Nuclear Collisions Research Articles

Probing initial state effects in nuclear collisions via jet and top-quark measurements with the ATLAS detector

Proton-lead collisions at the Large Hadron Collider energies offer a unique possibility to investigate initial state effects in nuclear collisions. Thanks to its wide acceptance, ATLAS can measure several probes that can help characterise these effects over a wide kinematic range. By analyzing the centrality dependence of dijet production, it is possible to investigate the suppression of dijet events in central collisions compared to peripheral ones and correlate it to the kinematic of the parton-parton scattering, accessible via the measurement of both jets in the final state. Also, the first measurement of the inclusive crosssection for top-quark pair production in dilepton and lepton+jet decay modes is reported. This process is sensitive to effects at high Bjorken-x values, which are hard to access experimentally using other available probes, and thus provides complementary information on the behavior of nuclear parton densities. In 2016, the ATLAS experiment collected 165 nb−1 of proton-lead collisions at a center-of-mass energy of 8.16 TeV per nucleon pair. In this article, new measurements of the centrality dependence of dijet production and the observation of top-quark pair production using this dataset are presented.

Recent results on the analysis of the 18O+48Ti collision at 275 MeV: Single charge exchange reaction

The present work is inherent to the NUMEN project that aims at providing data-driven information for the nuclear matrix elements of the neutrinoless double beta decay through the study of heavy-ion induced double charge exchange reactions. This is a formidable task since during a nuclear collision, the same final states may be populated through various reaction mechanisms. In this respect, understanding the degree of competition between successive nucleon transfer and charge exchange reactions is crucial for the proper description of the meson-exchange mechanism. To this purpose, the reaction dynamics in the 18O+48Ti collision were sought by measuring a plethora of reaction channels under the same experimental conditions. The 48Ti was chosen as target since it is the daughter nucleus of 48Ca in double beta decay. The relevant experiment was performed at the MAGNEX facility of INFN-LNS in Catania. In this contribution, the status of the analysis for the 48Ti(18O,18F)48Sc single charge exchange reaction will be presented.

Extracting the speed of sound in the strongly interacting matter created in relativistic nuclear collisions with the CMS experiment

A hot and dense matter exhibiting collective flow behavior with almost no viscous dissipation has been discovered in ultrarelativistic nuclear collisions. To constrain the fundamental degrees of freedom and equation of state of this matter, these proceedings present an extraction of its speed of sound using head-on lead-lead collision data collected by the CMS experiment at a center-of-mass energy per nucleon pair of 5.02 TeV. The measurement is based on an analysis of the observed charged multiplicity dependence of the average particle transverse momentum in ultra-central events (impact parameter of nearly zero). This variable probes the system temperature as a function of entropy density at a fixed volume. Results are compared with hydrodynamic simulations and lattice quantum chromodynamics (QCD) predictions of the equation of state at high temperatures and small chemical potential. Implications to search for QCD phase transition and the critical point are discussed.

Experimental search for the QCD critical endpoint

The phase diagram of strong interaction has been a subject of intense theoretical and experimental research. One of the big questions in this regard is whether a critical endpoint is associated with the phase transition of strongly interacting matter. There have been rapid theoretical developments to answer this question. On the experimental side, a tremendous effort is put forward to hunt for the critical point in collisions of atomic nuclei. This article gives an experimental overview of some of the key results for critical point searches, mainly focusing on net-proton number fluctuations in nuclear collisions.

Studying exotic hadrons in high energy nuclear collisions

Studies of exotic hadrons such as the χc1 (3872) state provide crucial insights into the fundamental force governing the strong interaction dynamics, with an emerging new frontier to investigate their production in high energy nuclear collisions where a partonic medium is present. This contribution discusses the production mechanisms of exotic hadrons in such collisions and analyzes novel e_ects from the partonic medium, demonstrating the potential to use heavy ion measurements for deciphering their internal structure and understanding their in-medium evolutions.

Collective dynamics - Theoretical overview

I overview the recent progress of phenomenological studies exploring collective dynamics in relativistic nuclear collisions to understand various QCD properties. Originally, collectivity was interpreted as a manifestation of the hydrodynamic behaviour of the QGP as a response to the initial collision geometry. Over the past decade, however, particularly following the experimental observation of momentum azimuthal anisotropy in small colliding systems, pioneering studies have demonstrated the possibility of other interpretations. In this talk, I highlight recent studies aimed at understanding various QCD properties at different collision stages through the lens of collectivity and emphasize the importance of establishing Monte Carlo event generators for relativistic nuclear collisions.

3D multi-system Bayesian calibration with energy conservation to study rapidity-dependent dynamics of nuclear collisions

Considerable information about the early-stage dynamics of heavyion collisions is encoded in the rapidity dependence of measurements. To leverage the large amount of experimental data, we perform a systematic analysis using three-dimensional hydrodynamic simulations of multiple collision systems — large and small, symmetric and asymmetric. Specifically, we perform fully 3D multi-stage hydrodynamic simulations initialized by a parameterized model for rapidity-dependent energy deposition, which we calibrate on the hadron multiplicity and anisotropic flow coefficients. We utilize Bayesian inference to constrain properties of the early- and late-time dynamics of the system, and highlight the impact of enforcing global energy conservation in our 3D model.

Exploring the freeze-out hypersurface of relativistic nuclear collisions with a rapidity-dependent thermal model

Considering applications to relativistic heavy-ion collisions, we develop a rapidity-dependent thermal model that includes thermal smearing effect and longitudinal boost. We calibrate the model with thermal yields obtained from a multistage hydrodynamic simulation. Through Bayesian analysis, we find that our model extracts freeze-out thermodynamics with better precision than rapidity-independent models. The potential of such a model to constrain longitudinal flow from data is also demonstrated.

Exploring the hadron gas phase of relativistic heavy-ion collisions at the LHC

The lifetimes of hadronic resonances are comparable to the one of the hadron gas phase created in the late stages of high-energy nuclear collisions. Therefore, a significant fraction of resonances decay inside a high-density medium and their decay daughters rescatter with nearby hadrons destroying their initial kinematic correlations. The competing effect of resonance regeneration via pseudo-elastic interactions of hadrons interplays between the rescattering effect, modifying the measured yields and transverse-momentum spectra of hadronic resonances. These effects are studied by measuring the production yield ratio of resonances to the corresponding long-lived particle as a function of the hadronic lifetime, i.e. charged-particle multiplicity. In addition, measurements of the differential yields of resonances with different masses, quark content, and quantum numbers help in understanding particle production mechanisms, strangeness production, and parton energy loss. This article reports recent ALICE measurements on resonances and the use of the experimental input in a partial chemical equilibrium-based thermal model to constrain the kinetic freezeout temperature. This is a novel procedure that is independent of assumptions on the flow velocity profile and the freeze-out hypersurface.

Probing nuclear structure at the Electron-Ion Collider and in ultra-peripheral nuclear collisions

Within the Color Glass Condensate framework, we demonstrate that exclusive vector meson production at high energy is sensitive to the geometric deformation of the target nucleus and subnucleon scale fluctuations. Deformation of the nucleus enhances the incoherent cross section in the small |t| region. Subnucleon scale fluctuations increase the incoherent cross section in the large |t| region. In ultra-peripheral collisions (UPCs), larger deformation leads to a wider distribution of the minimal impact parameter Bmin required to produce a UPC. This, together with larger incoherent cross sections for larger deformation, results in smaller extracted radii. Our results demonstrate great potential for future studies of nuclear structure in UPCs and electron-ion collisions.

Many-body correlations for nuclear physics across scales: from nuclei to quark-gluon plasmas to hadron distributions

It is an experimental fact that multi-particle correlations in the final states of high-energy nucleus-nucleus collisions are sensitive to collective correlations of nucleons in the wave functions of the colliding nuclei. Here, I show that this connection is more direct than it intuitively seems. With an energy deposition scheme inspired by high-energy quantum chromodynamics, and within a linearized description of initial-state fluctuations in the quark-gluon plasma, I exhibit relations between N-particle correlations in the final states of nuclear collisions and N-nucleon density distributions in the colliding nuclei. This result formally justifies the sensitivity of the outcome of high-energy collisions to features such as nuclear deformations. It paves the way, thus, to systematic studies of the impact of state-of-the-art nuclear interactions in such processes.

Hyperon polarization and its relation with directed flow in high-energy nuclear collisions

Hyperon polarization and its relation with directed flow in high-energy nuclear collisions

Parameterizing smooth viscous fluid dynamics with a viscous blast wave

Blast wave fits are widely used in high energy nuclear collisions to capture essential features of global properties of systems near kinetic equilibrium. They usually provide temperature fields and collective velocity fields on a given hypersurface. We systematically compare blast wave fits of fluid dynamic simulations for Au+Au collisions at and Pb+Pb collisions at TeV with the original simulations. In particular, we investigate how faithful the viscous blast wave introduced in Yang and Fries (2022 Phys. Rev. C 105 014910) can reproduce the given temperature and specific shear viscosity fixed at freeze-out of a viscous fluid dynamic calculation, if the final spectrum and elliptic flow of several particle species are fitted. We find that viscous blast wave fits describe fluid dynamic pseudodata rather well and reproduce the specific shear viscosities to good accuracy. However, extracted temperatures tend to be underpredicted, especially for peripheral collisions. We investigate possible reasons for these deviations. We establish maps from true to fitted values. These maps can be used to improve raw fit results from viscous blast wave fits. Although our work is limited to two specific, albeit important, parameters and two collision systems, the same procedure can be easily generalized to other parameters and collision systems.

Transverse momentum distributions of the identified particles in mini–bias non-single diffracted p＋p collisions at 200 GeV

The transverse momentum (pT) distributions of hadrons stem from the probability distributions of deconfined colored partons known as pT distributions. This provides insights into various stages of nuclear collisions, ranging from the initial partonic phase to the later stages, including the directly measurable hadronization process. The objective is to analyze the pT distributions of identified particles: π+, π−, K+, K−, K0s, p, p̄, Λ, Λ̄, Σ+, Σ−. These analyses are conducted in mini-bias non-single diffracted p+p collisions at an energy of 200GeV. The goal is to accurately determine the kinetic freezeout parameters and the extent of statistical nonextensivity. The measurements from the STAR experiment are merged with simulations using DPMJET, PYTHIA, and Sibyll.3d Monte–Carlo Event Generators. The predictions from these models and the actual measurements are compared using statistical distributions employing the Tsallis-type of non-extensive statistics. While Pythia8 displays a closer alignment with the data in certain instances, it was observed that none of the event generators could accurately replicate the data for all charged particles across the entire range of pT. We examined how the effective temperature correlates with the rest mass of different particles. Our analysis revealed that the freezeout process for heavier particles tends to occur at lower temperatures. This suggests that heavier particles might reach an equilibrium state relatively rapidly during the freezeout stage. Additionally, we observed a pattern where the non-extensive parameter q decreases as the mass of the produced particles increases. This implies that lighter particles tend to reach equilibrium after the heavier ones. Furthermore, the average transverse momentum (<pT>) is greater for heavier particles, leading to an enhanced radial flow compared to lighter particles. This rising trend also indicates the likelihood of mini-jet production, contributing to color string fragmentation that becomes more pronounced with the increasing mass of the particles.

Determination of the Neutron Skin of ^{208}Pb from Ultrarelativistic Nuclear Collisions.

Emergent bulk properties of matter governed by the strong nuclear force give rise to physical phenomena across vastly different scales, ranging from the shape of atomic nuclei to the masses and radii of neutron stars. They can be accessed on Earth by measuring the spatial extent of the outer skin made of neutrons that characterizes the surface of heavy nuclei. The isotope ^{208}Pb, owing to its simple structure and neutron excess, has been in this context the target of many dedicated efforts. Here, we determine the neutron skin from measurements of particle distributions and their collective flow in ^{208}Pb+^{208}Pb collisions at ultrarelativistic energy performed at the Large Hadron Collider, which are mediated by interactions of gluons and thus sensitive to the overall size of the colliding ^{208}Pb ions. By means of state-of-the-art global analysis tools within the hydrodynamic model of heavy-ion collisions, we infer a neutron skin Δr_{np}=0.217±0.058 fm, consistent with nuclear theory predictions, and competitive in accuracy with a recent determination from parity-violating asymmetries in polarized electron scattering. We establish thus a new experimental method to systematically measure neutron distributions in the ground state of atomic nuclei.

Multiplicity Dependence of the Freeze-Out Parameters in Symmetric and Asymmetric Nuclear Collisions at Large Hadron Collider Energies

Strange hadron transverse momentum spectra are analyzed in symmetric pp and PbPb and asymmetric pPb collision systems for their dependence on rapidity and event charged-particle multiplicity. The thermodynamically consistent Tsallis models with and without flow velocity are used to reproduce the experimental data, extracting the freeze-out parameters to gain insights into the underlying physics of the collision processes by looking into the parameters change with different multiplicities, particle types, and collision geometries. We found that with an increase in the event multiplicity, the average transverse flow velocity, effective, and kinetic freezeout temperatures increase, with heavier strange particle species exhibiting a more significant increase. The value of the non-extensivity parameter decreases with an increase in the multiplicity of the particles. For heavier particles, larger Teff and T0 and smaller q have been observed, confirming the quick thermalization and equilibrium for massive particles. Furthermore, the differences in parameter values for particle species are more significant in pp and pPb collisions than in PbPb collisions. In addition, in symmetric pp and PbPb collisions, parameter values (q,T0,βT) show more significant shifts for heavier particles compared to the lighter ones. In contrast, in asymmetric pPb collisions, both heavier and lighter particles display uniform linear progression.

Impact of gold ion irradiation on the initial alteration rate of the International Simple Glass

This study addresses the impact of 7 MeV Au irradiation on the initial alteration rate of International Simple Glass (ISG), considered as a reference for nuclear glasses. This irradiation scenario simulates the effect of nuclear collisions of the recoil nuclei of α decays. In order to study the very first moments of glass alteration, non-irradiated and Au-ion irradiated ISG glass monoliths were altered in pure water at 90 °C, pH 9 and with a ratio of glass surface area to solution volume of 0.01 cm−1. Glass alteration was monitored by measuring the releases of the glass elements in solution by UV–visible spectrophotometry and Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES). Under these alteration conditions, the releases of the elements in solution are congruent. The initial alteration rate was found to be (2.0 ± 0.8) times higher for the irradiated glass monoliths than for non-irradiated samples.

Mass Spectra of Diphotons at Finite Chemical Potential in Massive Nuclear Collisions

Mass Spectra of Diphotons at Finite Chemical Potential in Massive Nuclear Collisions

A review on anisotropic flow in heavy-ion collisions

We present a review of published experimental measurements on anisotropic flow in heavy-ion collisions. Flow coefficients [Formula: see text] are presented for charged and identified hadrons at various beam energies ranging from [Formula: see text][Formula: see text]GeV to 200[Formula: see text]GeV at the Relativistic Heavy Ion Collider (RHIC) and [Formula: see text][Formula: see text]GeV at the Large Hadron Collider (LHC). We present [Formula: see text] and its slope [Formula: see text] and discuss possible first-order Quantum chromodynamics (QCD) phase transition. [Formula: see text] of particles containing heavy quarks (Charm quarks) is also discussed to illustrate the effect of the magnetic field produced in relativistic heavy-ion collisions. We also present flow harmonics ([Formula: see text], [Formula: see text] and [Formula: see text]) for inclusive charged and identified hadrons. We compare the charged particle [Formula: see text] coefficients with hydrodynamics to study and constrain initial conditions in the transport models. The results of identified particle [Formula: see text] from ideal and viscous hydrodynamics are discussed to understand the dynamics and mechanism of particle production in high-energy nuclear collisions.

Nuclear viscosity estimated by dynamics of neck formation in the early stage of nuclear collision

The very early stage of the coalescence of two nuclei is studied and used to estimate the nuclear viscosity. The time evolution of the neck region has been simulated by the unified Langevin equation method, which is used in the analysis of heavy-ion collisions from the approaching stage to the fusion-fission stage. It is found that the transition from viscous to inertial coalescence that appeared in the neck growth of macroscopic drops can also be seen in the present simulation in nucleus–nucleus collisions. The dynamics of neck growth is analyzed in terms of the hydrodynamical formula and the viscosity coefficient of nuclear matter is estimated using the analogy of macroscopic drops.