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.

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.

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.

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).

An Explanation of a Phenomenon in Relativistic Heavy-Ion Collisions

在相对论的重离子的碰撞的多来源介子 interferometry 被介绍，为加 quark-gluon 血浆的强子气体和强子气体的二来源的模型被建议。模型能解决 HBT 难题。为一样的 q 和不同方向，二介子的关联功能表演特征摆动行为，它可以习惯于区分二来源的模型。我们的研究另外多来源介子关联能解决的 showes HBT 难题。

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.

Collision off-axis position dependence of relativistic nonlinear Thomson inverse scattering of an excited electron in a tightly focused circular polarized laser pulse

This paper presents a novel view of the impact of electron collision off-axis positions on the dynamic properties and relativistic nonlinear Thomson inverse scattering of excited electrons within tightly focused, circularly polarized laser pulses of varying intensities. We examine the effects of the transverse ponderomotive force, specifically how the deviation angle and speed of electron motion are affected by the initial off-axis position of the electron and the peak amplitude of the laser pulse. When the laser pulse intensity is low, an increase in the electron's initial off-axis distance results in reduced spatial radiation power, improved collimation, super-continuum phenomena generation, red-shifting of the spectrum's harmonic peak, and significant symmetry in the radiation radial direction. However, in contradiction to conventional understandings,when the laser pulse intensity is relatively high, the properties of the relativistic nonlinear Thomson inverse scattering of the electron deviate from the central axis, changing direction in opposition to the aforementioned effects. After reaching a peak, these properties then shift again, aligning with the previous direction. The complex interplay of these effects suggests a greater nuance and intricacy in the relationship between laser pulse intensity, electron position, and scattering properties than previously thought.

System scan of the multiplicity correlation between forward and backward rapidities in relativistic heavy-ion collisions using a multi-phase transport model

A systematic study on forward–backward(FB)multiplicity correlations from large systems to small ones through a multi-phase transport model(AMPT)has been performed and the phenomenon that correlation strength increases with centrality can be explained by taking the distribution of events as the superposition of a series of Gaussian distributions.It is also found that correlations in the η−ϕ plane can imply the shape of the event.Furthermore,long-range correlations originate from the fluctuations associated with the source information.FB correlations allow us to decouple long-range correlations from short-range correlations,and may provide a chance to investigate the α-clustering structure in initial colliding light nuclei as well.It seems the tetrahedron ^(16)O+^(16)O collision gives a more uniform and symmetrical fireball,that emits the final particles more isotropically or independently in the longitudinal direction,indicating that the forward-backward multiplicity correlation could be used to identify the pattern of α-clustered ^(16)O in future experiments.

Different production sources of light nuclei in ultra-relativistic heavy-ion collisions

We systematically study different production sources of light nuclei in ultra-relativistic heavy-ion collisions with a new method, an exclusive quark combination model + an inclusive hadron recombination model. We take deuterons and ~3 He produced in Pb-Pb collisions at ■= 2.76 TeV as examples to show the contribution of different production sources by studying their rapidity densities dN/dy, yield ratios and transverse momentum(PT)spectra just after hadronization and at the final kinetic freeze-out. We find that about a half of d and a fourth of ~3 He created just after hadronization can survive after the hadronic evolution process. Nucleons from A resonance decays make a much larger contribution to the regeneration of light nuclei at the hadronic phase stage, and this contribution is about 77% and 90% for d and ~3 He, respectively, calculated at the final kinetic freeze-out. In addition, we give an explanation for the constant behaviors of yield ratios d/p and ~3 He/p as a function of the averaged charged multiplicity in Pb-Pb collisions and also provide a possible explanation for the observation that d/p in Pb-Pb collisions is larger by a factor of about two than in pp collisions at LHC energies.

Medium modifications of girth distributions for inclusive jets and Z0+jets in relativistic heavy-ion collisions at the LHC

In this paper,we investigate the medium modifications of girth distributions for inclusive jets and Z^0 tagged jets with a small radius( R=0.2 )in Pb+Pb collisions with √s==2.76 TeV at the LHC.The partonic spectra in the initial hard scattering of elementary collisions are obtained by an event generator POWHEG+PYTHIA,which matches the next-to-leading order(NLO)matrix elements with parton showering,and the energy loss of a fast parton traversing the hot/dense QCD medium is calculated by Monte Carlo simulation within the higher-twist formalism of jet quenching in heavy-ion collisions.We present the model calculations of event normalized girth distributions for inclusive jets in p+p and Pb+Pb collisions at √s=2.76 TeV,which give good descriptions of ALICE measurements.It is shown that the girth distributions of inclusive jets in Pb+Pb are shifted to lower girth regions relative to those in p+p.Thus,the nuclear modification factor of girth distributions for inclusive jets is larger than unity at small girth regions and smaller than unity at large girth regions.This behavior results from softer fragments inside a jet as well as the fraction alteration of gluon/quark initiated jets in heavy-ion collisions.We further predict the girth distributions for Z^0 boson tagged jets in Pb+Pb collisions at √s=2.76 TeV and demonstrate that the medium modification on girth distributions for Z^0 tagged jets is less pronounced compared to that for inclusive jets because the dominant components of Z^0 tagged jets are quark-initiated jets.

Net-Baryon and Net-Kaon Rapidity Distributions in Ultrarelativistic Heavy-Ion Collisions

In this paper,the gluon distribution is extracted from the KLR-AdS/CFT saturation model and used to investigate net-baryon and net-kaon rapidity distributions in ultrarelativistic heavy-ion collisions.With the same parameters of the saturation model fitting to HERA data and an χ 2 analysis of the overall constant C,the theoretical results are in good agreement with RHIC data in Au+Au collisions at √ s = 0.2 TeV.Then,we present the predictive results for net-baryon rapidity distributions in central Pb+Pb collisions at LHC energies of √ s = 2.76,3.94,and 5.52 TeV,and give the corresponding values of dN/dy for net-baryon at y = 0.

Multiplicity fluctuation and correlation of mesons and baryons in ultra-relativistic heavy-ion collisions at the LHC

We study the multiplicity fluctuation and correlation of identified mesons and baryons formed at hadronization by the quark combination mechanism in the context of ultra-relativistic heavy-ion collisions. Based on the statistical method of free quark combination, we derive the two-hadron multiplicity correlations, including meson-meson and meson-baryon correlations, and take the effects of quark number fluctuation at hadronization into account by a Taylor expansion method. After including the decay contributions, we calculate the dynamical fluctuation observable ν_(dyn) for Kπ, pπ and Kp pairs and discuss what underlying physics can be obtained by comparing with data from Pb-Pb collisions at sNN^(1/2)=2.76 Te V and simulations from the HIJING and AMPT event generators.

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.

Slow Particle Production in Nucleus-Nucleus Collisions at Relativistic Energies

In this paper an effort has been made to study the general characteristics of slow particles produced in the interactions of 32S-Em at 200 AGeV to extract the information about the mechanism of particle production. The results have been compared with the experimental results obtained by other workers. The multiplicity distributions of the slow target associated particles (black, grey and heavy tracks) produced by 32S-beam with different targets have been studied. Also several types of correlations among them have been investigated. The variation of the produced particles with projectile mass number and target size has been studied. Also the multiplicity distributions of slow particles with NBD fits are presented and scaling multiplicity distributions of slow particles produced have been studied in order to check the validity of KNO-scaling.

Further developments of a multi-phase transport model for relativistic nuclear collisions

A multi-phase transport(AMPT)model was constructed as a self-contained kinetic theory-based description of relativistic nuclear collisions as it contains four main components:the fluctuating initial condition,a parton cascade,hadronization,and a hadron cascade.Here,we review the main developments after the first public release of the AMPT source code in 2004 and the corre-sponding publication that described the physics details of the model at that time.We also discuss possible directions for future developments of the AMPT model to better study the properties of the dense matter created in relativistic collisions of small or large systems.

Multiplicity Distribution of Evaporated Fragments in Relativistic Nucleus-Nucleus Collisions

The multiplicity distribtuion of evaporated fragments in relativistic nucleus-nucleus collisions are discussed.The calculated results are in agreement with the exper-imental data in the energy range from 3.7 to 200 GeV/N.

A Theoretical Approach to Study J/Ψ Suppression in Relativistic Heavy Ion Collisions with Ellipsoidal Evolution

With a view to understanding J/Ψ suppression in relativistic heavy ion collisions, we compute the suppression rate within the framework of hy-drodynamical evolution model. For this, we consider an ellipsoidal flow and use an ansatz for temperature profile function which accounts for time and the three dimensional space evolution of the quark-gluon plas-ma. We have calculated the survival probability separately as the func-tion of transverse and longitudinal momentum. We have shown that previous calculations are special cases of this model.

Determination of Absorption Cross Section of J/ψ in Relativistic Heavy Ion Collisions

The absorption cross section σabs of J/ψ produced in relativistic heavy ioncollisions is deduced based on the analysis of the absorption length through which theproduced J/ψ passes in the colliding nuclei.The obtained σabs in A-A collisions is con-siderably higher than that in h-A processes.The reason for this different result is dis-cussed in terms of an analysis of the pT-dependence of the produced J/ψ.

A Calculation of Excitation Energy of Spectator in Relativistic Nucleus-Nucleus Collisions

The excited degree of spectator in relativistic nucleus-nucleus collisions is investig-ated.The heat of friction due to the relative motion beween the participant and the spectatorand the superfluous surface energy due to the spectator deformation are calculated.