Top RHIC Research Articles

Controlling volume fluctuations for studies of critical phenomena in nuclear collisions

We generalize and extend the recently proposed method [1] to account for contributions of system size (or volume/participant) fluctuations to the experimentally measured moments of particle multiplicity distributions. We find that in the general case there are additional biases which are not directly accessible to experiment. These biases are, however, parametrically suppressed if the multiplicity of the particles of interest is small compared to the total charged-particle multiplicity, e.g., in the case of proton number fluctuations at top RHIC and LHC energies. They are also small if the multiplicity distribution of charged particles per wounded nucleon is close to the Poissonian limit, which is the case at low energy nuclear collisions, e.g., at GSI/SIS18. We further find that mixed events are not necessarily needed to extract the correction for volume fluctuations. We provide the formulas to correct pure and mixed cumulants of particle multiplicity distributions up to any order together with their associated biases.

Recent results from fixed-target collisions at LHCb

The LHCb spectrometer has the unique capability to function as a fixed-target experiment by injecting gas into the LHC beam pipe while proton or ion beams are circulating. The resulting beam+gas collisions have a center of mass energy intermediate to previous fixed-target experiments and the top RHIC energy for AA collisions, cover an unexplored kinematic range and allow systems of different size to be studied. Here we present new results on open charm, J/ψ, and ψ(2S) production from pNe and PbNe fixed-target collisions at LHCb. Comparisons with various theoretical models of particle production and transport through the nucleus will be discussed.

STAR Overview

The STAR experiment covers a wide range of energies and collisions thanks to RHIC’s versatility and STAR’s recent detector upgrades. In this contribution to the proceedings of the 2023 QuarkMatter conference, we highlight our new results from the Beam Energy Scan, system-size scans at top RHIC energies using a broad range of experimental probes.

Measuring deformed neutron skin with free spectator nucleons in relativistic heavy-ion collisions

The neutron skin in deformed nuclei is generally not uniformly distributed but has an angular distribution, depending on both the spin-dependent nuclear interaction and the nuclear symmetry energy. To extract the information of the deformed neutron skin, we have explored the possibility of using free spectator nucleons in central tip-tip and body-body collisions at top RHIC energy with four typical deformed nuclei. The density distributions of neutrons and protons are consistently obtained from the Skyrme-Hartree-Fock-Bogolyubov calculation, and the angular distribution of the neutron skin can be varied by adjusting the strength of the nuclear spin-orbit coupling. With the information of spectator nucleons obtained based on a Monte-Carlo Glauber model, the free spectator nucleons are generated from a multifragmentation process. By investigating the results from different systems and with different collision configurations, we found that although it is difficult to probe the deformed neutron skin in 96Zr and 238U by their collisions, it is promising to extract the polar angular distributions of the neutron skin in 96Ru and 197Au by comparing the yield ratios of free spectator neutrons to protons in their central tip-tip and body-body collisions. The proposed observables can be measured by dedicated zero-degree calorimeters in heavy-ion collision experiments that have been carried out in recent years by RHIC.

Experimental Status of the Chiral Magnetic Effect

Experimental searches for definitive evidence of the Chiral Magnetic Effect in heavy-ion collisions have become increasingly ingenious over the past decade, but a clear separation of any CME signal from flow-related backgrounds remains elusive. Although the isobar analysis by STAR does not appear to show a signal given the current preliminary background estimate, there is a tantalizing statistically significant signal in Au-Au at top RHIC energy using a spectator plane versus participant plane analysis. I discuss the status of these works in some detail, and discuss the possible advantage of an analysis that takes advantage of the expected correlation between the parity-odd event-by-event CME signal and other parity-odd measures such as the net hyperon helicity.

Femtoscopy of Proton, Light Nuclei, and Strange Hadrons in Au\(+\)Au Collisions at STAR

In these proceedings, we present the measurements of proton, light nuclei, and strange particle with neutral kaons correlation functions in Au+Au collisions at the BES program and top RHIC energy. The experimental results will be compared with model calculations to extract the size of emitting source and the properties of final state interactions. The collision energy and centrality dependence of the source size will be studied. Further, the implications for the production mechanism of light nuclei will be discussed.

Longitudinal dynamics and particle production in relativistic nuclear collisions

This work presents a three-dimensional dynamical initialization model for relativistic heavy-ion collisions, implementing local energy-momentum conservation and baryon charge fluctuations at string junctions. Constraining parameters using experimental data from p+p collisions at various collision energies, the model provides a very good description of the charged hadron and net proton rapidity distributions in Au+Au collisions from 7.7 to 200 GeV and Pb+Pb collisions at 8.77 and 17.3 GeV. We demonstrate the importance of fluctuations of baryon densities to string junctions for describing net-proton distributions at collision energies of 62.4 and 200 GeV. Including this improved baryon stopping description along with the requirement of strangeness neutrality also yields a good description of identified particle yields as functions of the collision energy above 7.7 GeV. We further study asymmetric p+Al and (p, d, $^3$He)+Au collisions at the top RHIC energy and p+Pb, Xe+Xe, and Pb+Pb collisions at LHC energies. We identify the produced particle rapidity distributions in asymmetric collision systems as particularly useful for constraining models of the early-time longitudinal dynamics.

Insight from the elliptic flow of identified hadrons measured in relativistic heavy-ion collisions

In this paper, we have discussed the transverse momentum dependence of elliptic flow measured in relativistic heavy-ion collisions. Under the assumption of quark recombination model, [Formula: see text] of light and strange quarks is obtained from the measured [Formula: see text] of identified hadrons. The [Formula: see text] of strange quarks obtained from [Formula: see text]-meson [Formula: see text] is found to be similar to that of from [Formula: see text] [Formula: see text] in Au+Au collisions at [Formula: see text][Formula: see text]GeV. This may indicate that both [Formula: see text] and [Formula: see text] are produced through quark recombination at top RHIC energy. However, we find that the [Formula: see text] of light quarks is consistently higher than that of strange quarks at the low transverse momentum. We have shown that [Formula: see text] of light and strange quarks is consistent with each other when plotted as a function of its transverse kinetic energy. The observed scaling of [Formula: see text] works at both RHIC and LHC energies. It is also shown that the observed scaling holds for other flow harmonics, e.g., [Formula: see text]. The new observed scaling is purely empirical and needs more theoretical input to understand the physics behind it.

Triple nuclear collisions – a new method to explore the matter properties under new extreme conditions

We suggest to explore an entirely new method to experimentally and theoretically study the phase diagram of strongly interacting matter based on the triple nuclear collisions (TNC).We simulated the TNC using the UrQMD 3.4 model at the beam center of- mass collision energies √SNN = 200 GeV and √SNN = 2.76 TeV. It is found that in the most central and simultaneous TNC the initial baryonic charge density is about 3 times higher than the one achieved in the usual binary nuclear collisions at the same energies. As a consequence, the production of protons and Λ-hyperons is increased by a factor of 2 and 1.5, respectively. Using the MIT Bag model equation we study the evolution of the central cell in TNC and demonstrate that for the top RHIC energy of collision the baryonic chemical potential is 2-2.5 times larger than the one achieved in the binary nuclear collision at the same time of reaction. Based on these estimates, we show that TNC offers an entirely new possibility to study the QCD phase diagram at very high baryonic charge densities.

Influence of correlations between yields on the chemical freeze-out temperature

A statistical (thermal) model is applied to the description of hadron yields measured at central nucleus-nucleus collisions at the top RHIC energy sNN=200 GeV and the LHC energy sNN=2.76 TeV. In contrast to the previous analyzes a more general form of the least squares test statistic is used, which takes into account also possible correlations between different species of yields. In addition to hadrons also light nuclei are included in the fits. Because of the lack of data, a toy model is constructed where the correlation coefficients are just free parameters. Because of this the presented considerations are speculative and should be treated as an imperfect illustration of the problem. Within these limitations it is impossible to formulate any definite conclusion, it might be only mentioned that for the considered examples the dependence of the freeze-out temperature and baryon chemical potential on correlations turned out to be weak.

Extraction of the specific shear viscosity of quark-gluon plasma from two-particle transverse momentum correlations

The specific shear viscosity, eta /s, of the quark-gluon plasma formed in ultrarelativistic heavy-ion collisions at RHIC and LHC is estimated based on the progressive longitudinal broadening of transverse momentum two-particle correlators, G_2, reported as a function of collision centrality by the STAR and ALICE experiments. Estimates are computed as a function of collision centrality using the Gavin ansatz which relates the G_2 longitudinal broadening to the specific shear viscosity. Freeze out times required for the use of the ansatz are computed using a linear fit of freeze out times reported as a function of the cubic root of the charged particle pseudorapidity density ({mathrm{d}}N_{mathrm{ch}}/deta )^{1/3}. Estimates of eta /s based on ALICE data exhibit little to no dependence on collision centrality at LHC energy, while estimates obtained from STAR data hint that eta /s might be a function of collision centrality at top RHIC energy.

Electromagnetic fields from the extended Kharzeev-McLerran-Warringa model in relativistic heavy-ion collisions

Based on the Kharzeev-McLerran-Warringa (KMW) model that estimates strong electromagnetic (EM) fields generated in relativistic heavy-ion collisions, we generalize the formulas of EM fields in the vacuum by incorporating the longitudinal position dependence, the generalized charge distributions and retardation correction. We further generalize the formulas of EM fields in the pure quark-gluon plasma (QGP) medium by incorporating a constant Ohm electric conductivity and also during the realistic early-time stages QGP evolution by using a time-dependent electric conductivity. Using the extended KMW model, we observe a slower time evolution and a more reasonable impact parameter b dependence of the magnetic field strength than those from the original KMW model in the vacuum. The inclusion of medium effects by using the lattice data helps to further prolong the time evolution of magnetic field, such that the magnetic field strength during the realistic QGP evolution at thermal freeze-out time can meet the 1σ bound constrained from experimentally measured difference in global polarizations of Λ and Λ¯ hyperons in Au+Au collisions at top RHIC energy. These generalized formulations in the extended KMW model will be potentially useful for many EM fields relevant studies in heavy-ion collisions, especially at lower colliding energies and for various species of colliding nuclei.

Baseline predictions of elliptic flow and fluctuations for the RHIC Beam Energy Scan using response coefficients

Currently the RHIC Beam Energy Scan is exploring a new region of the Quantum Chromodynamic phase diagram at large baryon densities that approaches nuclear astrophysics regimes. This provides an opportunity to study relativistic hydrodynamics in a regime where the net conserved charges of baryon number, strangeness, and electric charge play a role, which will significantly change the theoretical approach to simulating the baryon-dense Quark-Gluon Plasma. Here we detail many of the important changes needed to adapt both initial conditions and the medium to baryon-rich matter. Then, we make baseline predictions for the elliptical flow and fluctuations based on extrapolating the physics at LHC and top RHIC energies to support future analyses of where and how the new baryon-dense physics causes these extrapolations to break down. First we compare eccentricities across beam energies, exploring their underlying assumptions; we find the the extrapolated initial state is predicted to be nearly identical to that at AuAu $\sqrt{s_{NN}}=200$ GeV. Then the final flow harmonic predictions are based on linear+cubic response. We discuss preliminary STAR results in order to determine the implications that they have for linear+cubic response coefficients at the lowest beam energy of AuAu $\sqrt{s_{NN}}=7$ GeV.

Correlation analysis of high-energy heavy-ion collisions within the integrated hydrokinetic model

The interferometry radii, obtained from the Gaussian fits to the two-particle momentum correlation functions for different high-energy heavy-ion collision experiments at the LHC and the top RHIC energies, are described in the integrated hydrokinetic model. The role of the different geometrical sizes of the colliding ions in the formation of the observed femtoscopy scales is investigated by comparing the results for Au+Au, Pb+Pb and Xe+Xe collisions at close values of the mean charged particle multiplicity.

Modification of hadron production in small and large systems observed by PHENIX

Hadron production in p + p, p+A, A+A collisions gives possibility to look inside Quark Gluon Plasma, allowing to study its properties and characteristics. It is one of the main objectives for the high-energy nuclear physics. PHENIX has performed measurements of π 0, η, Ks , K*, φ and ω meson production in p + p, p+Al, p(d,3He)+Au, Cu+Cu, Cu+Au, Au+Au and U+U collisions at top RHIC energies. This rich collection of data give opportunity for detailed studies of the cold and hot nuclear matter effects from small to large systems. The paper presents obtained spectra and nuclear modification factors and theoretical model predictions where available.

Energy-Dependent Chemical Potentials of Light Hadrons and Quarks Based on Transverse Momentum Spectra and Yield Ratios of Negative to Positive Particles

We describe the transverse momentum spectra or transverse mass spectra of π ± , K ± , p , and p ¯ produced in central gold-gold (Au-Au), central lead-lead (Pb-Pb), and inelastic proton-proton (pp) collisions at different collision energies range from the AGS to LHC by using a two-component (in most cases) Erlang distribution in the framework of multisource thermal model. The fitting results are consistent with the experimental data, and the final-state yield ratios of negative to positive particles are obtained based on the normalization constants from the above describing the transverse momentum (or mass) spectra. The energy-dependent chemical potentials of light hadrons ( π , K , and p ) and quarks ( u , d , and s ) in central Au-Au, central Pb-Pb, and inelastic (pp) collisions are then extracted from the modified yield ratios in which the contributions of strong decay from high-mass resonance and weak decay from heavy flavor hadrons are removed. The study shows that most types of energy-dependent chemical potentials decrease with increase of collision energy over a range from the AGS to LHC. The curves of all types of energy-dependent chemical potentials, obtained from the fits of yield ratios vs. energy, have the maximum at about 3.510 GeV, which possibly is the critical energy of phase transition from a liquid-like hadron state to a gas-like quark state in the collision system. At the top RHIC and LHC, all types of chemical potentials become small and tend to zero at very high energy, which confirms that the high energy collision system possibly changes completely from the liquid-like hadron-dominant state to the gas-like quark-dominant state and the partonic interactions possibly play a dominant role at the LHC.

Femtoscopy scales and particle production in the relativistic heavy ion collisions from Au+Au at 200 AGeV to Xe+Xe at 5.44 ATeV within the integrated hydrokinetic model

The recent results on the main soft observables, including hadron and photon yields and particle number ratios, $p_T$ spectra, flow harmonics, as well as the femtoscopy radii, obtained within the integrated hydrokinetic model (iHKM) for high-energy heavy-ion collisions are reviewed and re-examined. The cases of different nuclei colliding at different energies are considered: Au+Au collisions at the top RHIC energy $\sqrt{s_{NN}}=200$ GeV, Pb+Pb collisions at the LHC energies $\sqrt{s_{NN}}=2.76$ TeV and $\sqrt{s_{NN}}=5.02$ TeV, and the LHC Xe+Xe collisions at $\sqrt{s_{NN}}=5.44$ TeV. The effect of the initial conditions and the model parameters, including the utilized equation of state (EoS) for quark-gluon phase, on the simulation results, as well as the role of the final afterburner stage of the matter evolution are discussed. The possible solution of the so-called ``photon puzzle'' is considered. The attention is also paid to the dependency of the interferometry volume and individual interferometry radii on the initial transverse geometrical size of the system formed in the collision.

Influence of the electromagnetic fields on hadronic observables in proton-induced collisions

We investigate proton-gold collisions at top RHIC energy GeV within the Parton-Hadron-String Dynamics (PHSD) approach, focusing on the influence of the electromagnetic fields on final hadronic observable. We discuss the space-time distribution of the magnetic and electric field components, showing that the electric field Ex along the impact parameter direction is comparable in magnitude to the magnetic field By perpendicular to the reaction plane. We discuss the effect of these fields on the directed flow υ1 of pions and kaons. While there is no visible effect of electromagnetic fields in 5% central collisions, the result for collisions at fixed impact parameter clearly indicates an electromagnetically-induced splitting in the v1 of positively and negatively charged mesons; the effect, driven by the huge Ex component, is larger for kaons than to pions and increases with the impact parameter of the collision.

Feeddown contributions from unstable nuclei in relativistic heavy-ion collisions

We estimate the feeddown contributions from decays of unstable A=4 and A=5 nuclei to the final yields of protons, deuterons, tritons, 3He, and 4He produced in relativistic heavy-ion collisions at sNN>2.4 GeV, using the statistical model. The feeddown contribution effects do not exceed 5% at LHC and top RHIC energies due to the large penalty factors involved, but are substantial at intermediate collision energies. We observe large feeddown contributions for tritons, 3He, and 4He at sNN≲10 GeV, where they may account for as much as 70% of the final yield at the lower end of the collision energies considered. Sizable (>10%) effects for deuteron yields are observed at sNN≲4 GeV. The results suggest that the excited nuclei feeddown cannot be neglected in the ongoing and future analysis of light nuclei production at intermediate collision energies, including HADES and CBM experiments at FAIR, NICA at JINR, RHIC beam energy scan and fixed-target programmes, and NA61/SHINE at CERN. We further show that the freeze-out curve in the T-μB plane itself is affected significantly by the light nuclei at high baryochemical potential.

Status of CME Search Before Isobar Collisions and Methods of Blind Analysis From STAR

The STAR collaboration is currently pursuing the blind analysis of the data for isobar collisions that was performed at RHIC in the year 2018 to make a decisive test of the Chiral Magnetic Effect (CME). Why is it so difficult to detect signals of CME in the experiment? Do we really understand different sources of background? Why observing similar charge separation between p/d + A and A + A does not stop us from pursuing the search for CME? In this contribution, I attempt to address some of these questions and briefly outline a few recent STAR analyses based on new methods and observables to isolate the possible CME-driven signal and non-CME background contributions at the top RHIC energy. Finally, I describe the procedure for the blind analysis of the isobar data. An outstanding question remains – what happens if we go down in energy? I address this by discussing how the new event-plane detector (EPD) upgrade provides a new capability at STAR towards CME search using the data from the RHIC BES-II program.