Relativistic Quantum Molecular Dynamics Research Articles

Properties of collective flow and pion production in intermediate-energy heavy-ion collisions with a relativistic quantum molecular dynamics model

Properties of collective flow and pion production in intermediate-energy heavy-ion collisions with a relativistic quantum molecular dynamics model

Directed flow of Λ in high-energy heavy-ion collisions and Λ potential in dense nuclear matter

We investigate the sensitivity of the $\Lambda$ directed flow to the $\Lambda$ potential in mid-central Au + Au collisions at $\sqrt{s_{NN}}\approx3.0$--$30$ GeV. The $\Lambda$ potential obtained from the chiral effective field theory ($\chi$EFT) is used in a microscopic transport model, a vector version of relativistic quantum molecular dynamics (RQMDv). We find that the density-dependent $\Lambda$ potentials, obtained from the $\chi$EFT assuming weak momentum dependence of the potential, reproduce the rapidity and the beam-energy dependence of the $\Lambda$ directed flow measured by the STAR collaboration in the Beam Energy Scan program. Although the $\Lambda$ directed flow is insensitive to the density dependence of the potential, it is susceptible to the momentum dependence. We also show that a hydrodynamics picture based on the blast-wave model predicts a similarity of the proton, $\Lambda$, and $\Xi$ directed flows, but the directed flow of $\Omega$ baryons slightly deviates from other baryons. We also show that the quark coalescence predicts different rapidity dependence of the directed flows for hyperons. These investigations suggest that measurements of a wide range of the rapidity dependence of the directed flow of hyperons may provide important information about the properties of hot and dense matter created in high-energy heavy-ion collisions.

利用机器学习研究原子核四极形变在中能重离子碰撞中的效应

The influence of quadrupole deformation on the projectile and target nucleus in $^{238}$U + $^{238}$U collisions at beam energies of 0.4, 1.0, and 1.5 GeV/nucleon is investigated using the ultra relativistic quantum molecular dynamics (UrQMD) model. The effect of quadrupole deformation is investigated using the LightGBM decision tree algorithm. LightGBM's ability to identify nuclear deformation from particle spectra is demonstrated on an event-by-event basis by learning the two-dimensional (transverse momentum and rapidity) distributions of free protons, charged fragments (mass number $A>$1), and charged $\pi$ mesons produced in $^{238}$U + $^{238}$U collisions with and without deformed initializations. Using the important characteristic analysis of LightGBM, the yield of charged fragments around target/projectile rapidities is found to be sensitive to the quadrupole deformation of both the projectile and the target.It can be used as a promising observable to probe quadrupole deformation with heavy-ion collision.

Mean-field update in the JAM microscopic transport model: Mean-field effects on collective flow in high-energy heavy-ion collisions atsNN=2–20 GeV energies

The beam energy dependence of the directed flow is a sensitive probe for the properties of strongly interacting matter. Hybrid models that simulate dense (core) and dilute (corona) parts of the system by combining the fluid dynamics and hadronic cascade model describe well the bulk observables, strangeness productions, and radial and elliptic flows in high-energy heavy-ion collisions in the high baryon density region. However, the beam energy dependence of the directed flow cannot be described in existing hybrid models. We focus on improving the corona part, i.e., the nonequilibrium evolution part of the system, by introducing the mean-field potentials into a hadronic cascade model. For this purpose, we consider different implementations of momentum-dependent hadronic mean fields in the relativistic quantum molecular dynamics (RQMD) framework. First, Lorentz scalar implementation of a Skyrme type potential is examined. Then, full implementation of the Skyrme type potential as a Lorentz vector in the RQMD approach is proposed. We find that scalar implementation of the Skyrme force is too weak to generate repulsion explaining observed data of sideward flows at $\sqrt{{s}_{NN}}<10$ GeV, while vector implementation gives collective flows compatible with the data for a wide range of beam energies $2.7<\sqrt{{s}_{NN}}<20$ GeV. We show that our approach reproduces the negative proton directed flow at $\sqrt{{s}_{NN}}>10$ GeV discovered by experiments. We discuss the dynamical generation mechanisms of the directed flow within a conventional hadronic mean field. A positive slope of proton directed flow is generated predominantly during compression stages of heavy-ion collisions by the strong repulsive interaction due to high baryon densities. In contrast, at the expansion stages of the collision, the negative directed flow is generated more strongly than the positive one by the tilted expansion and shadowing by the spectator matter. At lower collision energies $\sqrt{{s}_{NN}}<10$ GeV, the positive flow wins against the negative flow because of a long compression time. On the other hand, at higher energies $\sqrt{{s}_{NN}}>10$ GeV, negative flow wins because of shorter compression time and longer expansion time. A transition beam energy from positive to negative flow is highly sensitive to the strength of the interaction.

Directed flow of Λ from heavy-ion collisions and hyperon puzzle of neutron stars

We examine the Λ potential from the chiral effective field theory (χEFT) via the Λ directed flow from heavy-ion collisions. We implement the Λ potential obtained from the χEFT in a vector potential version of relativistic quantum molecular dynamics. We find that the Λ potentials obtained from the χEFT assuming weak momentum dependence reproduce the Λ directed flow measured by the STAR collaboration in the Beam Energy Scan program. While the Λ directed flow is not very sensitive to the density dependence of the potential, the directed flow at large rapidities is susceptible to the momentum dependence. Thus understanding the directed flow of hyperons in a wide range of beam energy and rapidity is helpful in understanding hyperon potentials in dense matter.

Hawking radiation from acoustic black holes in relativistic heavy ion collisions

We propose a new analogue model of gravity - the evolving quark gluon plasma (QGP) produced in relativistic heavy ion collisions. This quark gluon plasma is the “most inviscid” fluid known. Such low kinematic viscosity is believed to reflect strongly correlated nature for QGP in these experiments. Hence, it may provide a good example of a quantum fluid naturally suited to studies of acoustic Hawking radiation. Due to rapid longitudinal expansion, presence of a sonic horizon is also naturally guaranteed here, though, in general, this horizon is not static. Using Ultra relativistic quantum molecular dynamics (UrQMD) simulations, we show that, under certain conditions, the longitudinal velocity of the plasma, near the sonic horizon, can become time independent for a short span during the evolution of the system. During this period, we can have a conformally static acoustic metric with a (conformal) Killing horizon coinciding with the apparent horizon. An asymptotic observer will then see a thermal flux of phonons, constituting the Hawking radiation, coming from the horizon. For the relatively low energy collision considered here, where the resulting QCD system is governed by non-relativistic hydrodynamics, we estimate the Hawking temperature to be about 4-5 MeV (with the temperature of the QCD fluid being about 135 MeV). We discuss the experimental signatures of this Hawking radiation in terms of a thermal component in the rapidity dependence of the transverse momentum distribution of detected particles. We also discuss extension to ultra-relativistic case which should lead to a higher Hawking temperature, along with the effects of dynamical horizon leading to blue/red shift of the temperature.

Constraining resonance properties through kaon production in pion–nucleus collisions at low energies

Hadronic interactions are crucial for the dynamical description of heavy-ion reactions at low collision energies and in the late dilute stages at high collision energies. In particular, the properties and decay channels of resonances are an essential ingredient of hadronic transport approaches. The HADES collaboration measured particle production in collisions of pions with carbon and tungsten nuclei at E kin = 1.7 GeV (Adamczewski-Musch et al 2019 Phys. Rev. Lett. 123 022002). Such reactions are of high interest, because they allow to probe the properties of baryonic resonances produced in a much cleaner environment than the usual nucleus–nucleus collisions. We study these reactions with two transport approaches: SMASH (simulating many accelerated strongly-interacting hadrons) and UrQMD (ultra relativistic quantum molecular dynamics) which follow the same underlying concept but with different implementations. The differential spectra in rapidity and transverse momentum are used to show how model parameters, as the decay channels of high mass resonances and angular distributions of kaon elastic scattering, can be constrained. It is found that the data favor the production of more particles with lower momentum over the production of few particles with higher momentum in these decays. In addition, the shape of the rapidity distribution of the kaons strongly depends on the angular distribution of the elastic kaon–nucleon cross section.

Momentum-dependent potential and collective flows within the relativistic quantum molecular dynamics approach based on relativistic mean-field theory

Relativistic quantum molecular dynamics based on the relativistic mean field theory (RQMD.RMF) is extended by including momentum-dependent potential. The equation of state (EoS) dependence of the directed and the elliptic flow of protons in the beam energy range of $2.3 < \sqrt{s_{NN}}< 20$ GeV is examined. It is found that the directed flow depends strongly on the optical potential at high energies,$\sqrt{s_{NN}} > 3 $ GeV, where no information is available experimentally. The correlation between effective mass at saturation density and the optical potential is found: smaller values of effective mass require smaller strengths of the optical potential to describe the directed flow data.This correlation can also be seen in the beam energy dependence of the elliptic flow at $\sqrt{s_{NN}}>3$ GeV, although its effect is rather weak. On the other hand, stiff EoS is required to describe the elliptic flow at lower energies.Experimental constraints on the optical potential from $pA$ collisions will provide important information on the EoS at high energies.The proton directed and the elliptic flow are well described in the RQMD.RMF model from $\sqrt{s_{NN}}=2.3$ to 8.8 GeV. In contrast,to reproduce the collapse of the directed flow above 10 GeV, pressure has to be reduced, which indicates a softening of the EoS around $\sqrt{s_{NN}} =10 $ GeV.

Centrality Dependence of Multiplicity Fluctuations in Ion-Ion Collisions from the Beam Energy Scan at FAIR

Multiplicity distributions and event-by-event multiplicity fluctuations in AuAu collisions at energies in future heavy-ion experiment at the Facility for Anti-proton and Ion Research (FAIR) are investigated. Events corresponding to FAIR energies are simulated in the frame work of Ultra Relativistic Quantum Molecular Dynamics (URQMD) model. It is observed that the mean and the width of multiplicity distributions monotonically increase with beam energy. The trend of variations of dispersion with mean number of participating nucleons for the centrality-bin width of 5% are in accord with the Central Limit Theorem. The multiplicity distributions in various centrality bins as well as for full event samples are observed to obey Koba, Nielsen and Olesen (KNO) scaling. The trends of variations of scaled variance with beam energy are also found to support the KNO scaling predictions for larger collision centrality. The findings also reveal that the statistical fluctuations in 5% centrality-bin width appear to be under control.

Sensitivity of the excitation functions of collective flow to relativistic scalar and vector meson interactions in the relativistic quantum molecular dynamics model RQMD.RMF

Relativistic quantum molecular dynamics with scalar and vector interactions based on the relativistic mean meson field theory, RQMD.RMF, is developed.It is implemented into the microscopic transport code JAM.The sensitivity of the directed and of the elliptic proton flow in high energy heavy-ion collisions on the stiffness of the RMF equation of state (EoS) is examined. These new calculations are compared to experimental data at mid-central Au + Au collisions in the beam energy range $2.5 < \sqrt{s_{NN}} < 20$ GeV. This new RQMD model with the relativistic mean field scalar and vector meson interactions does describe consistently, with one RMF parameter set,the beam energy dependence of both the directed flow and the elliptic flow,from SIS18 to AGS and RHIC BES-II energies, $\sqrt{s_{NN}}< 10$ GeV.There are different sensitivities of these different kinds of flow to the EoS: elliptic flow is most sensitive to the nuclear incompressibility constant,at the moderate beam energies $\sqrt{s_{NN}}<3$ GeV,whereas the directed flow is most sensitive to the effective baryon mass at saturation density at $3< \sqrt{s_{NN}}<5 $ GeV. Matters abruptly change in the next higher energy range,$\sqrt{s_{NN}}\gtrsim 10-20$ GeV:the directed flow data show a double change of sign of the slope of $v_1$, inverting twice in this energy range,in sudden contradiction to the RQMD.RMF calculation for a monotonous, stiff EoS. This surprising oscillating behavior,a double change of sign of the $v_1$ slope, points to the appearance of a hitherto unknown first-order phase transition in excited QCD matter at high baryon densities in mid-central Au + Au collisions.

A hadronic transport analysis on the Δ mass in Au + Au reactions at 1.23 AGeV

AbstractThis study is motivated by upcoming data from the high acceptance di‐electron spektrometer (HADES) collaboration at gesellschaft für schwerionenforschung, which presented preliminary data on a possible mass shift of the Δ(1232) resonance in central Au + Au reactions at Elab = 1.23 AGeV. This effect is explored via the Ultra Relativistic Quantum Molecular Dynamics transport model, allowing us to track down each decaying Δ(1232) resonance and check whether it can be reconstructed. The obtained results show that the apparent mass shift on the order of 50 MeV can be explained by a late‐stage formation effect and a long‐evolving Δ ↔ N + π cycle.

Delta mass shift as a thermometer of kinetic decoupling in Au + Au reactions at 1.23 AGeV

The HADES experiment at GSI will soon provide data on the production and properties of Δ(1232) baryons from Au + Au reactions at 1.23 AGeV. Using the Ultra Relativistic Quantum Molecular Dynamics model, we predict the yield and spectra of Δ(1232) resonances. In addition we show that one expects to observe a mass shift of the Δ(1232) resonance on the order of 50 MeV in the reconstructable Δ(1232) mass distribution. This mass shift can be understood in terms of late stage Δ(1232) formation with limited kinetic energy. We show how the mass shift can be used to constrain the kinetic decoupling temperature of the system.

Event-by-event analysis of maximum pseudo-rapidity gap fluctuation in high-energy nucleus-nucleus collisions

A detailed study of event-by-event maximum pseudo-rapidity gap fluctuations in relativistic heavy-ion collisions in terms of the scaled variance ω has been carried out for 16O-AgBr, 28Si-AgBr and 32S-AgBr interactions at an incident momentum of 4.5 AGeV/c applying the multiplicity cut (Ns > 10). For all the interactions the values of scaled variance are found to be greater than zero indicating the presence of strong fluctuation of maximum pseudo-rapidity gap values in the multiparticle production process. The event-by-event fluctuations are found to decrease with the increase of average multiplicity of the interactions. Experimental analysis has been compared with the results obtained from the analysis of events simulated by the Ultra Relativistic Quantum Molecular Dynamics (UrQMD) model. The UrQMD-model–simulated values of event-by-event fluctuations of the maximum pseudo-rapidity gap are less than the corresponding experimental values.

Hadron yields and fluctuations at energies available at the CERN Super Proton Synchrotron: System-size dependence from Pb + Pb to p+p collisions

The kaon to pion ratio $K^+/\pi^+$ and the scaled variance $\omega^-$ for fluctuations of negatively charged particles are studied within the statistical hadron resonance gas (HRG) model and the Ultra relativistic Quantum Molecular Dynamics (UrQMD) transport model. The calculations are done for p+p, Be+Be, Ar+Sc, and Pb+Pb collisions at the CERN Super Proton Synchrotron energy range to reveal the system size dependence of hadron production. For the HRG calculations the canonical ensemble is imposed for all conserved charges. In the UrQMD simulations the centrality selection in nucleus-nucleus collisions is done by calculating the forward energy $E_{\rm F}$ deposited in the Projectile Spectator Detector, and the acceptance maps of the NA61/SHINE detectors are used. A comparison of the HRG and UrQMD results with the data of the NA61/SHINE Collaboration is done. To understand a difference of the event-by-event fluctuations in p+p and heavy ion collisions the centrality selection procedure in the sample of all inelastic p+p events is proposed and analyzed within the UrQMD simulations.

Observation of universality for high pT distribution at LHC energies

We have studied the distributions of the yield of primary charged particles produced in the asymmetric [Formula: see text]–[Formula: see text] collisions at [Formula: see text] for the three pseudorapidity regions: [Formula: see text] and [Formula: see text] and the transverse momentum range of [Formula: see text]. Heavy ion jet interaction generator (HIJING) and Ultra relativistic quantum molecular dynamics (UrQMD) models are used to produce simulated data and the results are compared with the CMS and ATLAS data. The comparison of models and data shows the existence of high [Formula: see text] area with boundary values that depend upon pseudorapidity ([Formula: see text]). At high [Formula: see text] values, the behavior of the distributions shows some universality, which does not depend upon the models. The reason of the universality could be the string dynamics for the parton hadronization at high [Formula: see text] values.

Higher order net-proton number cumulants dependence on the centrality definition and other spurious effects

We study the dependence of the normalized moments of the net-proton multiplicity distributions on the definition of centrality in relativistic nuclear collisions at a beam energy of . Using the ultra relativistic quantum molecular dynamics model as event generator we find that the centrality definition has a large effect on the extracted cumulant ratios. Furthermore we find that the finite efficiency for the determination of the centrality introduces an additional systematic uncertainty. Finally, we quantitatively investigate the effects of event-pile up and other possible spurious effects which may change the measured proton number. We find that pile-up alone is not sufficient to describe the data and show that a random double counting of events, adding significantly to the measured proton number, affects mainly the higher order cumulants in most central collisions.

Comparison study of the pT distributions of the charged particles in p–Pb interactions at LHC energies

Transverse momentum [Formula: see text] distributions of primary charged particles were compared to simulations using the Ultra Relativistic Quantum Molecular Dynamics (UrQMD) transport model and the HIJING 1.0 model in minimum bias p–Pb collisions at [Formula: see text] in the pseudorapidity [Formula: see text] regions: [Formula: see text], [Formula: see text] and [Formula: see text] and in the transverse momentum range [Formula: see text]. The simulated distributions were then compared with the ALICE data and it was observed that UrQMD predicts systematically higher yields than HIJING 1.0. Both codes cannot describe the experimental data in the range of [Formula: see text], though in the region of [Formula: see text] the model predictions are very close to the experimental results for particles with [Formula: see text], [Formula: see text]. The ratio of the yield at forward pseudorapidity to that at [Formula: see text] was also studied. It was observed that the predictions of the models depend on [Formula: see text]. In the experiment there is no essential difference of yields for particles from the intervals of [Formula: see text], [Formula: see text] and [Formula: see text]. The differences are significant for the models where the ratios are systematically less than 1. This means that the results are not connected to a medium effect but reflect the Cronin effect. We are led to conclude that the codes cannot take into account satisfactorily the leading effect due to the asymmetric p–Pb fragmentation.

Cumulative production of pions by heavy baryonic resonances in proton–nucleus collisions

Pion production in proton–nucleus (p+A) collisions outside the kinematical boundary of proton–nucleon (p+N) reactions and the so-called cumulative effect are studied. Restrictions from energy-momentum conservation of the energy of pions emitted in the backward direction in the target rest frame are analyzed. It is assumed that the cumulative pions are produced in p+A reactions by heavy baryonic resonances. The baryonic resonances are first created in p+N reactions. Due to successive collisions with nuclear nucleons, the masses of these resonances may then increase and, simultaneously, their longitudinal velocities decrease. We also use the ultra relativistic quantum molecular dynamics model to reveal the key role of successive collisions of baryonic resonances with nuclear nucleons for cumulative pion production in p+A reactions. Further experimental studies of cumulative hadron production in p+A reactions at high collision energies are needed to search for heavy hadron-like objects and investigate their properties.

Cumulative Pion Production via Successive Collisions in Nuclear Medium

Production of pions in proton-nucleus (p+A) reactions outside of a kinematical boundary of proton-nucleon collisions, the so-called cumulative effect, is studied. The kinematical restrictions on pions emitted in backward direction in the target rest frame are analyzed. It is shown that cumulative pion production requires a presence of massive baryonic resonances that are produced during successive collisions of projectile with nuclear nucleons. After each successive collision the mass of created resonance may increase and, simultaneously, its longitudinal velocity decreases. Simulations within Ultra relativistic Quantum Molecular Dynamics model reveals that successive collisions of baryonic resonances with nuclear nucleons plays the dominant role in cumulative pion production in p+A reactions.

Modeling of two-particle femtoscopic correlations at top RHIC energy

The spatial and temporal characteristics of particle emitting source produced in particle and/or nuclear collisions can be measured by using two-particle femtoscopic correlations. These correlations arise due to quantum statistics, Coulomb and strong final state interactions. In this paper we report on the calculations of like-sign pion femtoscopic correlations produced in p+p, p+Au, d+Au, Au+Au at top RHIC energy using Ultra Relativistic Quantum Molecular Dynamics Model (UrQMD). Three-dimensional correlation functions are constructed using the Bertsch-Pratt parametrization of the two-particle relative momentum. The correlation functions are studied in several transverse mass ranges. The emitting source radii of charged pions, Rout, Rside, Rlong, are obtained from Gaussian fit to the correlation functions and compared to data from the STAR and PHENIX experiments.