium energy loss for heavy quarks.

The post Heavy flavours probe QGP geometry appeared first on CERN Courier.

After seven years of construction at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, the Booster synchrotron at the brand-new NICA (Nuclotron-Based Ion Collider Facility) Complex has accelerated its first beam. On 19 December helium ions were injected into the synchrotron and a stable circulation of the beam was obtained at an energy of 3.2 MeV. The milestone marks an important step in establishing the NICA facility, which is estimated to be completed by 2022.

At this energy, ordinary matter and the quark-gluon plasma coexist in a mixed phase

The NICA accelerator complex will allow studies of the properties of nuclear matter in the region of maximum baryonic density. By colliding heavy gold ions at energies corresponding to the deconfinement phase transition (4.5 GeV), NICA will access the transition of the quark-gluon plasma (QGP) into hadrons. At this energy, ordinary matter and the QGP are able to exist in a so-called mixed phase – complementing studies at higher energy colliders such as the LHC.

The NICA booster is a 211 m circumference superconducting synchrotron which will accelerate beams to 500 MeV. It uses 2.2 m-long dipole and quadrupole magnets made up of a window frame iron yoke and a winding made of a hollow niobium-titanium superconducting cable cooled with a two-phase helium flow. Beams will then be transported to a separate ring surrounding the booster, the “nuclotron”, and accelerated to the GeV range. The nuclotron was originally built between 1987 and 1992 as part of the Dubna “syncrophasotron modernisation” programme, and was Europe’s first superconducting accelerator of heavy ions to high energies. Finally, beams will be injected into two identical 503 m storage rings, which will collide the beams at two detectors: the Multi-Purpose Detector (MPD) and the Spin-Physics Detector (SPD). The MPD facility is designed to study dense baryonic matter, while SPD will study collisions between polarised beams of protons and deuterons.

The complex is one of six Russian “megascience” facilities that are part of the CREMLIN project, which aims to use large-scale science facilities to improve and strengthen relations and networks between European and Russian research infrastructures. The CREMLIN consortium comprises 19 European and Russian research infrastructures, including CERN, and DESY. Other “megascience” facilities included in this project are the Super-Charm-Tau Factory at the Budker Institute of Nuclear Physics, and the Special-purpose Synchrotron-Radiation Source (SSRS-4) at the NRC Kurchatov Institute.

“This is a historical moment for our Laboratory and a great milestone in realization of our flagship megascience project – we have to thank the grant programme CREMLIN helping us in these challenges,” says Vladimir Kekelidze, the NICA project leader. “The final step before the physical launch of the Booster will be the adjustment of the beam acceleration mode, which will then allow focus to switch to the construction of the beam transport systems from the Booster to the Nuclotron.”

The post NICA booster achieves first beam appeared first on CERN Courier.

The nuclear physics division of the European Physical Society today awarded the 2020 Lise Meitner Prize to three physicists who have played a decisive role in turning a small-scale nuclear-physics experiment at CERN into a world-leading facility for the investigation of nuclear structure.

Klaus Blaum (Max Planck Institute for Nuclear Physics), Björn Jonson (Chalmers University of Technology) and Piet Van Duppen (KU Leuven) are recognised for the development and application of online instrumentation and techniques, and for the precise and systematic investigation of properties of nuclei far from stability at CERN’s Isotope mass Separator On-Line facility (ISOLDE).

Blaum has made key contributions to the high-precision determination of nuclear ground state properties with laser and mass spectroscopic methods and to the development of new techniques in this field, while Jonson was acknowledged for his studies of the lightest exotic nuclei, namely halo nuclei, where he was the first to explain its surprisingly large matter radius. Van Duppen was recognised for his push in the production and investigation of post-accelerated radioactive beams with REX-ISOLDE. Since the 1960s, the ISOLDE user facility has produced extreme nuclear systems to help physicists understand how the strong interaction binds the ingredients of atomic nuclei, with advanced traps and lasers recently offering new ways to look for physics beyond the Standard Model.

I’m very impressed by the breadth of the recent prize winners

Eckhard Elsen

The biennial Lise Meitner prize, named after one of the pioneers in the discovery of nuclear fission in 1939, was established in 2000 to acknowledge outstanding work in the fields of experimental, theoretical or applied nuclear science. Former winners include a quartet of physicists (Johanna Stachel, Peter Braun-Munzinger, Paolo Giubellino and Jürgen Schukraft) from the ALICE collaboration in 2014, for the experimental exploration of the quark-gluon plasma using ultra-relativistic nucleus-nucleus collisions, and for the design and construction of the ALICE detector.

This year’s awards were officially presented during the 2020 ISOLDE workshop and users meeting held online on 26-27 November. “I’m very impressed by the breadth of the recent prize winners….covering a range of topics and varying between individuals and teams,” said CERN director for research and computing Eckhard Elsen during the award ceremony. “It is a good indicator of the health and the push of the field – it is truly alive.”

The post Nuclear win for ISOLDE physicists appeared first on CERN Courier.

Nuclear physics is as wide-ranging and relevant today as ever before in the century-long history of the subject. Researchers study exotic systems from hydrogen-7 to the heaviest nuclides at the boundaries of the nuclear landscape. By constraining the nuclear equation of state using heavy-ion collisions, they peer inside stars in controlled laboratory tests. By studying weak nuclear processes such as beta decays, they can even probe the Standard Model of particle physics. And this is not to mention numerous applications in accelerator-based atomic and condensed-matter physics, radiobiology and industry. These nuclear-physics research areas are just a selection of the diverse work done at the Grand Accélérateur National d’Ions Lourds (GANIL), in Caen, France.

GANIL has been operating since 1983, initially using four cyclotrons, with a fifth Cyclotron pour Ions de Moyenne Energie (CIME) added in 2001. The latter is used to reaccelerate short-lived nuclei produced using beams from the other cyclotrons – the Système de Production d’Ions Radioactifs en Ligne (SPIRAL1) facility. The various beams produced by these cyclotrons drive eight beams with specialised instrumentation. Parallel operation allows the running of three experiments simultaneously, thereby optimising the available beam time. These facilities enable both high-intensity stable-ion beams, from carbon-12 to uranium-238, and lower intensity radioactive-ion beams of short-lived nuclei, with lifetimes from microseconds to milliseconds, such as helium-6, helium-8, silicon-42 and nickel-68. Coupled with advanced detectors, all these beams allow nuclei to be explored in terms of excitation energy, angular momentum and isospin.

The new SPIRAL2 facility, which is currently being commissioned, will take this work into the next decade and beyond. The most recent step forward is the beam commissioning of a new superconducting linac – a major upgrade to the existing infrastructure. Its maximum beam intensity of 5 mA, or 3 × 10¹⁶ particles per second, is more than two orders of magnitude higher than at the previous facility. The new beams and state-of-the-art detectors will allow physicists to explore phenomena at the femtoscale right up to the astrophysical scale.

Landmark facility

SPIRAL2 was approved in 2005. It now joins a roster of cutting-edge European nuclear-physics-research facilities which also features the Facility for Antiproton and Ion Research (FAIR), in Darmstadt, Germany, ISOLDE and nTOF at CERN, and the Joint Institute for Nuclear Research (JINR) in Russia. Due to their importance in the European nuclear-physics roadmap, SPIRAL2 and FAIR are both now recognised as European Strategy Forum on Research Infrastructures (ESFRI) Landmark projects, alongside 11 other facilities, including accelerator complexes such as the European X-Ray Free-Electron Laser, and telescopes such as the Square Kilometre Array.

Construction began in 2011. The project was planned in two phases: the construction of a linac for very-high-intensity stable beams, and the associated experimental halls (see “High intensity” figure); and infrastructure for the reacceleration of short-lived fission fragments, produced using deuteron beams on a uranium target through one of the GANIL cyclotrons. Though the second phase is currently on hold, SPIRAL2’s new superconducting linac is now in a first phase of commissioning.

Most linacs are optimised for a beam with specific characteristics, which is supplied time and again by an injector. The particle species, velocity profile of the particles being accelerated and beam intensity all tend to be fixed. By tuning the phase of the electric fields in the accelerating structures, charged particles surf on the radio-frequency waves in the cavities with optimal efficiency in a single pass. Though this is the case for most large projects, such as Linac4 at CERN, the Spallation Neutron Source (SNS) in the US and the European Spallation Source in Sweden, SPIRAL2’s linac (see “Multitasking” figure) has been designed for a wide range of ions, energies and intensities.

The multifaceted physics criteria called for an original design featuring a compact multi-cryostat structure for the superconducting cavities, which was developed in collaboration with fellow French national organisations CEA and CNRS. Though the 19 cryomodules are comparable in number to the 23 employed by the larger and more powerful SNS accelerator, the new SPIRAL2 linac has far fewer accelerating gaps. On the other hand, compared to normal-conducting cavities such as those used by Linac4, the power consumption of the superconducting structures at SPIRAL2 is significantly lower, and the linac conforms to additional constraints on the cryostat’s design, operation and cleanliness. The choice of superconducting rather than room-temperature cavities is ultimately linked not only to the need for higher beam intensities and energies, but also to the potential for the larger apertures needed to reduce beam losses.

SPIRAL2 joins a roster of cutting-edge European nuclear-physics-research facilities

Beams are produced using two specialised ion sources. At 200 kW in continuous-wave (CW) mode, the beam power is high enough to make a hole in the vacuum chamber in less than 35 µs, placing additional severe restrictions on the beam dynamics. The operation of high beam intensities, up to 5 mA, also causes space-charge effects that need to be controlled to avoid a beam halo which could activate accelerator components and generate neutrons – a greater difficulty in the case of deuteron beams.

For human safety and ease of technical maintenance, beam losses need to be kept below 1 W/m. Here, the SPIRAL2 design has synergies with several other high-power accelerators, leading to improvements in the design of quarter-wave resonator cavities. These are used at heavy-ion accelerators such as the Facility for Rare Isotope Beams in the US and the Rare Isotope Science Project in Korea; for producing radioactive-ion beams and improving beam dynamics at intense-light particle accelerators worldwide; for producing neutrons at the International Fusion Materials Irradiation Facility, the ESS, the Myrrha Multi-purpose Hybrid Research Reactor for High-tech Applications, and the SNS; and for a large range of studies relating to materials properties and the generation of nuclear power.

Beam commissioning

Initial commissioning of the linac began by sending beams from the injector to a dedicated system with various diagnostic elements. The injector was successfully commissioned with a range of CW beams, including a 5 mA proton beam, a 2 mA alpha-particle beam, a 0.8 mA oxygen–ion beam and a 25 µA argon–ion beam. In each case, almost 100% transmission was achieved through the radio-frequency quadrupoles. Components of the linac were installed, the cryomodules cooled to liquid-helium temperatures (4.5 K), and the mechanical stability required to operate the 26 superconducting cavities at their design specifications demonstrated.

As GANIL is a nuclear installation, the injection of beams into the linac required permission from the French nuclear-safety authority. Following a rigorous six-year authorisation process, commissioning through the linac began in July 2019. An additional prerequisite was that a large number of safety systems be validated and put into operation. The key commissioning step completed so far is the demonstration of the cavity performance at 8 MV/m – a competitive electric field well above the required 6.5 MV/m. The first beam was injected into the linac in late October 2019. The cavities were tuned and a low-intensity 200 µA beam of protons accelerated to the design value of 33 MeV and sent to a first test experiment in the neutrons for science (NFS) area. A team from the Nuclear Physics Institute in Prague irradiated copper and iron targets and the products formed in the reaction were transported by a fast-automatic system 40 m away, where their characteristic γ-decay was measured. Precise measurements of such cross-sections are important in order to benchmark safety codes required for the operation of nuclear reactors.

SPIRAL2 is now moving towards its design power by gradually increasing the proton beam current and subsequently the duty cycle of the beam – the ratio of pulse duration to the period of the waveform. A similar procedure with alpha particles and deuteron beams will then follow. Physics programmes will begin in autumn next year.

Future physics

With the new superconducting linac, SPIRAL2 will provide intense beams from protons to nickel – up to 14.5 MeV/A for heavy ions – and continuous and quasi-mono energetic beams of neutrons up to 40 MeV. With state-of-the-art instrumentation such as the Super Separator Spectrometer (S³), the charged particle beams will allow the study of very rare events in the intense background of the unreacted beam with a signal to background fraction of 1 in 10¹³. The charged particle beams will also characterise exotic nuclei with properties very different from those found in nature. This will address questions related to heavy and super-heavy element/isotope synthesis at the extreme boundaries of the periodic table, and the properties of nuclei such as tin-100, which have the same number of neutrons and protons – a far cry from naturally existing isotopes such as tin-112 and tin-124. Here, ground-state properties such as the mass of nuclei must be measured with a precision of one part in 10⁹ – a level of precision equivalent to observing the addition of a pea to the weight of an Airbus A380. SPIRAL2’s low-energy experimental hall for the disintegration, excitation and storage of radioactive ions (DESIR), which is currently under construction, will further facilitate detailed studies of the ground-state properties of exotic nuclei fed both by S³ and SPIRAL1, the existing upgraded reaccelerated exotic-beams facility. The commissioning of S³ is expected in 2023 and experiments in DESIR in 2025. In parallel, a continuous improvement in the SPIRAL2 facility will begin with the integration of a new injector to substantially increase the intensity of heavy-ion beams.

Properties must be measured with a level of precision equivalent to observing the addition of a pea to the weight of an Airbus A380

Thanks to its very high neutron flux – up to two orders of magnitude higher, in the energy range between 1 and 40 MeV, than at facilities like LANSCE at Los Alamos, nTOF at CERN and GELINA in Belgium – SPIRAL2 is also well suited for applications such as the transmutation of nuclear waste in accelerator-driven systems, the design of present and next-generation nuclear reactors, and the effect of neutrons on materials and biological systems. Light-ion beams from the linac, including alpha particles and lithium-6 and lithium-7 impinging on lead and bismuth targets, will also be used to investigate more efficient methods for the production of certain radioisotopes for cancer therapy.

Developments at SPIRAL2 are quickly moving forwards. In September, the control of the full emittance and space–charge effects was demonstrated – a crucial step to reach the design performance of the linac – and a first neutron beam was produced at NFS, using proton beams. The future looks bright. With the new SPIRAL2 superconducting linac now supplementing the existing cyclotrons, GANIL provides an intensity and variety of beams that is unmatched in a single laboratory, making it a uniquely multi-disciplinary facility in the world today.

The post Spiralling into the femtoscale appeared first on CERN Courier.

Ultra-relativistic heavy-ion collisions create a system of deconfined quarks and gluons known as the quark–gluon plasma (QGP). Among other particles, a large number of light nuclei such as the deuteron, triton, helium-3, helium-4 and their corresponding antinuclei are produced, and can be measured with very good precision by the ALICE experiment at the LHC thanks to its excellent tracking and particle-identification capabilities via specific energy loss and time-of-flight measurements. Considering that the binding energies of light (anti)nuclei do not exceed a few MeV, it is not clear how such fragile objects can survive the hadron gas phase created after the phase transition from the QGP to hadrons, where particles rescatter with a typical momentum transfer in excess of 100 MeV. The production mechanism of light (anti)nuclei in these collisions is still not understood and is under intense debate in the scientific community. Constraining models of light antinuclei production is also important for predicting the backgrounds to indirect dark-matter searches using cosmic rays, as performed by experiments in space and in hot-air balloons, for which light antinuclei are promising signals.

The measured elliptic flow of light nuclei is bracketed by the simple coalescence approach and the blast-wave model

Azimuthal anisotropies of light (anti)nuclei production with respect to the symmetry plane of the collision are key observables to study interactions in the hadron-gas phase, and can shed light on the production mechanism of these fragile objects. The ALICE collaboration has recently reported the measurements of two harmonic coefficients (v_n) in a Fourier decomposition of the azimuthal distribution of deuterons in Pb–Pb collisions at √s_NN = 5.02 TeV: their elliptic flow, v₂, and the first measurement of their triangular flow, v₃. A clear mass ordering is observed in the elliptic flow of non-central Pb–Pb collisions at low p_T when the deuteron results are compared with other particle species, as expected for an expanding hydrodynamic system (figure 1, left).

Blast wave is best

The results are often compared to three phenomenological models, namely the statistical hadronisation model, the coalescence model, and the blast-wave model. In the statistical hadronisation model, light (anti)nuclei are assumed to be emitted by a source of thermal and hydrochemical equilibrium, like other hadron species, and their abundances fixed at the chemical freeze-out – the time at which inelastic interactions cease. However, this model only describes their yields, and not their flow. On the other hand, the coalescence model predicts that light nuclei are formed by the coalescence of protons and neutrons that are close in phase space at the kinetic freeze-out – the time at which elastic interactions cease. The blast-wave model, which is based on a simplified version of relativistic hydrodynamics, describes their transverse momentum spectra with just a few parameters, such as the kinetic freeze-out temperatures and transverse velocity.

In the new ALICE results, the measured elliptic flow of light nuclei is bracketed by the simple coalescence approach and the blast-wave model, which describe the data in different multiplicity regimes (figure 1, middle). The deuteron triangular flow is consistent with the coalescence model predictions, but large uncertainties do not allow a conclusive statement (figure 1, right). This specific aspect will be addressed with the larger data sample that ALICE will record in Run 3, which will also allow measurement of the flow of heavier nuclei. These results will contribute to shed light on their production mechanism and to study the properties of the hadron gas phase.

The post Fragile light nuclei flow through freeze-out appeared first on CERN Courier.

Assuming DM is a material substance comprised of particles – not an illusion resulting from an imperfect understanding of gravity – there are three independent ways to search for it. One is to directly measure the production of DM particles in a high-energy collider such as the LHC. Another is to infer the presence of DM particles via their scattering off nuclei, as investigated by large underground detectors such as XENON1T and LUX. A third, similarly indirect, strategy is to search for the annihilation or decay of DM particles into ordinary (anti) particles such as positrons or antinuclei – as employed by the AMS experiment on board the International Space Station and in balloon-borne experiments such as GAPS. Low-energy light antinuclei, such as antideuterons and antihelium, are particularly promising signals for such indirect DM searches, since the background stemming from ordinary collisions between cosmic rays and the interstellar medium is expected to be low with respect to the DM signal.

ALICE is the only experiment at the LHC that is able to study the production and annihilation of low-energy antinuclei

The ability to interpret any future observation of antinuclei in our galaxy – especially when trying to identify their creation in exotic processes like DM annihilations – requires a quantitative understanding of light antinuclei production and annihilation mechanisms within the interstellar medium. However, the production of light antinuclei in hadronic collisions between cosmic rays and the interstellar medium is still not fully understood: different models compete to explain how these loosely bound objects can be formed in such high-energy collisions. Furthermore, the inelastic annihilation cross section of light antinuclei with matter is completely unknown in the kinematic region relevant for indirect DM searches, forcing current estimates to rely on extrapolations and modelling.

Fortunately, both the antinuclei production mechanism and the interactions between antinuclei and ordinary matter can be studied on Earth using large accelerators. The main contributions so far have come from the LHC at CERN and from the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. Thanks to its unique low-material-budget tracker, which provides excellent tracking and particle-identification performance for low-momentum particles, ALICE is the only experiment at the LHC that is able to study the production and annihilation of low-energy antinuclei.

Antinucleosynthesis in the lab
While antinuclei can also be produced at lower collision energies, only at the LHC are matter and antimatter generated in equal abundances in the region transverse to the beam direction. The most abundant non-trivial antinucleus produced is the antideuteron, which consists of an antineutron and an antiproton. At low momentum, deuterons and antideuterons can be clearly identified thanks to their high energy loss in the ALICE detector’s time-projection chamber. At larger momenta, a clean identification of antideuterons is possible using the ALICE time-of-flight detector. This information, combined with the measured track length and the particle momentum, provide a precise determination of the particle mass. Using these and other techniques, the ALICE collaboration has recently measured the production of (anti)deuterons in proton–proton collisions, as well as in other colliding systems, and set tight constraints on the production models of (anti)nuclei.

There are two main ways to model the production mechanism of (anti)nuclei. Coalescence models assume that primary (anti)neutrons and (anti)protons can bind if they are close enough in phase space. Statistical hadronisation models, on the other hand, assume that hadrons and (anti) nuclei emerge when the collision system reaches thermodynamical equilibrium, making the temperature and the volume of the system the key parameters. Measurements of nuclei-to-proton ratios in various colliding systems have recently enabled the ALICE collaboration to compare the two model approaches in detail (see “Competing models” figure). As can be seen, the two models give different predictions for the evolution of the nuclei-to-proton ratio versus particle multiplicity, with the latest ALICE measurements slightly favouring the coalescence approach.

Similar conclusions about the two models can be drawn using heavier antinuclei, like ³He and ⁴He, which were already measured by ALICE in p–Pb and Pb–Pb collisions. The achievable precision of the measurement is limited by the available data: the antinuclei production rate in pp collisions goes down by a factor of about 1000 for every additional antinucleon in the antinucleus.

The precision of the measurements from proton–proton collisions places strong constraints on the production models, which can then be used to predict the antinuclei fluxes in space. Indeed, the ALICE measurements combined with different coalescence models have already been employed to estimate the antideuteron and antihelium flux from cosmic-ray interactions measurable by the AMS and GAPS experiments. These predictions will allow correct interpretations of the eventual antinuclei signal that might be observed in the future by the two collaborations.

Further helping clarify the results of indirect DM searches, ALICE has recently performed the first measurement of the antideuteron inelastic cross section in the momentum range 0.3 < p < 4 GeV/c – significantly extending our knowledge about this cross section from previous measurements at momenta of 13 and 25 GeV/c at the Serpukhov accelerator complex in Russia in the early 1970s. The collaboration took advantage of the ability of antideuterons produced at the LHC to interact inelastically with the detector material. To quantify this process, ALICE has employed a novel approach based on the antideuteron- to-deuteron ratio reconstructed in collisions of protons and heavy ions at a centre-of-mass energy per nucleon–nucleon pair of 5.02 TeV. Such a ratio depends on both of the inelastic cross sections of deuterons and antideuterons. The former has been measured in various previous experiments at different momenta; the latter can be constrained from the ALICE data by comparing the measured ratio with detailed Monte Carlo simulations.

The resulting antideuteron inelastic cross section is shown (see “Interaction probability” figure), where the two panels correspond to the different sub-detectors employed in the analysis and therefore to different average material crossed by (anti)deuterons – corresponding to a difference of about a factor two in average mass number. The inelastic cross sections include all possible inelastic antideuteron processes such as break-up, annihilation or charge exchange, and the analysis procedure was validated by demonstrating consistency with existing antiproton results from traditional scattering experiments.

The momentum range covered is of particular importance to evaluate the signal predictions for indirect dark-matter searches

The momentum range covered in this latest analysis is of particular importance to evaluate the signal predictions for indirect dark-matter searches. Additionally, these measurements can help researchers to understand the low-energy antideuteron inelastic processes and to model better the inelastic antideuteron cross sections in widely-used toolkits such as Geant4. Together with the proper modelling of antinuclei formation, the obtained results will impact the antideuteron flux expectations at low momentum for ongoing and future satellite- and balloon-borne experiments.

The heavier, the better
ALICE is studying the full range of antinuclei physics with unprecedented precision. These results, which have started to emerge only since 2015, are contributing significantly to our understanding of antinuclei formation and annihilation processes, with important ramifications for DM searches. Both the statistical hadronisation and coalescence models can describe antideuteron production at the LHC, while the detector material can be used as an absorber to study the antinuclei inelastic cross section at low energies relevant for the astrophysical applications.

For the foreseeable future, ALICE will continue to provide an essential reference for the interpretation of astrophysics measurements of antinuclei in space. With the increased integrated luminosity that will be acquired by ALICE during LHC Run 3 from early 2022, it will be possible to extend the current analyses to heavier (anti)nuclei, such as ³He and ⁴He, with even better precision than the currently available measurements for (anti)deuterons. This will allow the collaboration to perform fundamental tests of the production and annihilation mechanisms with heavier, doubly-charged antinuclei, which are more easily identified by satellite-borne experiments and thus expected to provide an even clearer DM signature.

The post ALICE’s dark side appeared first on CERN Courier.

Quarkonia, the bound states of charm and anti-charm or bottom and anti-bottom quarks, are an important tool to test our knowledge of quantum chromodynamics (QCD). At the LHC, the study of quarkonia polarisations offers a valuable new window onto how heavy quarks bind together in such states. Understanding quarkonium polarisation has already proven to be difficult at lower energies, however, and measurements at the LHC pose significant further challenges.

ALICE measures quarkonia spin orientations with respect to a chosen axis via a measurement of the anisotropy in the angular distribution of the decay products. The angular distribution is parametrised in terms of the polarisation parameters, λ_θ, λ_φ and λ_θφ, where θ and φ are the polar and azimuthal emission angles. If all of them are null, no polarisation is present, whereas (λ_θ = 1, λ_φ = 0, λ_θφ = 0) and (λ_θ = –1, λ_φ = 0, λ_θφ = 0) indicate a polarisation of the spin in the transverse and longitudinal directions, respectively.

Polarisation studies represent a valuable tool for the study of the properties of quark–gluon plasma

In pp collisions, polarisation has been mainly used to investigate the J/ψ production mechanism. Reproducing the small values of polarisation parameter λ_θ observed at the LHC is a challenge for many theoretical models. Until recently, no corresponding results were available for nucleus–nucleus collisions, and in this domain polarisation studies represent a valuable tool for the study of the properties of quark–gluon plasma (QGP). The formation of this deconfined, strongly interacting medium impacts differently on the various quarkonium resonances, inducing a larger suppression on the less bound excited states ψ(2S) and χ_c, and modifying their feed-down fractions into the ground state, J/ψ. This effect may lead to a variation of the overall polarisation values since different charmonium states are expected to be produced with different polarisations. In addition, the recombination of uncorrelated heavy-quark pairs inside the QGP gives rise to an extra source of J/ψ, which can further modify the overall polarisation with respect to pp collisions.

The ALICE experiment has recently made the first measurements of the J/ψ and ϒ(1S) polarisation in Pb–Pb collisions. The data correspond to a centre-of-mass energy √(s_NN) = 5.02 TeV, and the rapidity range 2.5 < y < 4. The measurements were carried out in the dimuon decay channel, and results were obtained in two different reference frames, helicity and Collins–Soper, each of them with its own definition of the quantisation axis. In the helicity frame, the quarkonium momentum direction in the laboratory is chosen, while the bisector of the angle formed by the two colliding beams boosted in the quarkonium rest frame is used in the Collins–Soper frame. The J/ψ polarisation parameters, evaluated in three p_T bins covering the range between 2 and 10 GeV, are close to zero, but with a maximum positive deviation for λ_θ (corresponding to a transverse polarisation) of 2σ for 2 < p_T < 4 GeV in the helicity reference frame. Interestingly, the corresponding LHCb pp result for prompt J/ψ at √(s_NN) = 7 TeV instead shows a small but significant longitudinal polarisation.

The observation of a significant difference between J/ψ polarisation results in pp and Pb–Pb collisions motivates further experimental and theoretical studies, with the main goal of connecting this observable with the known suppression and regeneration mechanisms in heavy-ion collisions. For the rarer ϒ(1S), a bound state of a bottom and an antibottom quark, the inclusive polarisation parameters were found to be compatible with zero within sizeable uncertainties. A higher precision and momentum-differential measurement will be enabled by the ten-fold larger Pb–Pb luminosity expected in Run 3 of the LHC.

The post J/ψ polarisation differs in lead collisions appeared first on CERN Courier.

Exotic charmonium-like states are a very active field of study at the LHC. These states have atypical properties such as non-zero electric charges and strong decays that violate isospin symmetry. The exotic X(3872) charmonium state discovered by the Belle collaboration in 2003 displays such isospin-violating strong decays and has a natural width of about 1 MeV, which is unexpectedly narrow for a state with mass very close to the D^*0 D⁰ threshold.

Luciano Maiani and collaborators pointed out that the new CMS measurement can naturally be explained by a tetraquark model of X(3872)

Several theoretical interpretations of the internal structure of these charmonium-like states have been proposed to explain their peculiar properties. To choose the most adequate model for each state, we must continue studying their properties and improving the determination of their parameters. As for the X(3872), although it is inconsistent with the predicted conventional charmonium states and does not have a definite isospin, its production partially resembles that of ordinary charmonium states such as ψ(2S) or χ_c1(1P). One of the ways to evaluate the degree of similarity between X(3872) and ψ(2S) is to compare their production rates in exclusive b-hadron decays. In the case of ψ(2S), which is a conventional charmonium state, the branching fractions of the decays B⁰_s → ψ(2S)φ, B⁺ → ψ(2S)K⁺, and B⁰ → ψ(2S)K⁰, are approximately equal to each other. Recent CMS measurements of the corresponding rates for decays to X(3872) show differences, however, which may provide a clue to the nature of this exotic charmonium-like state.

Recently the CMS collaboration observed the decay B⁰_s → X(3872)φ for the first time, with a significance exceeding five standard deviations. The X(3872) is reconstructed via its decay to J/ψπ⁺π^–, followed by a decay of the J/ψ meson into a pair of muons, and of the φ meson to a pair of charged kaons (figure 1).

Diquark hypothesis

At a simple Feynman-diagram level, this decay is a close analogue to the B⁺ → X(3872)K⁺ and B⁰ → X(3872)K⁰ decays that have previously been observed. The ratio of the branching fractions of this new B⁰_s decay to that of the B⁺ decay is significantly below unity at 0.48 ± 0.10, while a similar ratio for the decays involving ψ(2S) is consistent with unity. This is not expected from naive “spectator-quark” considerations, based on a simple tree-level diagram, and assuming X(3872) is a pure charmonium state. The measured ratio also happens to be consistent with the analogous ratio for the B⁰ → X(3872)K⁰ to B⁺ → X(3872)K⁺ decays, though the latter ratio has not yet been measured with high accuracy. The results suggest that spectator quarks behave differently in the B⁺ and B⁰_(s) two-body decays into X(3872) and a light meson. In a recent theoretical paper, former CERN Director-General Luciano Maiani and collaborators pointed out that the new CMS measurement can naturally be explained by a tetraquark model of X(3872), which describes this exotic particle as a bound state of a diquark (charm and up quarks) and its anti-diquark.

Further studies of X(3872) are now important in order to gain a deeper understanding of its exotic properties and uncover its mysterious nature. The results may have interesting consequences for our understanding of quantum chromodynamics.

The post CMS reaffirms exotic nature of the X(3872) appeared first on CERN Courier.

Understanding how the strong interaction binds the ingredients of atomic nuclei is the central quest of nuclear physics. Since the 1960s CERN’s ISOLDE facility has been at the forefront of this quest, producing the most extreme nuclear systems for examination of their basic characteristic properties.

A chemical element is defined by the number of protons in its nucleus, with the number of neutrons defining its isotopes. Apart from a few interesting exceptions, all elements in nature have at least one stable isotope. These form the so-called valley of stability in the nuclear chart of atomic number versus neutron number (see “Nuclear landscape” figure). Adding or removing neutrons disturbs the nuclear equilibrium and creates isotopes that are generally radio­active; the greater the proton–neutron imbalance, the faster the radioactive decay.

Most of the developments have been exported to other radioactive beam facilities around the world

The mass of a nucleus reveals its binding energy, which reflects the interplay of all forces at work within the nucleus from the strong, weak and electromagnetic interactions. Indications of sudden changes in the nuclear shape, when adding neutrons, are often revealed first indirectly as a sudden change in the mass, and can then be probed in detail by measurements of the charge radius and electromagnetic moments. Such diagnosis – performed by ion-trapping and laser-spectroscopy experiments on short-lived (from a few milliseconds upwards) isotopes – provides the first vital signs concerning the nature of nuclides with extreme proton-to-neutron ratios.

Recent mass-spectrometry measurements and high-precision measurements of nuclear moments and radii at ISOLDE demonstrate the rapid progress being made in understanding the stubborn mysteries of the nucleus. ISOLDE’s state-of-the-art laser-spectroscopy tools are also opening an era where molecular radioisotopes can be used as sensitive probes for physics beyond the Standard Model.

Tools of the trade

Progress in understanding the nucleus has gone hand in hand with the advancement of new techniques. Mass measurements of stable nuclei pioneered by Francis Aston nearly a century ago revealed a near-constant binding energy per nucleon. This pointed to a characteristic saturation of the nuclear force, which underlies the liquid-drop model and led to the semi-empirical mass formula for the nucleus developed by Bethe and von Weizsäcker. With the advent of particle accelerators in the 1930s, more isotopic mass data became available from reactions and decays, bringing new surprises. In particular, comparisons with the liquid drop revealed conspicuous peaks at certain so-called “magic” numbers (8, 20, 28, 50, 82, 126), analogous to the high atomic-ionisation potentials of the closed electron-shell noble-gas elements. These findings inspired the nuclear-shell model, developed by Maria Goeppert-Mayer and Hans Jensen, which is still used as an important benchmark today. The difference with the atomic system is that the force that governs the nuclear shells is poorly understood. This is because nucleons are themselves composite particles that interact through the complex interplay of three fundamental forces, rather than the single electromagnetic force governing atomic structure. The most important question in nuclear physics today is to describe these closed shells from fundamental principles (e.g. the strong interaction between quarks and gluons inside nucleons), to understand why shell structure erodes and how new shells arise far from stability.

A key to reaching a deeper understanding of nuclear structure is the ability to measure the size and shape of nuclei. This was made possible using the precision technique of laser spectroscopy, which was pioneered with tremendous success at ISOLDE in the late 1970s. While increased binding energy is a tell-tale sign of a deforming nucleus, it gives no specific information concerning nuclear size or shape. Closed-shell configurations tend to favour spherical nuclei, but since these are rather rare, a particularly important feature of nuclei is their deformation. Inspecting electromagnetic moments derived from the measured atomic hyperfine structure and the change in charge radii derived from its isotopic shift provides detailed information about nuclear shapes and deformation, beautifully complementing mass measurements.

During the past half-century, nuclear science at ISOLDE has expanded beyond fundamental studies to applications involving radioactive tracers in materials (including biomaterials) and the fabrication of isotopes for medicine (with the MEDICIS facility). But the bulk of the ISOLDE physics programme, around 70%, is still devoted to the elucidation of nuclear structure and the properties of fundamental interactions. These studies are carried out through nuclear reactions, by decay spectroscopy, or by measuring the basic global properties – mass and size – of the most exotic species possible.

Half a century of history

The fabrication of extreme nuclear systems requires a driver accelerator of considerable energy, and CERN’s expertise here has been instrumental. After many years receiving proton beams from a 600 MeV synchrocyclotron (the SC, now a museum piece at CERN), ISOLDE now lies just off the beam line to the Proton Synchrotron (PS), receiving 1.4 GeV beam pulses from the PS Booster (see “ISOLDE from above” figure). ISOLDE in fact receives typically 50% of the pulses in the so-called super-cycle that links the intricate complex of CERN’s injectors for the LHC.

The heart of ISOLDE is a cylindrical target that can contain various different materials. The stable nuclei in the target are dissociated by the proton impact and form exotic combinations of protons and neutrons. Heating the target (up to 2000 degrees) helps these fleeting nuclides to escape into an ionisation chamber, in which they form 1⁺ ions that are electrostatically accelerated to around 50 keV. Isotopes of one particular mass are selected using one of two available mass separators, and subsequently delivered to the experiments through more than a dozen beamlines. A similar number of permanent experimental setups are operated by several small international collaborations. Each year, more than 40 experiments are performed at ISOLDE by more than 500 users. More than 900 users from 26 European and 17 non-European countries around the world are registered as members of the ISOLDE collaboration.

A new era for fundamental physics research has opened up

ISOLDE sets the global standard for the production of exotic nuclear species at low energies, producing beams that are particularly amenable to study using precision lasers and traps developed for atomic physics. Hence, ISOLDE is complementary to higher energy, heavy-ion facilities such as the Radioactive Isotope Beam Factory (RIBF) at RIKEN in Japan, the future Facility for Rare Isotope Beams (FRIB) in the US, and the Facility for Antiproton and Ion Research (FAIR/GSI) in Europe. These installations produce even more exotic nuclides by fragmenting heavy GeV projectiles on a thin target, and are more suitable for studying high-energy reactions such as breakup and knock-out. Since 2001, ISOLDE has also driven low-energy nuclear-reaction studies by installing a post-accelerator that enables exotic nuclides to be delivered at MeV energies for the study of more subtle nuclear reactions, such as Coulomb excitation and transfer. Post-accelerated radioactive beams have superior optical quality compared to the GeV beams from fragment separators so that the radioactive beams accelerated in the REX and more recent HIE-ISOLDE superconducting linacs enable tailored reactions to reveal novel aspects of nuclear structure.

ISOLDE’s state-of-the art experimental facilities have evolved from more than 50 years of innovation from a dedicated and close-knit community, which is continuously expanding and also includes material scientists and biochemists. The pioneering experiments concerning binding energies, charge radii and moments were all performed at CERN during the 1970s. This work, spearheaded by the Orsay group of the late Robert Klapisch, saw the first use of on-line mass separation for the identification of many new exotic species, such as ³¹Na. This particular success led to the first precision mass measurements in 1975 that hinted at the surprising disappearance of the N = 20 shell closure, eight neutrons heavier than the stable nucleus ²³Na. In collaboration with atomic physicists at Orsay, Klapisch’s team also performed the first laser spectroscopy of ³¹Na in 1978, revealing the unexpected large size of this exotic isotope. To reach heavier nuclides, a mass spectrometer with higher resolution was required, so the work naturally continued at the expanding ISOLDE facility in the early 1980s.

Meanwhile, another pioneering experiment was initiated by the group of the late Ernst-Wilhelm Otten. After having developed the use of optical pumping with spectral lamps in Mainz to measure charge radii, Otten’s group exploited ISOLDE’s first offerings of neutron-deficient Hg isotopes and discovered the unique feature of shape-staggering in 1972. Through continued technical improvements, the Mainz group established the collinear laser spectroscopy (COLLAPS) programme at ISOLDE in 1979, with results on barium and ytterbium isotopes. When tunable lasers and ion traps became available in the early 1980s, the era of high-precision measurements of radii and masses began. These atomic-physics inventions have revolutionised the study of isotopes far from stability and the initial experimental set-ups are still in use today thanks to continuous upgrades and the introduction of new measurement methods. Most of these developments have been exported to other radioactive beam facilities around the world.

Mass measurements with ISOLTRAP

ISOLTRAP is one of the longest established experiments at ISOLDE. Installed in 1985 by the group of Hans-Jürgen Kluge from Mainz, it was the first Penning trap on-line at a radioactive beam facility, spawning a new era of mass spectrometry. The mass is determined from the cyclotron frequency of the trapped ion, and bringing the technique on line required significant and continuous development, notably with buffer-gas cooling techniques for ion manipulation. Today, ISOLTRAP is composed of four ion traps, each of which has a specific function for preparing the ion of interest to be weighed.

Since the first results on caesium, published in 1987, ISOLTRAP has measured the masses of more than 500 species spanning the entire nuclear chart. The most recent results, published this year by Vladimir Manea (Paris-Saclay), Jonas Karthein (Heidelberg) and colleagues, concern the strength of the N = 82 shell closure below the magic (Z = 50) ¹³²Sn from the masses of (Z = 48) ^132,130Cd. The team found that the binding energy only two protons below the closed shell was much less than what was predicted by global microscopic models, stimulating new ab-initio calculations based on a nucleon–nucleon interaction derived from QCD through chiral effective-field theory. These calculations were previously available for lighter systems but are now, for the first time, feasible in the region just south-east of ¹³²Sn, which is of particular interest for the rapid neutron-capture process creating elements in merging neutron stars.

The other iconic doubly magic nucleus ⁷⁸Ni (Z = 28, N = 50) is not yet available at ISOLDE due to the refractory nature of nickel, which slows its release from the thick target so that it decays on the way out. However, the production of copper – just one proton above – is so good that CERN’s Andree Welker and his colleagues at ISOLTRAP were recently able to probe the N = 50 shell by measuring the mass of its nuclear neighbour ⁷⁹Cu, finding it to be consistent with that of the doubly magic ⁷⁸Ni nucleus. Masses from large-scale shell-model calculations were in excellent agreement with the observed copper masses, indicating the preservation of the N = 50 shell strength but with some deformation energy creeping in to help. Complementary observables from laser spectroscopy helped to tell the full story, with results on moments and radii from the COLLAPS and the more recent Collinear Resonance Ionization Spectroscopy (CRIS) experiments adding an interesting twist.

Laser spectroscopy with COLLAPS and CRIS

Quantum electrodynamics provides its predictions of atomic energy levels mostly by assuming the nucleus is point-like and infinitely heavy. However, the nucleus indeed has a finite mass as well as non-zero charge and current distributions, which impact the fine structure. Thus, complementary to the high-energy scattering experiments used to probe nuclear sizes, the energy levels of orbiting electrons offer a marvellous probe of the electric and magnetic properties of the nucleus. This fact is exploited by the elegant technique of laser spectroscopy, a fruitful marriage of atomic and nuclear physics realised by the COLLAPS collaboration since the late 1970s. COLLAPS uses tunable continuous-wave lasers for high-precision studies of exotic nuclear radii and moments, and similar setups are now running at other facilities, such as Jyvaskyla in Finland, TRIUMF in Canada and NSCL-MSU in the US.

A recent highlight from COLLAPS, obtained this year by Simon Kaufmann of TU Darmstadt and co-workers, is the measurement of the charge radius of the exotic, semi-magic isotope ⁶⁸Ni. Such medium-mass exotic nuclei are now in reach of the modern ab-initio chiral effective-field theories, which reveal a strong correlation between the nuclear charge radius and its dipole polarisability. With both measured for ⁶⁸Ni, the data provide a stringent benchmark for theory, and allow researchers to constrain the point-neutron radius and the neutron skin of ⁶⁸Ni. The latter, in turn, is related to the nuclear equation-of-state, which plays a key role in supernova explosions and compact-object mergers, such as the recent neutron-star merger GW170817.