Au Collisions Research Articles

Collective flow and fluid behavior in p/d/3He+Au collisions at GeV*

Abstract By varying the intrinsic initial geometry, p/d/3He+Au collisions at the Relativistic Heavy Ion Collider (RHIC) provide a unique opportunity to understand the collective behavior and probe possible sub-nucleon fluctuations in small systems. In this study, we employed the hybrid model under TRENTo initial conditions to study the collective flow and fluid behavior in p/d/3He+Au collisions. With fine-tuned parameters, can describe the and data from the PHENIX and STAR collaborations. However, for certain parameter sets with initial sub-nucleon fluctuations, the hydrodynamic simulations already go beyond their limits with an average Knudsen number clearly larger than unity. Our calculations demonstrate that, for a meaningful evaluation of the fluid behavior in small systems, model simulations must also pay attention to the validity range of hydrodynamics.

Collision-energy dependence of deuteron cumulants and proton-deuteron correlations in Au+Au collisions at RHIC

We report the first measurements of cumulants, up to 4th order, of deuteron number distributions and proton-deuteron correlations in Au+Au collisions recorded by the STAR experiment in phase-I of Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider. Deuteron cumulants, their ratios, and proton-deuteron mixed cumulants are presented for different collision centralities covering a range of center-of-mass energy per nucleon pair sNN=7.7 to 200 GeV. It is found that the cumulant ratios at lower collision energies favor a canonical ensemble over a grand canonical ensemble in thermal models. An anti-correlation between proton and deuteron multiplicity is observed across all collision energies and centralities, consistent with the expectation from global baryon number conservation. The UrQMD model coupled with a phase-space coalescence mechanism qualitatively reproduces the collision-energy dependence of cumulant ratios and proton-deuteron correlations.

Initial and final state temperature of $$K^{*0}$$ in Beam Energy Scan of Au–Au collisions at RHIC energies

Initial and final state temperature of $$K^{*0}$$ in Beam Energy Scan of Au–Au collisions at RHIC energies

Thermodynamic signatures and phase transitions in high-energy Au–Au collision

In this study, we systematically investigate the dynamics of various hadrons namely π +, π −, K +, K −, p, p¯ , Λ, Λ¯ , Ξ− and Ξ¯+ produced in central Au–Au collisions. We analyze data of AGS and RHIC, which span a broad range of collision energies, ranging from sNN = 1.9 to 200 GeV. To analyze the transverse momentum (p T ) and transverse mass (m T ) distributions, we employ a two-component standard distribution function, achieving a very good representation of the experimental data across these energy regimes. We extract key thermodynamic parameters, including the effective temperature T, the mean transverse momentum 〈p T 〉, and the initial temperature T i , and analyze their dependence on the values of collision energy and particle mass. Our findings reveal a distinct transition behaviour around sNN=19.6GeV . Below sNN=19.6GeV , the values of T, 〈p T 〉, and T i increase monotonically for all hadrons due to higher energy transfer into the system. Above this energy threshold, these extracted parameters plateau, suggesting that the additional energy is utilized as latent heat for phase transition rather than increasing the system’s temperature. These observations delineate two distinct regions: a hadron-dominated region at lower energies and a parton-dominated region at higher energies, each potentially indicative of different phases of matter, with the latter possibly signalling the onset of a quark–gluon plasma. The study thus provides critical insights into the complex interplay of thermodynamics, phase transitions, and particle interactions in high-energy Au–Au collisions.

Centrality dependency of proton, deuteron, and triton’s temperatures in Au+Au collisions at 200 GeV

The transverse momentum (pT) spectra of protons (p), deuterons (d), and tritons (t) in 200 GeV gold–gold (Au + Au) collisions at RHIC are examined across a range of centrality bins using the Levy Tsallis (TS) statistical model. The model's predictions closely match the experimental results from PHENIX (p) and STAR (d and t) Collaborations. Kinetic freeze-out temperatures of hadrons are obtained from particle spectra. The results showed that the kinetic freeze-out temperature decreases as collisions move from center to the periphery. This work found mass-dependent kinetic freeze-out temperatures, heavier particles arrive to the freeze-out phase before lighter ones. Comparison with same data fitted by blast wave function with Tsallis statistics (BWTS) showed that T0 values are increasing from central to peripheral collisions, while in case of TS function (current analysis) it decreases. This behavior puts a question mark on the reliability of using such functions for temperature extraction.

Virtual Photons Shed Light on the Early Temperature of Dense QCD Matter.

Dileptons produced during heavy-ion collisions represent a unique probe of the QCD phase diagram, and convey information about the state of the strongly interacting system at the moment their preceding off-shell photon is created. In this study, we compute thermal dilepton yields from Au+Au collisions performed at different beam energies, employing a (3+1)-dimensional dynamic framework combined with emission rates accurate at next-to-leading order in perturbation theory and which include baryon chemical potential dependencies. By comparing the effective temperature extracted from the thermal dilepton invariant mass spectrum with the average temperature of the fluid, we offer a robust quantitative validation of dileptons as an effective probe of the early quark-gluon plasma stage.

Two-Pion Bose–Einstein Correlations in Au+Au Collisions at sNN = 3 GeV in the STAR Experiment

The correlation femtoscopy technique makes it possible to estimate the geometric dimensions and lifetime of the particle emission region after the collision of ions. Measurements of the emission region characteristics not only at midrapidity but also at backward (forward) rapidity can provide new information about the source and make it possible to impose constraints on the heavy-ion collision models. This work is devoted to revealing the dependence of the spatial and temporal parameters of the emission region of identical pions in Au+Au collisions at sNN = 3 GeV from the fixed-target program of the STAR experiment. The extracted femtoscopic radii, Rout, Rside, Rlong, Rout−long2, and the correlation strength, λ, are presented as a function of collision centrality, pair rapidity, and transverse momentum. Physics implications will be discussed.

Extracting Kinetic Freeze-Out Properties in High-Energy Collisions Using a Multisource Thermal Model

We study the transverse momentum (pT) spectra of neutral pions and identified charged hadrons produced in proton–proton (pp), deuteron–gold (d–Au), and gold–gold (Au–Au) collisions at the center of mass energy sNN=200 GeV. The study is made in the framework of a multisource thermal model used in the partonic level. It is assumed that the contribution to the pT-value of any hadron comes from two or three partons with an isotropic distribution of the azimuthal angle. The contribution of each parton to the pT-value of a given hadron is assumed to obey any one of the standard (Maxwell-Boltzmann, Fermi-Dirac, and Bose-Einstein) distributions with the kinetic freeze-out temperature and average transverse flow velocity. The pT spectra of the final-state hadrons can be fitted by the superposition of two or three components. The results obtained from our Monte Carlo method are used to fit the experimental results of the PHENIX and STAR Collaborations. The results of the present work serve as a suitable reference baseline for other experiments and simulation studies.

Evolution of topological charge through chiral anomaly transport

Built upon the state-of-the-art model a multiphase transport, we develop a new module of chiral anomaly transport (CAT) to trace the evolution of the initial topological charge of the gauge field created through sphaleron transition at finite temperature and the external magnetic field in heavy-ion collisions. The eventual experimental signals of the chiral magnetic effect (CME) have been measured. The CAT explicitly shows the generation and evolution of the charge separation, the signals of the CME through the CAT are quantitatively in agreement with the experimental measurements in Au+Au collisions at s=200 GeV, and the centrality dependence of the CME fraction follows that of the fireball temperature. Our simulation supports the generation of the CME. Published by the American Physical Society 2024

Extracting the hyperon-nucleon interaction via collective flows in heavy-ion collisions

The 2-body hyperon-nucleon interaction softens the equation of state of neutron star matter and leads to the maximum mass of neutron star to be 1.3-1.6M⊙, which deviates from the recent observation with the masses of PSR J0348+0432 (2.01±0.04M⊙) and J0740+6620 (2.14−0.09+0.10M⊙). One attempt is to introduce the more repulsive hyperon-nucleon interaction above the saturation density. A phenomenological Λ potential by fitting the 3-body ΛNN results of chiral effective field theory is implemented into the quantum molecular dynamics transport model to solve the ‘hyperon puzzle’ in neutron stars. It is found that the directed and elliptic flows are sensitive to the high-density hyperon-nucleon potentials in collisions of 197Au + 197Au and 124Sn + 124Sn. The influence of the phenomenological potential on the Λ yields, rapidity distributions and transverse momentum spectra is negligible in comparison with the ones calculated by the well-known relativistic mean-field model. The inclusion of the ΛN, ΣN and ΞN potentials leads to the reduction of hyperon production in the midrapidity region. The Λ directed flows and the slope in the midrapidity region are enhanced with the phenomenological potential and more consistent with the STAR data in collisions of 197Au + 197Au at sNN=3 GeV.

System-size dependence of collective transverse flow in relativistic nuclear collisions

The transverse momentum distributions of identified hadrons (π±, K±, p, p, K0S, Λ, Λ, Ξ−, Ξ+, φ, Ω−, and Ω+) produced in Au+Au collisions at RHIC-BES energies are analyzed with Blast-Wave model and with Boltzmann-Gibbs statistics. The kinetic freeze-out temperature, the average transverse flow velocity and the flow profile exponent are extracted from the transverse momentum spectra of the particles. The study of system-size dependence of the kinetic freeze-out parameters showed that kinetic freeze-out temperature increases, while average transverse flow ve- locity decreases from central to peripheral collisions. The larger freeze-out temperature for peripheral collisions may indicate an earlier decoupling of the expanding system. For the same centrality class, it is observed that the transverse flow velocity and flow profile exponent have an increasing trend with increasing collision energy.

Nuclear Modification Factor of Inclusive Charged Particles in Au+Au Collisions at sNN = 27 GeV with the STAR Experiment

The Beam Energy Scan (BES) program at RHIC aims to explore the QCD phase diagram, including the search for the evidence of the 1st order phase transition from hadronic matter to Quark-Gluon Plasma (QGP) and the location of the QCD critical point. One of the features previously observed in the study of QGP is the effect of suppression of particle production with high transverse momenta pT (>2 GeV/c) at energies sNN = 62.4–200 GeV, which was deduced from the charged-particle nuclear modification factor (RCP) measured using the data from Beam Energy Scan Program Phase I (BES-I) of STAR experiment. In 2018, STAR has collected over 500 million events from Au+Au collisions at sNN = 27 GeV as a part of the STAR BES-II program, which is about a factor of 10 higher than BES-I 27 GeV data size. In this report, we present new measurements of charged particle production and the nuclear modification factor RCP, from this new 27 GeV data set and compare them with the BES-I results. The new measurements extend the previous BES-I results to higher transverse momentum range, which allows better exploration of the jet quenching effects at low RHIC energies, and may help to understand the effects of the formation and properties of QGP at these energies.

Examining the influence of hadronic interactions on the directed flow of identified particles in RHIC Beam Energy Scan energies using UrQMD model

The directed flow of identified particles can serve as a sensitive tool for investigating the interactions during initial and final states in heavy ion collisions. This study examines the rapidity distribution ([Formula: see text]), rapidity-odd directed flow ([Formula: see text]) and its slope ([Formula: see text]) for [Formula: see text], [Formula: see text], p, and [Formula: see text] in Au+Au collisions at different collision centralities and beam energies ([Formula: see text], 11.5, 14.5, 19.6, 27, and 39[Formula: see text]GeV) using the UrQMD model. We investigate the impact of late-stage hadronic interactions on charge-dependent [Formula: see text] and its slope by modifying the duration of the hadronic cascade lifetime ([Formula: see text]). The energy dependence of [Formula: see text] for p ([Formula: see text]) exhibits distinct pattern compared to [Formula: see text] and [Formula: see text]. Notably, we observe a change in the sign reversal position of proton [Formula: see text] at different beam energies with varying [Formula: see text] in central and mid-central collisions. Moreover, the difference in [Formula: see text] between positively and negatively charged hadrons ([Formula: see text]) demonstrates a stark centrality dependence for different particle species. The deuteron displays a significant increase in [Formula: see text] with increasing [Formula: see text] compared to p and n. This investigation underscores the importance of considering the temporal evolution and duration of the hadronic phase when interpreting the sign reversal, charge splitting of [Formula: see text] and light nuclei formation at lower RHIC energies.

Analysis of [formula omitted] spectra for [formula omitted] mesons in [formula omitted]–[formula omitted] collisions at 200 GeV, using PYTHIA and Tsallis function

The ϕ(1020)0 mesons produced by asymmetric Cu–Au collisions at sNN=200GeV in the pseudorapidity region of |η|<0.35 is studied in relation to the transverse momentum (pT). The nuclear modification factor (RAB) is also calculated for ϕ(1020)0 mesons. To validate the model’s predictions, the simulations are compared to the experimental data from PHENIX using the PYTHIA8 and tuned to color reconnection mode. A systematically close relationship exists between PYTHIA8 predictions and the PHENIX data. PYTHIA CR1 mode predictions do not agree well with PHENIX experimental data. In addition to the RAB predictions, we investigated the ratio between the production of ϕ(1020)0 mesons to K∗(892)0 mesons and Λ baryons in Cu–Au collisions at sNN=200GeV. A comparison of the ϕ(1020)0 mesons yield produced by symmetric Au–Au and Cu–Cu collisions at sNN=200GeV are conducted using PYTHIA8 for comparison against asymmetric Cu–Au collisions. In this study, we also fit thermodynamically consistent Tsallis distribution functions for ϕ(1020)0 mesons onto experimental RHIC (PHENIX) pT spectra obtained from Cu–Au collisions at sNN=200GeV energy.

First-Order Event Plane Correlated Directed and Triangular Flow from Fixed-Target Energies at RHIC-STAR

We report the measurement of first-order event plane-correlated directed flow (v1) and triangular flow (v3) for identified hadrons (π±, K±, and p), net particle (net-K, net-p), and light nuclei (d and t) in Au + Au collisions at sNN = 3.2, 3.5, and 3.9 GeV in the fixed-target mode from the second phase of the beam energy scan (BES-II) program at RHIC-STAR. The v1 slopes at mid-rapidity for identified hadrons and net particles except π+ are found to be positive, implying the effect of dominant repulsive baryonic interactions. The slope of v1 for net-kaon undergoes a sign change from negative to positive at a lower collision energy compared to net-proton. An approximate atomic mass number scaling is observed in the measured v1 slopes of light nuclei at mid-rapidity, which favors the nucleon coalescence mechanism for the production of light nuclei. The v3 slope for all particles decreases in magnitude with increasing collision energy, suggesting a notable integrated impact of the mean-field, baryon stopping, and collision geometry at lower collision energies.

Observation of the Electromagnetic Field Effect via Charge-Dependent Directed Flow in Heavy-Ion Collisions at the Relativistic Heavy Ion Collider

The deconfined quark-gluon plasma (QGP) created in relativistic heavy-ion collisions enables the exploration of the fundamental properties of matter under extreme conditions. Noncentral collisions can produce strong magnetic fields on the order of 1018 G, which offers a probe into the electrical conductivity of the QGP. In particular, quarks and antiquarks carry opposite charges and receive contrary electromagnetic forces that alter their momenta. This phenomenon can be manifested in the collective motion of final-state particles, specifically in the rapidity-odd directed flow, denoted as v1(y). Here, we present the charge-dependent measurements of dv1/dy near midrapidities for π±, K±, and p(p¯) in Au+Au and isobar (Ru4496+Ru4496 and Zr4096+Zr4096) collisions at sNN=200 GeV, and in Au+Au collisions at 27 GeV, recorded by the STAR detector at the Relativistic Heavy Ion Collider. The combined dependence of the v1 signal on collision system, particle species, and collision centrality can be qualitatively and semiquantitatively understood as several effects on constituent quarks. While the results in central events can be explained by the u and d quarks transported from initial-state nuclei, those in peripheral events reveal the impacts of the electromagnetic field on the QGP. Our data put valuable constraints on the electrical conductivity of the QGP in theoretical calculations. Published by the American Physical Society 2024

Pion Interferometry with Lévy-Stable Sources in sNN = 200 GeV Au + Au Collisions at STAR

Measurements of femtoscopic correlations in high-energy heavy-ion collisions are used to unravel the space–time structure of the particle-emitting source (the quark–gluon plasma). Recent results indicate that the pion pair source exhibits a power law behavior and can be described well by a Lévy distribution. In this study, Lévy fits were applied to the measured one-dimensional two-pion correlation functions in Au + Au collisions at sNN = 200 GeV. The three extracted source parameters are the Lévy scale parameter, R, which relates to the size of the source; the correlation strength parameter, λ; and the Lévy exponent, α, which characterizes the power law tail of the source. In this paper, we report the current status of the analysis of the extracted Lévy source parameters and present their dependence on average transverse mass, mT, and on centrality.

Momentum correlation of light nuclei in Au + Au collisions at [formula omitted] GeV

Within the Ultra-relativistic Quantum Molecular Dynamics model (UrQMD) coupled with nucleon coalescence model and Mini-Spanning-Tree model, the yields of light nuclei have been stimulated in Au + Au collisions over an energy range of sNN=2.0∼7.7GeV and the momentum correlation functions of light nuclei pairs have been calculated by both the Lednický-Lyuboshitz and the Correlation After Burner methods. We compare the yields of light nuclei and their momentum correlation functions at midrapidity in this energy region with experimental data. It is found that there are differences between the results of the two models, and the coalescence method seems less valid at low collision energy. Furthermore, both the peak values of proton-proton correlation functions and the transition point of elliptic flows from out-of-plane to in-plane emission show a turning point around 3-4 GeV, which suggests that there is a relation between momentum correlation function and collective flow of particles.

Describing the thermal radiation in Au+Au collisions at sNN=200 GeV by an analytic solution of relativistic hydrodynamics

In high-energy heavy-ion collisions, a nearly perfect fluid, the so-called strongly coupled quark–gluon plasma (QGP), forms. After the short period of thermalization, the evolution of this medium can be described by the laws of relativistic hydrodynamics. The time evolution of the QGP can be understood through direct photon spectra measurements, which are sensitive to the entire period between the thermalization and the freeze-out of the medium. I present a new analytic formula that describes the thermal photon radiation and it is derived from an exact and finite solution of relativistic hydrodynamics with accelerating velocity field. Then I compare my calculations to the most recent nonprompt spectrum of direct photons for [Formula: see text] at [Formula: see text][Formula: see text]GeV collisions. I have found a convincing agreement between the model and the data, which allows to give an estimate of the initial temperature in the center of the fireball. My results predict hydrodynamic scaling behavior for the thermal photon spectra of high-energy heavy-ion collisions.

Hyperon polarization in heavy ion collisions at STAR

In these proceedings, we present the measurements of global polarization for Λ, ¯Λ with the high-statistics data collected by the STAR experiment for isobar (Ru+Ru, Zr+Zr) collisions at √SNN = 200 GeV and Au+Au collisions at √SNN = 19.6, 27 GeV. These measurements allow us to study possible magnetic field driven effects through the polarization difference between Λ and ¯Λ and system size dependence of global polarization. Furthermore, we present the first measurements of Λ, ¯Λ hyperon local polarization in isobar collisions at √SNN = 200 GeV and Au+Au collisions at √SNN = 19.6, 27 GeV. Comparisons with previous measurements in Au+Au and Pb+Pb collisions at RHIC and LHC provide important insights into the collision system size and energy dependence of the vorticities. The local polarization measurements at lower beam energies can probe the predicted baryonic spin hall effect in a dense baryonic environment in heavy-ion collisions.