Electromagnetic fields in ultra-peripheral relativistic heavy-ion collisions

Ultra-peripheral heavy-ion collisions(UPCs)offer unique opportunities to study processes under strong electromagnetic fields.In these collisions,highly charged fast-moving ions carry strong electromagnetic fields that can be effectively treated as photon fluxes.The exchange of photons can induce photonuclear and two-photon interactions and excite ions.This excitation of the ions results in Coulomb dissociation with the emission of photons,neutrons,and other particles.Additionally,the electromagnetic fields generated by the ions can be sufficiently strong to enforce mutual interactions between the two colliding ions.Consequently,the two colliding ions experience an electromagnetic force that pushes them in opposite directions,causing a back-to-back correlation in the emitted neutrons.Using a Monte Carlo simulation,we qualitatively demonstrate that the above electromagnetic effect is large enough to be observed in UPCs,which would provide a clear means to study strong electromagnetic fields and their effects.

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

The relativistic mean-field approach was implemented in the Lanzhou quantum molecular dynamics transport model(LQMD.RMF). Using the LQMD.RMF, the properties of collective flow and pion production were investigated systematically for nuclear reactions with various isospin asymmetries. The directed and elliptic flows of the LQMD.RMF are able to describe the experimental data of STAR Collaboration. The directed flow difference between free neutrons and protons was associated with the stiffness of the symmetry energy, that is, a softer symmetry energy led to a larger flow difference. For various collision energies, the ratio between the π^(-) and π^(+) yields increased with a decrease in the slope parameter of the symmetry energy. When the collision energy was 270 MeV/nucleon, the single ratio of the pion transverse momentum spectra also increased with decreasing slope parameter of the symmetry energy in both nearly symmetric and neutron-rich systems.However, it is difficult to constrain the stiffness of the symmetry energy with the double ratio because of the lack of threshold energy correction on the pion production.

The study of intelligent algorithm in particle identification of heavy-ion collisions at low and intermediate energies

Traditional particle identification methods face timeconsuming,experience-dependent,and poor repeatability challenges in heavy-ion collisions at low and intermediate energies.Researchers urgently need solutions to the dilemma of traditional particle identification methods.This study explores the possibility of applying intelligent learning algorithms to the particle identification of heavy-ion collisions at low and intermediate energies.Multiple intelligent algorithms,including XgBoost and TabNet,were selected to test datasets from the neutron ion multi-detector for reaction-oriented dynamics(NIMROD-ISiS)and Geant4 simulation.Tree-based machine learning algorithms and deep learning algorithms e.g.TabNet show excellent performance and generalization ability.Adding additional data features besides energy deposition can improve the algorithm’s performance when the data distribution is nonuniform.Intelligent learning algorithms can be applied to solve the particle identification problem in heavy-ion collisions at low and intermediate energies.

Effects of sequential decay on collective flows and nuclear stopping power in heavy-ion collisions at intermediate energies

In this study, the rapidity distribution, collective flows, and nuclear stopping power in ^(197)Au+^(197)Au collisions at intermediate energies were investigated using the ultrarelativistic quantum molecular dynamics(UrQMD) model with GEMINI++ code. The UrQMD model was adopted to simulate the dynamic evolution of heavy-ion collisions, whereas the GEMINI++ code was used to simulate the decay of primary fragments produced by UrQMD. The calculated results were compared with the INDRA and FOPI experimental data. It was found that the rapidity distribution, collective flows, and nuclear stopping power were affected to a certain extent by the decay of primary fragments, especially at lower beam energies. Furthermore, the experimental data of the collective flows and nuclear stopping power at the investigated beam energies were better reproduced when the sequential decay effect was included.

Recent development of hydrodynamic modeling in heavy-ion collisions

We present a concise review of the recent development of relativistic hydrodynamics and its applications to heavy-ion collisions.Theoretical progress on the extended formulation of hydrodynamics toward out-ofequilibrium systems is addressed,with emphasis on the socalled attractor solution.Moreover,recent phenomenological improvements in the hydrodynamic modeling of heavy-ion collisions with respect to the ongoing beam energy scan program,the quantitative characterization of transport coefficients in three-dimensionally expanding quark–gluon plasma,the fluid description of small colliding systems,and certain other interdisciplinary connections are discussed.

Anomalous chiral transports and spin polarization in heavy-ion collisions

Relativistic heavy-ion collisions create hot quark–gluon plasma as well as very strong electromagnetic(EM)and fluid vortical fields.The strong EM field and vorticity can induce intriguing macroscopic quantum phenomena such as chiral magnetic,chiral separation,chiral electric separation,and chiral vortical effects as well as the spin polarization of hadrons.These phenomena provide us with experimentally feasible means to study the nontrivial topological sector of quantum chromodynamics,the possible parity violation of strong interaction at high temperature,and the subatomic spintronics of quark–gluon plasma.These studies,both in theory and in experiments,are strongly connected with other subfields of physics such as condensed matter physics,astrophysics,and cold atomic physics,and thus form an emerging interdisciplinary research area.We give an introduction to the aforementioned phenomena induced by the EM field and vorticity and an overview of the current status of experimental research in heavy-ion collisions.We also briefly discuss spin hydrodynamics as well as chiral and spin kinetic theories.

Recent developments in chiral and spin polarization effects in heavy-ion collisions

We give a brief overview of recent theoretical and experimental results on the chiral magnetic effect and spin polarization effect in heavy-ion collisions.We present updated experimental results for the chiral magnetic effect and related phenomena.The time evolution of the magnetic fields in different models is discussed.The newly developed quantum kinetic theory for massive fermions is reviewed.We present theoretical and experimental results for the polarization of K hyperons and the q00 value of vector mesons.

Effect of relaxation time on the squeezed correlations of bosons for evolving sources in relativistic heavy-ion collisions

The squeezed back-to-back correlation(SBBC)of a boson-antiboson pair is sensitive to the time distribution of the particle-emitting source,and the SBBC function for an evolving source is expected to be affected by the relaxation time of the system.In this study,we investigated the effect of relaxation time on the SBBC function.We propose a method for calculating the SBBC function with relaxation-time approximation for evolving sources.SBBC functions of D^(0)D^(-0)in relativistic heavy-ion collisions were investigated using a hydrodynamic model.We found that the relaxation time reduces the amplitudes of the SBBC functions.This becomes apparent for long relaxation times and large initial relative deviations of the chaotic and squeezed amplitudes from their equilibrium values in the temporal steps.

Squeezed Back-to-Back Correlation of D^0~0 in Relativistic Heavy-Ion Collisions

We investigate the squeezed back-to-back correlation（BBC） of D^0~0 in relativistic heavy-ion collisions, using the in-medium mass modification calculated with a self-energy in hot pion gas and the source space-time distributions provided by the viscous hydrodynamic code VISH2＋1. It is found that the squeezed BBC of D^0~0 is significant in peripheral Au＋Au collisions at the relativistic heavy ion collider energy. A possible way to detect the squeezed BBC in an experiment is presented.

An Explanation of a Phenomenon in Relativistic Heavy-Ion Collisions

The multi-source pion interferometry in relativistic heavy-ion collisions is presented and two-source models for hadron gas and hadron gas plus quark-gluon plasma are proposed. The models can resolve the HBT puzzle. For the same q and different direction of q, the two-pion correlation functions show characteristic oscillation behaviors, which may be used to distinguish the two-source models. Our research also showes that the multi-source pion correlations can resolve the HBT puzzle.

Novel quantum phenomena induced by strong magnetic fields in heavy-ion collisions

The relativistic heavy-ion collisions create both hot quark–gluon matter and strong magnetic fields, and provide an arena to study the interplay between quantum chromodynamics and quantum electrodynamics. In recent years, it has been shown that such an interplay can generate a number of interesting quantum phenomena in hadronic and quark–gluon matter. In this short review, we first discuss some properties of the magnetic fields in heavy-ion collisions and then give an overview of the magnetic fieldinduced novel quantum effects. In particular, we focus on the magnetic effect on the heavy flavor mesons, the heavyquark transports, and the phenomena closely related to chiral anomaly.

Nuclear dynamics and particle production near threshold energies in heavy-ion collisions

Recent progress of the quantum molecular dynamics model for describing the dynamics of heavy-ion collisions is viewed, in particular the nuclear fragmentation, isospin physics, particle production and in-medium effect, hadron-induced nuclear reactions, hypernucleus, etc.The neck fragmentation in Fermi-energy heavy-ion collisions is investigated for extracting the symmetry energy at subsaturation densities. The isospin effects, in-medium properties, and the behavior of high-density symmetry energy in medium-and high-energy heavy-ion collisions are thoroughly discussed. The hypernuclide dynamics formed in heavy-ion collisions and in hadron-induced reactions is analyzed and addressed in the future experiments at the high-intensity heavy-ion accelerator facility(HIAF).

Search for the QCD critical point with fluctuations of conserved quantities in relativistic heavy-ion collisions at RHIC： an overview

Fluctuations of conserved quantities, such as baryon, electric charge, and strangeness number, are sensitive observables in relativistic heavy-ion collisions to probe the QCD phase transition and search for the QCD critical point. In this paper, we review the experimental measurements of the cumulants(up to fourth order) of event-byevent net-proton(proxy for net-baryon), net-charge and netkaon(proxy for net-strangeness) multiplicity distributions Au+Au collisions at sNN^(1/2) 7:7; 11:5; 14:5; 19:6; 27;39; 62:4; 200 Ge V from the first phase of beam energy scan program at the relativistic heavy-ion collider(RHIC). We also summarize the data analysis methods of suppressing the volume fluctuations, auto-correlations, and the unified description of efficiency correction and error estimation.Based on theoretical and model calculations, we will discuss the characteristic signatures of critical point as well as backgrounds for the fluctuation observables in heavy-ion collisions. The physics implications and the future secondphase of the beam energy scan(2019–2020) at RHIC will also be discussed.

2-16 Symmetry Energy Extraction and Emitting Source Properties in Intermediate Heavy-ion Collisions

In Eq. (1), A?? and Nln(N/A)+Zln(Z/A) originate from the increases of the entropy and the mixing entropy at the time of the fragment formation, respectively. n (p) is the neutron (proton) chemical potential. is the critical exponent. In this work, the value of =2.3 is adopted from the previous studies[1]. In general coefficients, av, as, asym, ac, ap and the chemical potentials are temperature and density dependent.

Estimation of Local Energy Density in Relativistic Heavy-Ion Collisions

The energy density for the central region in relativistic heavy-ion collisions can be estimated via the pseudorapidity distribution of transverse energy.The way to estimate the local energy density for the central region in relativistic heavy-ion collisions is proposed,in which only final state particles emitted from the same source are included.The energy density arrived in the NA49 experiments is about 0.9 GeV/fm^(3).

Entrance Channel Dependence of the Isospin Effects of the Nuclear Stopping in Intermediate Energy Heavy-Ion Collisions

The entrance channel dependence of the isospin effects of nuclear stopping in intermediate energy heavy-ion collisions has been studied by using an isospin-dependent quantum molecular dynamics with three different kinds of symmetry potentials.It is shown that nuclear stopping is sensitive to the beam energy,the impact parameter and the mass of the colliding system,specially very sensitive to the isospin dependence of in-medium nucleon-nucleon cross section,but insensitive to symmetry potential and the ratio of neutron to proton of colliding system.From this investigation.It is proposed that nuclear stopping can be used as a new probe to extract the information on the isospin dependence of in-medium nucleon-nucleon cross section in intermediate energy heavy-ion collisions.

The Fluctuation of Mean Field and Multifragmentation in Intermediate Energy Heavy-Ion Collisions

The time evolution of the interaction potential contour plot in intermediate energy heavy ion col-lisions is investigated.The stochastic fluctuation appearing in the peak zone of the interaction potential,which is indicated by the deformation of equipotential lines,especially by the negative curvature of equipo-tential lines,contributes to initiating the multifragmentation.

Effect of Pion Mean-Field on Properties of Pions and Kaons From Heavy-Ion Collisions

The Relativistic Vlasov-Uehling-Uhlenbeck(RVUU)model is used to study the properties of pi-ons and kaons produced in heavy ion collisions.We include the nuclear medium effect on kaon and pionin the model,and simulate pion production and subthreshold kaon production in Kr+Zr reactions at 1GeV/u.The calculated results show that the attractive pion optical potential changes the final-state pionmomentum spectrum,enhancing the yield of pions with low transverse momenta.At the same ti1e it alsoincreases the kaon abundance and modifies the kaon momentum distribution.

On The Hardening of The Spectrum of High-Energy Particles Formed in Heavy-Ion Collisions Considered within The Framework of The Hydrodynamic Approach

The emission of high-energy particles in 16O + 197Au collisions at energy 20 MeV / nucleon is considered within the framework of the time evolution of a hot spot taking into account the hydrodynamic compression and expansion stages. In addition, the evaporation of the particles that are formed in the early (hot) stage of the evolution of the hot spot is included in the calculation of the spectrum. This leads to a hardening of the particle spectrum in its high-energy part, which is in agreement with experimental data.

Blind Spots of Probing the High-density Symmetry Energy in Heavy-ion Collisions

The nuclear symmetry energy,especially at suprasaturation densities,plays crucial roles in many astrophysical studies.However,nowadays the high-density behavior of the symmetry energy is still very controversial in nuclear community.To constrain the high-density behavior of the symmetry energy,neutron-rich nuclei collisions at medium energies are considered to be one of the most effective methods.While probing the high-density symmetry energy by using heavy-ion collisions,blind spots may exist.In the framework of the Isospin-dependent Boltzmann-Uehling-Uhlenbeck(IBUU)transport model,the blind spots of probing the high-density symmetry energy by the n/p ratio in the central Au+Au reaction at 300 MeV/u are demonstrated.