BNL Relativistic Heavy Ion Collider Research Articles

Hard jet substructure in a multistage approach

We present predictions and postdictions for a wide variety of hard jet-substructure observables using a multistage model within the framework. The details of the multistage model and the various parameter choices are described in []. A novel feature of this model is the presence of two stages of jet modification: a high-virtuality phase [modeled using the modular all twist transverse-scattering elastic-drag and radiation model ()], where modified coherence effects diminish medium-induced radiation, and a lower virtuality phase [modeled using the linear Boltzmann transport model ()], where parton splits are fully resolved by the medium as they endure multiple scattering induced energy loss. Energy-loss calculations are carried out on event-by-event viscous fluid dynamic backgrounds constrained by experimental data. The uniform and consistent descriptions of multiple experimental observables demonstrate the essential role of modified coherence effects and the multistage modeling of jet evolution. Using the best choice of parameters from [], and with no further tuning, we present calculations for the medium modified jet fragmentation function, the groomed jet momentum fraction zg and angular separation rg distributions, as well as the nuclear modification factor of groomed jets. These calculations provide accurate descriptions of published data from experiments at the Large Hadron Collider. Furthermore, we provide predictions from the multistage model for future measurements at the BNL Relativistic Heavy Ion Collider. Published by the American Physical Society 2024

Heavy-flavor hadron production in relativistic heavy ion collisions at energies available at BNL RHIC and at the CERN LHC in the EPOS4HQ framework

Employing the recently developed EPOS4HQ event generator, we study the production of different heavy-flavor mesons in relativistic heavy-ion collisions at BNL Relativistic Heavy Ion Collider and CERN Large Hadron Collider energies. The transverse momentum spectra, yield ratio, nuclear modification factor, and elliptic flow can be well described in the EPOS4HQ framework. We furthermore analyze the processes which modify these observables as compared to pp collisions and are at the origin of the experimentally determined nuclear modification factor RAA. Published by the American Physical Society 2024

Characterizing the initial- and final-state effects in isobaric collisions at energies available at the BNL Relativistic Heavy Ion Collider

Characterizing the initial- and final-state effects in isobaric collisions at energies available at the BNL Relativistic Heavy Ion Collider

Probing vortical structures in heavy-ion collisions at RHIC-BES energies through helicity polarization

We investigate the azimuthal angle dependent local hydrodynamic helicity polarization of Λ hyperons, defined as the projection of the spin polarization vector along the directions of particle momenta, at BNL Relativistic Heavy Ion Collider beam energy scan energies by utilizing the relativistic (3+1)D CLVisc hydrodynamics framework with SMASH initial conditions. As opposed to local spin polarization at high energy collisions, our hydrodynamic simulations demonstrate that the azimuthal angle dependent helicity polarization induced by the kinetic vorticity dominates over other contributions at intermediate and low collision energies. Our findings provide an opportunity to probe the fine structure of local kinetic vorticity as a function of azimuthal angle at intermediate and low collision energies by mapping our predictions to the future measurements in experiments. Published by the American Physical Society 2024

Search for the chiral magnetic wave using anisotropic flow of identified particles at energies available at the BNL Relativistic Heavy Ion Collider

Search for the chiral magnetic wave using anisotropic flow of identified particles at energies available at the BNL Relativistic Heavy Ion Collider

Dynamical mechanisms for deuteron production at mid-rapidity in relativistic heavy-ion collisions from energies available at the GSI Schwerionensynchrotron to those at the BNL Relativistic Heavy Ion Collider

Dynamical mechanisms for deuteron production at mid-rapidity in relativistic heavy-ion collisions from energies available at the GSI Schwerionensynchrotron to those at the BNL Relativistic Heavy Ion Collider

Cumulants of net-strangeness multiplicity distributions at energies available at the BNL Relativistic Heavy Ion Collider

The higher-order cumulants of net-proton number, net-charge, and net-strangeness multiplicity distributions are widely studied to search for the quantum-chromodynamics critical point and extract the chemical freeze-out parameters in heavy-ion collisions. In this context, the event-by-event fluctuations of the net-strangeness multiplicity distributions play important roles in extracting the chemical freeze-out parameter in the strangeness sector. Due to having difficulties in detecting all strange hadrons event by event, the kaon ($K$) and lambda ($\Lambda$) particles serve as a proxy for the strangeness-related observables in heavy-ion collisions. We have studied the net-$K$, net-$\Lambda$, and net-($K$ + $\Lambda$) multiplicity distributions and calculated their different order of cumulants using the ultrarelativistic quantum molecular dynamics model and hadron resonance gas calculation. To adequately account for the net-strangeness cumulants, it has been found that the inclusion of resonance decay contributions in $K$ and $\Lambda$ is necessary.

Momentum-dependent flow correlations in deformed nuclei at collision energies available at the BNL Relativistic Heavy Ion Collider

Flow fluctuations in ultrarelativistic heavy-ion collisions can be probed by studying the momentum dependent correlations or the factorization-breaking coefficients between flow harmonics in separate kinematic bins (transverse momentum $p$ or pseudorapidity $\ensuremath{\eta}$). We study such factorization-breaking coefficients for collisions of deformed $^{238}\mathrm{U}+^{238}\mathrm{U}$ nuclei to see the effect of the nuclear deformation on momentum dependent coefficients. We also study momentum dependent mixed-flow correlations for the isobar collision system: $^{96}\mathrm{Ru}+^{96}\mathrm{Ru}$ and $^{96}\mathrm{Zr}+^{96}\mathrm{Zr}$, which have the same mass number but different nuclear structure, thus providing the ideal scenario to study nuclear deformation effect on such observables. We use the TRENTO $+$ MUSIC model for simulations and event-by-event analysis of those observables. We find that these momentum dependent correlation coefficients are not only excellent candidates to probe the fluctuation in heavy-ion collision, but also show significant sensitivity to the nuclear deformation.

Evidence of Hexadecapole Deformation in Uranium-238 at the Relativistic Heavy Ion Collider.

State-of-the-art hydrodynamic simulations of the quark-gluon plasma are unable to reproduce the elliptic flow of particles observed at the BNL Relativistic Heavy Ion Collider (RHIC) in relativistic ^{238}U+^{238}U collisions when they rely on information obtained from low-energy experiments for the implementation of deformation in the colliding ^{238}U ions. We show that this is due to an inappropriate treatment of well-deformed nuclei in the modeling of the initial conditions of the quark-gluon plasma. Past studies have identified the deformation of the nuclear surface with that of the nuclear volume, though these are different concepts. In particular, a volume quadrupole moment can be generated by both a surface hexadecapole and a surface quadrupole moment. This feature was so far neglected in the modeling of heavy-ion collisions, and is particularly relevant for nuclei like ^{238}U, which is both quadrupole deformed and hexadecapole deformed. With rigorous input from Skyrme density functional calculations, we show that correcting for such effects in the implementation of nuclear deformations in hydrodynamic simulations restores agreement with BNL RHIC data. This brings consistency to the results of nuclear experiments across energy scales, and demonstrates the impact of the hexadecapole deformation of ^{238}U on high-energy collisions.

Impact of hadronic interactions and conservation laws on cumulants of conserved charges in a dynamical model

Understanding the phase diagram of QCD by measuring fluctuations of conserved charges in heavy-ion collision is one of the main goals of the beam energy scan program at the BNL Relativistic Heavy Ion Collider (RHIC). Within this work, we calculate the role of hadronic interactions and momentum cuts on cumulants of conserved charges up to fourth order in a system in equilibrium within a hadronic transport approach (smash). In our model the net baryon, net charge, and net strangeness are perfectly conserved on an event-by-event basis and the cumulants are calculated as a function of subvolume sizes and compared to analytic expectations. We find a modification of the kurtosis due to charge annihilation processes in systems with simplified degrees of freedom. Furthermore the result of the full smash hadron gas for the net baryon and net proton number fluctuations is presented for systems with zero and finite values of baryon chemical potential. Additionally we find that due to dynamical correlations the cumulants of the net baryon number cannot easily be recovered from the net protons. Finally the influence of deuteron cluster formation on the net proton and net baryon fluctuations in a simplified system is shown. This analysis is important to better understand the relation between measurements of fluctuations in heavy-ion collisions and theoretical calculations which are often performed in a grand canonical ensemble.

Features of hadronic and deconfined matter in a range of collision energies spanning from the BNL Alternating Gradient Synchrotron to the CERN Large Hadron Collider

Previous extensive studies on the dependence of the average transverse momentum, its slope as a function of the hadron mass, and the average transverse expansion on the particle multiplicity per unit rapidity and unit transverse overlap area of the colliding partners are extended to the ratio of the energy density to the entropy density. The behavior of the ratio between the average transverse momentum and the square root of the particle multiplicity per unit rapidity and unit transverse overlap area $\ensuremath{\langle}{p}_{T}\ensuremath{\rangle}/\sqrt{\ensuremath{\langle}dN/dy\ensuremath{\rangle}/{S}_{\ensuremath{\perp}}}$ as a function of collision energy for a given centrality or as a function of centrality for a given collision energy supports the predictions of color glass condensate and percolation based approaches. The dependence of the ratio of the energy density $\ensuremath{\langle}d{E}_{T}/dy\ensuremath{\rangle}/{S}_{\ensuremath{\perp}}$ to the entropy density $\ensuremath{\langle}dN/dy\ensuremath{\rangle}/{S}_{\ensuremath{\perp}}$ at different collision centralities for $A\text{\ensuremath{-}}A$ collisions from the Alternating Gradient Synchrotron, the Super Proton Synchrotron, the BNL Relativistic Heavy Ion Collider, and the CERN Large Hadron Collider energies is presented. The trend of this ratio towards a plateau at the highest RHIC energies followed by a steep rise at LHC energies is in agreement with theoretical predictions made 40 years ago that indicate this behavior as a signature of a phase transition. This pattern strongly depends on the collision geometry, converging towards the dependence that characterizes the $pp$ minimum bias collisions for the most peripheral $A\text{\ensuremath{-}}A$ collisions. Expected similarities between $pp$ and Pb-Pb collisions at LHC energies are confirmed.

Low- pT direct-photon production in Au+Au collisions at sNN=39 and 62.4 GeV

The measurement of direct photons from Au+Au collisions at sNN=39 and 62.4 GeV in the transverse-momentum range 0.4<pT<3Gev/c is presented by the PHENIX collaboration at the BNLRelativistic Heavy Ion Collider. A significant direct-photon yield is observed in both collision systems. A universal scaling is observed when the direct-photon pT spectra for different center-of-mass energies and for different centrality selections at sNN=62.4 GeV is scaled with (dNch/dη)α for α=1.21±0.04. This scaling also holds true for direct-photon spectra from Au+Au collisions at sNN=200 GeV measured earlier by PHENIX, as well as the spectra from Pb+Pb at sNN=2760 GeV published by ALICE. The scaling power α seems to be independent of pT, center of mass energy, and collision centrality. The spectra from different collision energies have a similar shape up to pT of 2 Gev/c. The spectra have a local inverse slope Teff increasing with pT of 0.174±0.018Gev/c in the range 0.4<pT<1.3Gev/c and increasing to 0.289±0.024Gev/c for 0.9<pT<2.1Gev/c. The observed similarity of low-pT direct-photon production from sNN=39 to 2760 GeV suggests a common source of direct photons for the different collision energies and event centrality selections, and suggests a comparable space-time evolution of direct-photon emission.9 MoreReceived 24 March 2022Accepted 21 October 2022DOI:https://doi.org/10.1103/PhysRevC.107.024914©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasPhoton, lepton & quark productionRelativistic heavy-ion collisionsTechniquesHadron collidersNuclear Physics

Effect of global momentum conservation on longitudinal flow decorrelation

We calculate the longitudinal flow decorrelation coefficients, i.e., $r_n(\eta,\eta_r)$ for $n=2,3$, in the presence of hydro-like flow and the global momentum conservation (GMC) constraint. The longitudinal flow decorrelation is weakened due to the GMC constraint. The GMC effect is sensitive to the total number of particles involved in GMC, the average longitudinal momentum, the transverse momentum, and the reference pseudorapidity. Our results of the $r_2(\eta,\eta_{rA})/r_2(\eta,\eta_{rB})$ ratio between two reference pseudorapidity bins are consistent with the experimental measurements. We predict that the modification effect of GMC on longitudinal flow decorrelation is more noticeable at BNL Relativistic Heavy Ion Collider energies than at CERN Large Hadron Collider energies. Our finding provides a new perspective for understanding the longitudinal flow decorrelation in relativistic heavy-ion collisions.

3D structure of anisotropic flow in small collision systems at energies available at the BNL Relativistic Heavy Ion Collider

We present (3+1)D dynamical simulations of asymmetric nuclear collisions at the Relativistic Heavy Ion Collider (RHIC). Employing a dynamical initial state model coupled to (3+1)D viscous relativistic hydrodynamics, we explore the rapidity dependence of anisotropic flow in the RHIC small system scan at 200 GeV center of mass energy. We calibrate parameters to describe central $^3$He+Au collisions and make extrapolations to d+Au and p+Au collisions. Our calculations demonstrate that approximately 50% of the $v_3(p_T)$ difference between the measurements by the STAR and PHENIX Collaborations can be explained by the use of reference flow vectors from different rapidity regions. This emphasizes the importance of longitudinal flow decorrelation for anisotropic flow measurements in asymmetric nuclear collisions, and the need for (3+1)D simulations. We also present results for the beam energy dependence of particle spectra and anisotropic flow in d+Au collisions.

Directed flow in relativistic resistive magneto-hydrodynamic expansion for symmetric and asymmetric collision systems

We construct a dynamical model for high-energy heavy-ion collisions based on the relativistic resistive magneto-hydrodynamic framework. Using our newly developed ($3+1$)-dimensional relativistic resistive magneto-hydrodynamics code, we investigate magneto-hydrodynamic expansion in symmetric and asymmetric collision systems as the first application to high-energy heavy-ion collisions. As a realistic initial condition for electromagnetic fields, we consider the solutions of the Maxwell equations with the source term of point charged particles moving in the direction of the beam axis, including finite constant electrical conductivity of the medium. We evaluate the directed flow in the symmetric and asymmetric collisions at BNL Relativistic Heavy Ion Collider energy. We find a significant effect of finite electrical conductivity on the directed flow in the asymmetric collision system. We confirm that a certain amount of energy transfer by dissipation associated with Ohmic conduction occurs in the asymmetric collision system because of the asymmetry of the electric field produced by two different colliding nuclei. Because this energy transfer makes the pressure gradient of the medium flatter, the growth of directed flow decreases.

Testing the collectivity in large and small colliding systems with test particles

We propose a test-particle method to probe the transport dynamics of the establishment and development of collective flow in large and small systems of heavy-ion collisions. We place test particles as passengers into the partonic medium created by $\mathrm{Au}+\mathrm{Au}$ midcentral collisions at $\sqrt{{s}_{NN}}=200$ GeV and $p+\mathrm{Pb}$ central collisions at $\sqrt{{s}_{NN}}=5.02$ TeV, using a multiphase transport model. With the help of test particles in two extreme test cases, we demonstrate that parton collisions play an important role in establishing and developing collectivity in large and small colliding systems. The collectivity established by final state parton collisions is much stronger in large colliding systems compared to small colliding systems. The collectivity from the initial state can persist or survive more easily in small colliding systems than in large colliding systems due to fewer parton collisions. Our study provides a new method to understand the origin of collectivity in large and small colliding systems at the BNL Relativistic Heavy Ion Collider and the CERN Large Hadron Collider.

Intermittency of charged particles in the hybridUrQMD+CMCmodel at energies available at the BNL Relativistic Heavy Ion Collider

Within the framework of intermittency analysis, a search for critical fluctuations is ongoing to locate the possible critical point in the quantum chromodynamics phase diagram. In this study, self-similar critical fluctuations from a critical Monte Carlo (CMC) model have been incorporated into the cascade ultrarelativistic quantum molecular dynamics (UrQMD) model. This hybrid UrQMD+CMC model exhibits a clear power-law behavior of scaled factorial moment for charged particles in Au+Au collisions at $\sqrt{s_\mathrm{NN}}$ = 7.7-200 GeV. By comparing the UrQMD+CMC model results with those from the STAR experiment, it is found that the value of a calculated scaling exponent falls in the range of the experimental measurement when 1-2 \% signal of intermittency fluctuations is added into the UrQMD sample.

Flow correlations from a hydrodynamics model with dynamical freeze-out and initial conditions based on perturbative QCD and saturation

We extend the applicability of the hydrodynamics, perturbative QCD and saturation -based EKRT (Eskola-Kajantie-Ruuskanen-Tuominen) framework for ultrarelativistic heavy-ion collisions to peripheral collisions by introducing dynamical freeze-out conditions. As a new ingredient compared to the previous EKRT computations we also introduce a nonzero bulk viscosity. We compute various hadronic observables and flow correlations, including normalized symmetric cumulants, mixed harmonic cumulants, and flow--transverse-momentum correlations, and compare them against measurements from the BNL Relativistic Heavy Ion Collider (RHIC) and the CERN Large Hadron Collider (LHC) . We demonstrate that the inclusion of the dynamical freeze-out and bulk viscosity allows a better description of the measured flow coefficients in peripheral collisions and enables the use of an extended centrality range when constraining the properties of QCD matter in the future.

Predictions and postdictions for relativistic lead and oxygen collisions with the computational simulation code Trajectum

We introduce a global analysis of relativistic heavy ion collisions using Trajectum of a significantly higher precision and including a new option to vary the normalization of the centrality estimator. We use the posterior distribution of our parameters to generate a set of high-statistics samples that allows us to make precise predictions including statistical and systematic uncertainties estimated from the model parameter distribution. The results are systematically compared with experiment whereby we also include many observables not included in the global analysis. This includes in particular (extremely) ultracentral anisotropic flow and mean transverse momentum, whereby we find satisfactory agreement with experiment where data are available. Lastly, we compute spectra and anisotropic flow for oxygen-oxygen collisions performed at the BNL Relativistic Heavy Ion Collider (RHIC) and to be performed at the the CERN Large Hadron Collider (LHC) and comment on how these collisions may inform us on properties of the quark-gluon plasma.

Free spectator nucleons in ultracentral relativistic heavy-ion collisions as a probe of neutron skin

Besides the yield ratio of free spectator neutrons produced in ultracentral $^{96}\mathrm{Zr}$+$^{96}\mathrm{Zr}$ to $^{96}\mathrm{Ru}$+$^{96}\mathrm{Ru}$ collisions [Liu et al., Phys. Lett. B 834, 137441 (2022)], we propose that the yield ratio ${N}_{n}/{N}_{p}$ of free spectator neutrons to protons in a single collision system at the BNL Relativistic Heavy Ion Collider or the CERN Large Hadron Collider can be a more sensitive probe of the neutron-skin thickness $\mathrm{\ensuremath{\Delta}}{r}_{\mathrm{np}}$ and the slope parameter $L$ of the symmetry energy. The idea is demonstrated based on the proton and neutron density distributions of colliding nuclei obtained from Skyrme-Hartree-Fock-Bogolyubov calculations, and on a Glauber model that provides information of spectator matter. The final spectator particles are produced from direct emission, clusterization by a minimum spanning tree algorithm or a Wigner function approach, and deexcitation of heavy clusters by gemini. A larger $\mathrm{\ensuremath{\Delta}}{r}_{\mathrm{np}}$ associated with a larger $L$ value increases the isospin asymmetry of spectator matter and thus leads to a larger ${N}_{n}/{N}_{p}$, especially in ultracentral collisions where the multiplicity of free nucleons are free from the uncertainties of cluster formation and deexcitation. We have further shown that the double ratio of ${N}_{n}/{N}_{p}$ in isobaric collision systems or in collisions by isotopes helps to cancel the detecting efficiency for protons. Effects from nuclear deformation and electromagnetic excitation are studied, and they are found to be subdominant compared to the expected sensitivity to $\mathrm{\ensuremath{\Delta}}{r}_{\mathrm{np}}$.