Search

Among the most iconic results of Run-1 and Run-2 of the LHC is the observation of enhanced production of (multi-)strange to non-strange particles, gradually rising from low-multiplicity to high-multiplicity pp or p–Pb collisions and reaching values close to those measured in peripheral Pb–Pb collisions [1]. The observed behaviour cannot be quantitatively reproduced by any of the available QCD-inspired MC generators. In this contribution two extensions of this study are presented: the measurement of Σ baryons and the first measurement of the full Probability Density Function (PDF) for KS0, Λ, Ξ− and Ω−, therefore extending the study of strangeness production beyond the average of the distribution. This novel method represents a unique opportunity to test the connection between charged and strange particle multiplicity production. [ABSTRACT FROM AUTHOR]

Ultraperipheral collisions of relativistic heavy ion beams lead to a diverse set of photon-nucleus (photonuclear) interactions. The measurements of particles and their interaction produced in photonuclear reactions can shed light on the QCD dynamics of these novel, extremely asymmetric colliding systems, with energies between those available at RHIC and the LHC. Previous studies by ATLAS indicate significant elliptic and triangular flow coefficients in these events [1]. Thus, it is imperative to check these events for other potential QGP signatures including radial flow, strangeness enhancement, and enhanced baryon/meson production. This proceeding presents the measurement of charged hadron yields in photonuclear collisions using 5.02 TeV Pb+Pb data collected in 2018 by ATLAS, with a dedicated photo-nuclear event trigger. The charged hadron yields are presented as a function of pseudorapidity and transverse momentum in different categories of event multiplicity. The results are compared with 5.02 TeV p+Pb data collected in 2016 by ATLAS, at the same event multiplicities. The results are also compared with calculations from DPMJET and hydrodynamic-based models. These comparisons enable detailed characterizations of photonuclear collision properties, including the photon energy distribution, and whether small QGP droplets may be formed. [ABSTRACT FROM AUTHOR]

We present new NLO pQCD predictions for photoproduction of dijets in ultraperipheral PbPb collisions at 5.02 TeV with a realistic photon flux and up-to-date nuclear PDFs. Our calculation of the impact parameter dependence of the photon flux includes the effects of the nuclear form factor in the photon-emitting nucleus and the spatial dependence of nuclear PDFs of the target nucleus, which are estimated using theWood-Saxon nuclear density profile. We show that a significant portion of the measured dijets at large zγ originate from events with impact parameters of the order of a few nuclear radii, and that the cross section predictions therefore become sensitive to the modelling of the nuclear geometry and photon flux close to the source nucleus. [ABSTRACT FROM AUTHOR]

Heavy meson production in reactions with nuclei is an active new frontier to understand QCD dynamics and the process of hadronization in nuclear matter. Measurements in various colliding systems at RHIC and LHC, including Pb-Pb, Xe-Xe, O-O, p-Pb, and p-O, enable precision tests of the medium-size, temperature, and mass dependencies of the in-medium parton propagation and shower formation. We employ a coupled DGLAP evolution framework that takes advantage of splitting functions recently obtained in softcollinear effective theory with Glauber gluons (SCETG) and hard thermal loop (HTL) motivated collisional energy loss effects. With jet quenching effects constrained to the nuclear modification factor of charged hadrons in Pb-Pb collisions at 5.02 TeV, we present predictions for light and heavy-meson in Xe-Xe, O-O and p-Pb collisions at the LHC.We find that the nuclear modification scales non-trivially with the quark mass and medium properties. In particular, there can be sizeable collision-induced attenuation of heavy mesons in small systems such as oxygen-oxygen and high-multiplicity p-Pb events. Finally, we analyze the impact of different models of initial-state parton dynamics on the search for QGP signatures in small colliding systems. [ABSTRACT FROM AUTHOR]

One of the main goals in the study of hadronic interactions at LHC energies is the attempt to characterize the mechanisms involved in particle production in different regimes. The charged-particle multiplicity is one of the most interesting observables in these kind of studies. Measurements of chargedparticle pseudorapidity densities in pp collisions at √s = 13.6 TeV are presented, for the first time. Furthermore, we present a method for better understanding the collision dynamics. On the one hand, the pseudorapidity dependence of charged-particle production provides information on the partonic structure of the colliding hadrons and is sensitive to non-linear QCD evolution in the initial state. On the other hand to understand the thermal-like behavior and particle yields in pp collisions, a possible approach is to address the principles of quantum states and their entanglement in the produced system. The entanglement in the initial state has a measurable effect on the evolution of the system and is the driving mechanism behind the thermal-like behavior and particle yields observed. We describe a method to understand the level of entanglement in the initial and final states of the collision using a calculation of entropy, the final-state entropy being calculated from multiplicity distributions. [ABSTRACT FROM AUTHOR]

The study of jet production in small collision systems is essential for testing our understanding of perturbative and non perturbative QCD and cold nuclear matter (CNM) effects. In addition, studies at high multiplicity in small collision systems exhibit signatures of collectivity, which is still not fully understood within a unified picture across system size. Jet quenching in small systems is not observed within current measurement precision, calling for more precise jet measurements. Results of charged particle and fully reconstructed jet production in pp and p-Pb collisions are presented at √SNN = 5.02 and 8 TeV, along with the corresponding nuclear modification factor RpPb at √SNN = 5.02 TeV. These results are the most precise measurements of the RpPb by ALICE to date. To investigate whether jet energy is redistributed in CNM, the crosssection ratios for different jet resolution parameters (R) are compared between pp and p–Pb collisions, as well as within each collision system. Finally, comparisons between data and model predictions are discussed. This result extends previous LHC measurements to lower jet transverse momentum, constraining hard parton production and fragmentation mechanisms applied in model calculations, and the impact of the nuclear-modified parton distribution function on jet production. This measurement also provides new constraints on jet quenching in small collision systems. [ABSTRACT FROM AUTHOR]

We report on measurements of light charged hadron production in Au+Au collisions at √SNN = 14.6, 19.6, 27, and 54.4 GeV from the second phase of the beam energy scan (BES-II) program at RHIC. Transverse mass spectra and rapidity density distributions of π±, K±, p, and ¯p are measured as a function of collision energy, centrality, and rapidity. This work extends the previously published BES-I mid-rapidity (|y| < 0.1) measurement to a rapidity of 0.9. This new rapidity dependence highlights the effects of baryon stopping for proton yields and the different production mechanisms of π± and K±. The relative abundance of these light hadrons indicates different thermodynamic environments at different energies, centralities, and rapidities. The temperature (T) and chemical potentials (μB and μS) at chemical freeze-out are reported with their dependence on rapidity to further understand the QCD phase diagram. [ABSTRACT FROM AUTHOR]

We present first results for leading hadron suppression in small collision systems, from a convolved radiative and collisional pQCD energy loss model which receives a short path length correction to the radiative energy loss. We find that the short path length correction is exceptionally large for light flavor final states at high-pT , for all system sizes. We numerically investigate the self-consistency of various assumptions underlying the radiative energy loss model, in an effort to understand the size of the short path length correction. This calculation shows that the large formation time assumption, which is utilized by most contemporary energy loss models, is invalid for a large portion of the phenomenologically relevant parameter space. [ABSTRACT FROM AUTHOR]

We employ an Einstein-Maxwell-dilaton model, based on the gauge/gravity correspondence, to obtain the thermodynamics and transport properties for the hot and dense quark-gluon plasma. The model, which is constrained to reproduce lattice QCD thermodynamics at zero density, predicts a critical point and a first order line at finite temperature and density, is used to quantify jet energy loss through simulations of high-energy collision events. [ABSTRACT FROM AUTHOR]

We reveal that the criticality of the chiral phase transition in QCD at the macroscale arises from the microscopic energy levels of its fundamental constituents, the quarks. We establish a novel relation between cumulants of the chiral order parameter (i.e., chiral condensate) and correlations among the energy levels of quarks (i.e., eigenspectra of the massless Dirac operator), which naturally leads to a generalization of the Banks-Casher relation. Based on this novel relation and through (2+1)-flavor lattice QCD calculations using the HISQ action with varying light quark masses in the vicinity of the chiral phase transition, we demonstrate that the correlations among the infrared part of the Dirac eigenspectra exhibit same universal scaling behaviors as expected of the cumulants of the chiral condensate. We find that these universal scaling behaviors extend up to the physical values of the up and down quark masses. [ABSTRACT FROM AUTHOR]

Using an eighth-order Taylor expansion in baryon chemical potential, we recently obtained the (2+1)-flavor QCD equation of state (EoS) at nonzero conserved charge chemical potentials from the lattice. We focused on strangeness-neutral, isospin-symmetric QCD matter, which closely resembles the situation encountered in heavy-ion collision experiments. Using this EoS, we present here results on various QCD material parameters; in particular we compute the specific heat, speed of sound, and compressibility along appropriate lines of constant physics. We show that in the entire range relevant for the beam energy scan at RHIC, the specific heat, speed of sound, and compressibility show no indication for an approach to critical behavior that one would expect close to a possibly existing critical endpoint. [ABSTRACT FROM AUTHOR]

The equation of state of Quantum Chromodynamics has been in recent years the focus of intense effort from first principle methods, mostly lattice simulations, with particular interest to the finite baryon density regime. Because of the sign problem, various extrapolation methods have been used to reconstruct bulk properties of the theory up to as far as μB=T ≃ 3:5. However, said efforts rely on the equation of state at vanishing baryon density as an integration constant, which up to μB=T ≃ 2 - 2:5 proves to be the dominant source of uncertainty at the level of precision currently available. In this contribution we present the update of our equation of state at zero net baryon density from 2014, performing a continuum limit from lattices with Nτ = 8; 10; 12; 16. We show how the improved precision is translated in a lower uncertainty on the extrapolated equation of state at finite chemical potential. [ABSTRACT FROM AUTHOR]

We calculate the grand canonical fluctuations of the net baryon number and net strangeness in the region of chiral crossover from lattice QCD. We continuum extrapolate the results in a finite volume using the new 4HEX discretization scheme up to χ6B and also for χ8B at a single temperature. Our results contradict recent estimates at a finite lattice spacing. See Ref. [1] for the full version of this work. [ABSTRACT FROM AUTHOR]

The effect of a finite volume presents itself both in heavy ion experiments as well as in recent model calculations. The magnitude is sensitive to the proximity of a nearby critical point. We calculate the finite volume effects at finite temperature in continuum QCD using lattice simulations. We focus on the vicinity of the chiral crossover. We investigate the impact of finite volumes at zero and small chemical potentials on the QCD transition though the chiral observables. [ABSTRACT FROM AUTHOR]

We discuss a quantitative "double copy" between radiation from shockwave collisions in Einstein gravity and in QCD. The correspondence extends to 2 → N amplitudes in Regge asymptotics. The classicalization and unitarization of these amplitudes at maximal occupancy, corresponding to black hole and Color Glass Condensate (CGC) states respectively, are described by the emergent Goldstone dynamics of wee partons. We outline some consequences of the universal dynamics on both sides of the correspondence. [ABSTRACT FROM AUTHOR]

Determining the existence and the location of the QCD critical point remains a major goal in the heavy-ion collision experiments. A crucial theoretical input for achieving this goal is mapping the QCD equation of state in the presence of baryon chemical potential (μ) which at the moment is limited to small values of μ, away from the critical point. We present a new framework for reconstructing the equation of state from a truncated Taylor series expansion for small μ by using novel resummation techniques. We show how this resummation method can be used to (i) determine the location of the critical point and (ii) constrain the form of the critical contribution to the QCD equation of state which has a direct impact on the shape of the experimental signatures of the critical point. Using the recent lattice data from the HotQCD collaboration we estimate that Tc ≈ 100 MeV, μc ≈ 580 MeV, the crossover slope at the critical point α1 ≈ 9°, and further constrain the non-universal mapping parameters between the Ising model and QCD (scaling) equations of state. [ABSTRACT FROM AUTHOR]

We study cold nuclear matter effects on Drell-Yan production at small and moderate pT in proton/pion-nucleus collisions using a new transversemomentum dependent (TMD) factorization framework. Both collisional broadening and medium-induced radiative corrections in the initial state are considered in the soft-collinear effective theory with Glauber gluons (SCETG) approach. We demonstrate that in-medium bremsstrahlung exhibits rapidity divergences as x → 1 and collinear divergences at the endpoints x = 0, 1 of the medium-induced emission spectra. We further show that the rapidity divergences lead to the Balitsky-Fadin-Kuraev-Lipatov (BFKL) evolution of the collision kernel and can be resummed into the transverse momentum broadening of particle production. In turn, the endpoints divergences of in-medium radiation can be resummed through the collinear evolution of parton densities in nuclear matter. The TMD factorization framework is applied to understand the transverse-momentum spectra of Drell-Yan pair production in pA and πA collisions and provides calculations with improved accuracy for hadron production in cold QCD processes at RHIC and LHC. [ABSTRACT FROM AUTHOR]

Various studies of jet substructure in heavy-ion collisions offer a consistent picture of QCD medium interactions and a diverse path towards further differentiating energy loss mechanisms. Some results, however, remain disjoint: the jet mass and jet angularities, including girth and thrust, are stronglycorrelated observables which have given seemingly conflicting answers on the angular quenching of jets traversing the quark–gluon plasma (QGP). ALICE has carried out new systematic measurements of these and other perturbativelycalculable angularities using consistent definitions, resolving the girth-mass problem, and revealing quenching effects at broad angles. Concurrently, applying Soft Drop grooming isolates the narrowing in the core of quenched jets. Grooming can also be employed to resolve medium scattering centers, with varying methods used to focus on regions of the splitting phase space. We present the first application of dynamical grooming in heavy-ion collisions to search for excess kT,g emissions as a signature of point-like scatters, providing new constraints on searches for in-medium Molière scattering. We also present a novel shift from studying jet-medium opacity from the angular perspective to a time-like one. By employing a new time reclustering strategy, we potentially enable a time-dependent study of jet substructure observables. We compare all results to assorted jet quenching models, providing new critical information on medium evolution as a function of angular, momentum, and time structure. [ABSTRACT FROM AUTHOR]

We study the time evolution of the density matrix of a high energy quark in the presence of a dense QCD background that is modeled as a stochastic Gaussian color field. At late times, we find that only the color singlet component of the quark's reduced density matrix survives the in-medium evolution and that the density matrix becomes asymptotically diagonal in both transverse position and momentum spaces. In addition, we observe an accelerated entropy growth due to the larger phase space being explored by the quark and that the quantum and classical quark entropies converge at late times. We further observe that the quark state loses all memory of the initial condition. Combined with the fact that the reduced density matrix satisfies Boltzmann-diffusion transport, we conclude that the quark reduced density matrix can be interpreted as a classical phase space distribution. [ABSTRACT FROM AUTHOR]

In this contribution to the Quark Matter 2023 proceedings, we study the hydrodynamization process in heavy-ion collisions using QCD kinetic theory and introduce the new concept of limiting attractors. They are defined via an extrapolation of observables to vanishing and infinite couplings. We find that the pressure ratio exhibits both a hydrodynamic and a bottom-up limiting attractor, while the ratios of hard probes transport coefficients q^zz/q^yy and κT/κz are better described in terms of the new bottom-up limiting attractor. [ABSTRACT FROM AUTHOR]

Results on collisions of 16O nuclei performed at the Relativistic Heavy Ion Collider (RHIC) have been presented for the first time at Quark Matter 2023 by the STAR collaboration. 16O+16O collisions are also expected to take place in the near future at the Large Hadron Collider (LHC) at much higher beam energies. We explore the potential of beam-energy-dependent studies for this system to probe small-x dynamics and QCD evolution. We perform 3 + 1D IP-Glasma simulations to predict the rapidity dependence of the initial geometry of light-ion collisions, focusing on 16O+16O and 20Ne+20Ne collisions at √SNN = 70 GeV and 7 TeV. The choice of 20Ne is motivated by its strongly elongated geometry, which may respond differently to the effect of the highenergy evolution compared to the more spherical 16O. We find that smearing induced by soft gluon production at high energy causes mild variations in the initial-state eccentricities as a function of the collision energy. These effects could be resolved in future experiments and deserve further investigation. [ABSTRACT FROM AUTHOR]

Incoherent and dissociative J/ψ photoproduction is sensitive to fluctuations of the gluonic structure of the target. Thus, the measurement of Jψ photoproduction of the colliding hadron sheds light on the initial state of QCD and provides important constraints on the initial conditions used in hydrodynamical models of heavy-ion collisions. The first measurement of the transverse momentum dependence of both coherent and incoherent Jψ photoproduction in ultraperipheral Pb–Pb collisions at midrapidity is presented. These new results provide, a clear indication of subnucleonic fluctuations of the lead target. We also present the new measurement of dissociative Jψ photoproduction cross section as a function of energy, in p–Pb collisions at forward rapidity. Dissociative results do not show any indication of saturation and agree with previous data. [ABSTRACT FROM AUTHOR]

We use QCD effective kinetic theory to calculate far-fromequilibrium dynamics on an event-by-event basis within the KøMPøST framework. We present non-equilibrium charge response functions and the dynamical evolution of the conserved charge current pertinent to the early-time dynamics of heavy-ion collisions at the highest energies. The KøMPøST framework with conserved baryon, strangeness, and electric charges can then be readily implemented into a multistage model allowing for the initialization of a nonequilibrium charge current in hydrodynamic simulations. [ABSTRACT FROM AUTHOR]

In heavy-ion and hadronic collisions, indications of thermalization are detected in the yields of produced hadrons: these observations call for a detailed study of the hadronization processes. Novel observables are required to discriminate between the different hadronization models that successfully describe the average yields of hadrons. The ALICE Collaboration reports on the studies of event-by-event fluctuations of the net-Ξ± number, and Pearson correlations of both the net-Ξ±–net-kaon numbers and the antideuteron–antiproton numbers, in pp, p–Pb, and Pb–Pb collisions. The measured fluctuations are compared with different hadronization models to explore the compatibility with a charge-equilibration scenario. In all cases, the experimental data agree with Thermal-FIST canonical-statistical calculations, allowing for the extraction of the correlation length for strange and baryon quantum number conservation. [ABSTRACT FROM AUTHOR]

The hadro-chemistry of bottom quarks produced in hadronic collisions encodes valuable information on the mechanism of color-neutralization in these reactions. We first compute the chemistry of bottom-hadrons in high-energy pp collisions employing statistical hadronization with a largely augmented set of states beyond the currently measured spectrum. This enables a comprehensive prediction of fragmentation fractions of weakly decaying bottom hadrons for the first time and a satisfactory explanation of the existing measurements in pp collisions at the LHC. Utilizing the bottom hadro-chemistry thus obtained as the baseline, we then perform transport simulations of bottom quarks in the hot QCD matter created in PbPb collisions at the LHC energy and calculate the pertinent bottom-hadron observables. The transverse momentum (pT) dependent modifications of the bottom baryon-to-meson ratio (Λb0/B-) relative to their pp counterparts are highlighted as a result of bottom quark diffusion and hadronization in the Quark-Gluon Plasma (QGP). We finally summarize the heavy quark (charm vs bottom) diffusion coefficients as extracted from transport simulations and compare them to result from recent full lattice QCD computations. [ABSTRACT FROM AUTHOR]

We report the measurements of D0 transverse momentum (pT) differential invariant spectra with pT < 8 GeV/c for 0–10%, 10–40%, and 40–80% centrality classes in isobar collisions (96Ru + 96Ru and 96Zr + 96Zr) at √SNN = 200 GeV with the STAR experiment. The strong suppression of D0 yield is observed for pT > 3 GeV/c in the central collisions, demonstrating that charm quarks suffer significant energy loss in the bulk QCD medium. In this proceeding, the measurements of D0 meson tagged jets at √SNN = 200 GeV in Au+Au collisions are reported as well. We present the nuclear modification factor, RCP, as a function of pT,Jet and zJet, and show the ratios of the D0 radial profile with 1

NA60+ is a new experiment designed to study the phase diagram of the strongly interacting matter at CERN SPS energies, where the values of the baryochemical potential μB approximately range between 200 and 550 MeV. It is focused on precision studies of thermal dimuons, heavy quarks, and strangeness production in Pb–Pb collisions at center of mass energies ranging from 6 to 17 GeV per nucleon pair. In this paper the apparatus concept and the physics reach will be discussed. [ABSTRACT FROM AUTHOR]

In these proceedings, we compute the heavy quark momentum diffusion coefficient using QCD effective kinetic theory for a plasma going through the bottom-up thermalization scenario until approximate hydrodynamization. This transport coefficient describes heavy quark momentum diffusion in the quark-gluon plasma and is used in many phenomenological frameworks, e.g. in the open quantum systems approach. Our extracted nonthermal diffusion coefficient matches the thermal one for the same energy density within 30%. At large occupation numbers in the earliest stage, the transverse diffusion coefficient dominates, while the longitudinal diffusion coefficient is larger for the underoccupied system in the later stage of hydrodynamization. [ABSTRACT FROM AUTHOR]

We study how the dilepton production rates and electric conductivity are affected by the phase transition to color superconductivity and the QCD critical point. Effects of the soft modes associated with these phase transitions are incorporated through the photon self-energy called the Aslamazov- Larkin, Maki-Thompson, and density-of-states terms, which are responsible for the paraconductivity in metallic superconductors. We show that anomalous enhancements of the production rate in the low energy/momentum region and the conductivity occur around the respective critical points. [ABSTRACT FROM AUTHOR]