Influence of Chiral Mean Field on Kaon In-plane Flow in Heavy Ion Collisions

The influence of the chiral mean field on the in-plane flow in heavy ion collisions at SIS energy is investigated within covariant kaon dynamics. For the kaon mesons inside the nuclear medium a quasi-particle picture including scalar and vector fields is adopted and compared to the standard treatment with a static potential. It is confirmed that a Lorentz force from spatial component of the vector field provides an important contribution to the in-medium kaon dynamics and strongly counterbalances the influence of the vector potential on the in-plane flow. The calculated results show that the new FOPI data can be reasonably described using the Brown & Rho parametrization, which partly takes into account the correction of higher order contributions in the chiral expansion. This indicates that one can abstract the information on the kaon potential in a nuclear medium from the analysis of the in-plane flow.

Anisotropic emission of charged mesons and structure characteristic of emission source in heavy ion collisions at 1-2A GeV

Angular distributions of pious and kaons produced in heavy ion collisions at the low-energy end of high energies （1-2 A GeV） have been investigated by using a multisource ideal gas model. The model covers the expansions and movements of the emission sources, and it is related to the collective flows. By using the analytic expression and the Monte Carlo method, the azimuthal and polar angle distributions of mesons are calculated by the model and compared with the experimental data of the KaoS Collaboration.

Unified Hydrodynamics and Pseudorapidity Distributions of Charged Particles Produced in Heavy Ion Collisions at Low Energies at RHIC

In the context of unified hydrodynamics, we discuss the pseudorapidity distributions of the charged particles produced in Au-Au and Cu-Cu collisions at the low RHIC energies of √SNN = 19.6 and 22.4 GeV, respectively. It is found that the unified hydrodynamics alone can give a good description to the experimental measurements. This is different from the collisions at the maximum RHIC energy of √SNN = 200 GeV or at LHC energy of √SNN= 2.76 TeV, in which the leading particles must be taken into account so that we can properly explain the experimental observations.

Momentum-Dependent Symmetry Potential from Nuclear Collective Flows in Heavy Ion Collisions at Intermediate Energies

Within the isospin-dependent quantum molecular dynamics model, we investigate the nuclear collective flows produced in semi-central 197 Au＋197 Au collisions at intermediate energies. The neutron proton differential flows and difference of neutron proton collective flows are sensitive to the momentum-dependent symmetry potential. This sensitivity is less affected by both the isoscalar part of nuclear equation of state and in-medium nucleon- nucleon cross sections. Moreover, this sensitivity becomes pronounced with increasing the rapidity cut.

Superheavy Particle Production in High Energy Heavy Ion Collisions

The existence of several TeV superheavy particles (SHPs) is predicted by theories beyond the Standard Model. Particles with a mass exceeding the energy in the center of mass in the collision of protons with protons can be produced in subthreshold heavy ion collisions at the LHC. The purpose of the performed research was to estimate the rate of a rare process of SHPs production. It was shown that the data on the subthreshold production of antiprotons can be explained by the phenomenological parton model. The obtained parton distribution function was used to determine the number of SHPs produced in subthreshold heavy ion collisions at the LHC. In one month of collision of lead with lead, the yield of 16 TeVparticles is about 70 per year. To study the kinematically forbidden phenomena in proton-proton interactions in collisions of heavy nuclei at the LHC, an experiment on the production of antiprotons is proposed in the ALICE fixed target project.

Correlations of baryon and charge stopping in heavy ion collisions

Baryon numbers are theorized to be carried by valence quarks in the standard QCD picture of the baryon structure.Another theory proposed an alternative baryon number carrier,a non-perturbative Y-shaped configuration of the gluon field,called the baryon junction in the 1970s.However,neither of these theories has been verified experimentally.Recently,searching for the baryon junction by investigating the correlation of net-charge and net-baryon yields at midrapidity in heavy-ion collisions has been suggested.This paper presents studies of such correlations in collisions of various heavy ions from oxygen to uranium with the UrQMD Monte Carlo model.The UrQMD model implements valence quark transport as the primary means of charge and baryon stopping at midrapidity.Detailed studies are also conducted for isobaric ^(96)_(40)Zr+^(96)_(40)Zr and ^(96)_(44)Ru+^(96)_(44)Ru collisions.We found a universal trend of charge stopping with respect to baryon stopping and discovered that the charge stopping is always greater than the baryon stopping.This study provides a model baseline in valence quark transport for what is expected in net-charge and net-baryon yields at the midrapidity of relativistic heavy-ion collisions.

Re-visit N／Z Ratio of Free Nucleons from Collisions of Neutron-Rich Nuclei as a Probe of EoS of Asymmetric Nuclear Matter

The N/Z ratio of free nucleons from collisions of neutron-rich nuclei as a function of their momentum is studied by means of isospin-dependent Quantum Molecular Dynamics. We find that this ratio is not only sensitive to the form of the density dependence of the symmetry potential energy but also its strength determined by the symmetry energy coefficient. The uncertainties about the symmetry energy coefficient influence the accuracy of probing the density dependence of the symmetry energy by means of the N/Z ratio of free nucleons of neutron-rich nuclei.

Studies of K-shell x-ray energy shifts induced by MeV/u heavy ions

This paper reports that the K x-ray spectra of the thin target 4TAg, 4sCd, 49In and 50Sn were measured by an HPGe semi-conductor detector in collisions with 84.5 MeV 6C4＋ ions. Our experiment revealed the Kα x-ray energy shifts were not obvious and the Kβ1 x-ray energy shifts were about 90-110 eV. The simple model of Burch et al has been previously used to calculate the K x-ray energy shifts due to an additional vacancy in 2p orbit. The present work extends the model of Burch to calculate the x-ray energy shifts of multiple ionized atoms induced by heavy ions with kinetic energy of MeV/u. In addition to our experimental results, many other experimental results are compared with the calculated values by using the model.

Shear Viscosity to Entropy Density Ratio in Au+Au Central Collisions

The shear viscosity （7） and entropy density （s） are studied for the central Au＋Au collisions with the help of a microscopic transport model, namely the isospin-dependent quantum molecular dynamic model （IQMD）. Employ the formula given in Ref. [1],η is calculated as a time dependent variable for different incident energies from 40 MeV/u to 120 MeV/u and the energy dependence of η/s of nuclear matter in the most compressed stage in collision process is displayed.

Entropy Production and Fractal Dimensions in Heavy Ion Nuclear Reaction at Intermediate Energies

The characteristics of the nonlinear dynamics in the Heavy Ion Collision (HIC) at intermediate energies have been studied by evaluating the productions of the Generalized Entropy (GE) and the Multifragmentation Entropy (ME) as well as the features of the information and fractal dimensions within the Isospin Quantum Molecular Dynamical Model compensated by the lattice methods. Results demonstrate from various views that the existence of deterministic chaos in the dynamical process of reaction.

Effects of a phase transition on two-pion interferometry in heavy ion collisions at √SNN=2.4-7.7 GeV

Hanbury-Brown-Twiss(HBT) correlations for charged pions in central Au+Au collisions at √SNN=2.4-7.7 GeV(corresponding to beam kinetic energies in the fixed target frame from Elab=1.23 to 30 GeV/nucleon) are calculated using the ultra-relativistic quantum molecular dynamics model with different equations of state(EoSs).The effects of a phase transition at high baryon densities are clearly observed in the explored HBT parameters.The results show that the available data on the HBT radii,RO/RSand R^(2)_(O)-R^(2)_(S),in the investigated energy region favor a relatively stiff EoS at low beam energies,which then turns into a soft EoS at high collision energies consistent with astrophysical constraints on the high-density EoS of quantum chromodynamics(QCD).The specific effects of two different phase transition scenarios on RO/RSand R^(2)_(O)-R^(2)_(S)are investigated.A phase transition with a significant softening of the EoS below four times the nuclear saturation density can be excluded using HBT data.Our results highlight that the pion’s RO/RSand R^(2)_(O)-R^(2)_(S)are sensitive to the stiffness of the EoS and can be used to constrain and understand the QCD EoS in a high baryon density region.

Isoscaling properties for neutron-rich fragments in highly asymmetric heavy ion collision systems

Traditionally,isoscaling has been interpreted and applied within the framework of the grand canonical ensemble,based on the assumption that fragment production occurs following the attainment of a statistical equilibrium state.However,the influence of the symmetry energy can lead to differences in the neutron and density distribution in neutron-rich nuclei.This in turn may impact the iso scaling parameters(usually denoted byαandβ).We examine the isoscaling properties for neutron-rich fragments produced in highly asymmetric systems on inverse kinematics,namely^(40,48)Ca and ^(58,64)Ni+^(9)Be at 140 MeV per nucleon.We evaluate α and β values and sort them as a function of the neutron excess I≡N-Z.The significant differences in a extracted from fragments within different ranges of I emphasize the importance of understanding the dependence of isoscaling parameters on fragments generated in various collision regions.Furthermore,the|β(N)|/α(Z)value for a specific fragment in small size and highly iso spin asymmetry systems can serve as a probe to detect the variations in neutron density and proton density in different regions of the nucleus and indicate the limitations of theoretical models in investigating these issues.

Different coalescence sources of light nucleus production in Au-Au collisions at √sNN=3 GeV

We study the production of light nuclei in the coalescence mechanism of Au-Au collisions at midrapidity at √sNN=3GeV.We derive analytic formulas of the momentum distributions of two bodies,three bodies,and four nucleons coalescing into light nuclei and naturally explain the transverse momentum spectra of the deuteron(d),triton(t),helium-3(3He),and helium-4(4He).We reproduce data on the yield rapidity densities,yield ratios,and averaged transverse momenta of d,t,3He,and 4He and provide the proportions of contributions from different coalescence sources for t,3He,and 4He in their production.We find that besides nucleon coalescence,nucleon+nucleus coalescence and nucleus+nucleus coalescence may play requisite roles in light nucleus production in Au-Au collisions at √sNN=3 GeV.

Production of X_(eses)in heavy ion collisions

The yields of X_(cscs)with its two possible configurations,i.e.,the hadronic molecular state and tetraquark state,for Pb-Pb collisions at√sNN=5.02 TeV is studied.A volume effect is found from the centrality distribution of X_(cscs),which could help to distinguish the inner structure of X_(cscs).We also show the rapidity and the transverse momentum distributions of X_(cscs) production as well as its elliptic flow coefficient as a function of the transverse momentum.

Interactions between heavy quarks and tilted QGP fireballs in 200 A GeV Au+Au collisions

Heavy quark observables are applied to probe the initial energy density distribution with violation of longitudinal boost invariance produced in relativistic heavy-ion collisions.Using an improved Langevin model coupled to a(3+1)-dimensional viscous hydrodynamic model,we study the nuclear modification factor(R_(AA)) and directed flow(v_(1)) and elliptic flow(v_(2)) coefficients of heavy mesons and their decayed electrons at an RHIC energy.We find that the counter-clockwise tilt of nuclear matter in the reaction plane results in a positive(negative) heavy flavor vi in the backward(forward) rapidity region,whose magnitude increases with the heavy quark transverse momentum.The difference in the heavy flavor R_(AA) between different angular regions is also proposed as a complementary tool to characterize the asymmetry of the medium profile.Our model results are consistent with currently available data at the RHIC and provide predictions that can be tested by future measurements.

Antiproton Production with a Fixed Target and Search for Superheavy Particles at the LHC

A proposal for an experiment to measure the cross section of antiproton production in a proton-nuclear collision in a kinematically forbidden region for nucleon-nucleon interaction on a fixed LHC target is considered. It is shown that this process can be separated from the kinematically allowed production process using the existing detectors of the ALICE facility at a proton energy of 7 TeV with a fixed nuclear target. Assuming the scale dependence of the cross section, the data obtained can be used to estimate the subthreshold cross section for the production of superheavy particles with a mass of several tens of TeV in the LHC lead nucleus beam.

Interaction of 28Si with Emulsion Nuclei at 4.5 AGeV/c in View of Thermo-Statistical Approach

In this work, we study some changes of nuclear matter in the interactions of 28Si with emulsion nuclei at 4.5 AGeV/c. From the experimental quantities investigated using Tsallis’ statistics, we deduced the temperature, entropy density and non-equilibrium factor of the nuclear medium. These obtained parameters were used to reveal variations in the nuclear matter at the stated interaction energy. The results that came up from this study were compared with their corresponding results obtained from other heavy ion collision experiments at wide energy range.

Hydrodynamic simulations of directed flow for light hadrons in Au+Au and isobarcollisions at √SNN=200 GeV

Using a(3+1)-D hydrodynamic model,CLVisc,we study the directed flow(vi)of light hadrons pro-duced in Au+Au,Ru+Ru,and Zr+Zr collisions at √SNN=200 GeV.The evolution of tilted energy density,pres-sure gradient,and radial flow along the x-direction is systematically investigated.The counter-clockwise tilt of the initial fireball is shown to be a vital source of directed flow for final light hadrons.A good description of directed flow is provided for light hadrons in central and mid-central Au+Au and isobar collisions at the RHIC.Our numeric-al results show a clear system size dependence for light hadron Vi across different collision systems.We further study the effect of nuclear structure on the directed flow and find that v1 for light hadrons is insensitive to nuclei with quadrupole deformation.

Signals of α clusters in ^(16)O+^(16)O collisions at the LHC from relativistic hydrodynamic simulations

In relativistic heavy ion collisions,the fluctuations of initial entropy density convert to the correlations of final state hadrons in momentum space through the collective expansion of strongly interacting QCD matter.Using a(3+1)D viscous hydrodynamic program,CL Visc,we consider whether the nuclear structure,which provides initial state fluctuations as well as correlations,can affect the final state of heavy ion collisions,and whether one can find signals of α cluster structures in oxygen using final state observables in ^(16)O+ ^(16)O collisions at the CERN Large Hadron Collider.For the initial nucleon distributions in oxygen nuclei,we compare three different configurations,a tetrahedral structure with four-α clusters,the deformed Woods-Saxon distribution,and a spherical symmetric Woods-Saxon distribution.Our results show that the charged multiplicity as a function of centrality and the elliptic flow at the most central collisions using the four-α structure differs from those with the Woods-Saxon and deformed Woods-Saxon distributions,which may help to identify α clustering structures in oxygen nuclei.

Progress of quantum molecular dynamics model and its applications in heavy ion collisions

In this review article,we first briefty introduce the transport theory and quantum molecular dynamics model applied in the study of the heavy ion collisions from low to intermediate energies.The developments of improved quantum molecular dynamics model(ImQMD)and ultra-relativistic quantum molecular dynamics model(UrQMD),are reviewed.The reaction mechanism and phenomena related to the fusion,multinucleon transrer,fragmentation,collective flow and particle production are reviewed and discussed within the framework of the two models.The constraints on the isospin asymmetric muclear equation of state and in-medium nucleon nucleon cross sections by comparing the heavy ion collision data with transport models calculations in last decades are also discussed,and the uncertainties of these constraints are analyzed as well.Finally,we discuss the future direction of the development of the transport models for improving the understanding of the reaction mechanism,the descriptions of various observables,the constraint on the nuclear equation of state,as well as for the constraint on in-medium nucleon-nucleon cross sections.