The Collective Flow Of ? From The Medium Energy Neutron Rich Collision

Nuclear equation of state(EOS)in high-density region not only for us to study the structure and properties of nuclei,but also to study the mass-radius relation of neutron stars in the universe is very important.Especially the isospin asymmetric nuclear matter EOS,in recent years become the focus of research,the density dependence of symmetry energy is an urgent problem at present.In the past for a long time,nuclear physicists had been made many attempts from the nuclear related physical quantities to extract information,but has a completely clear image of the nuclear matter density dependence of symmetry energy can not appear so far.We compare the transport model simulation results and experimental data to extract the symmetry energy information,therefore,the transport model plays a very important role in the process of extracting symmetric energy information.At the present stage,There are many transport models used in the study,and this paper we used Ultra-relativistic quantum molecular dynamics(Ur QMD)model,the model of the collision culvert cover 55 different states and 32 different mesons,greatly expand the research field.We through the improvement of transport model to extract valuable information.In this work,firstly by considering different values of the time-step for the potential updates in the Ultra-relativistic quantum molecular dynamics(UrQMD)model,we examine the influence of the time-step its influence on observables,such as the yield and collective flow of nucleons and pions from heavy-ion collisions around 1 GeV /nucleon.It is found that these observables are affected to some extent by the choice of the time-step,and the impact of the time-step on the pion-related observables is more visible than that on the nucleon-related ones.However,its effect on the ?-/ ? + yield ratio and elliptic flow difference between neutrons and protons,which have been taken as sensitive observables for probing the density-dependent nuclear symmetry energy,is fairly weakThe heavy-ion collision is the only way to generate dense nuclear matter.The pion produced from heavy-ion collisions at intermediate energies is one of the most important probe to investigate the equation of state of dense nuclear matter.However,the pion-nucleon potential which influences the pion propagation through the nuclear medium is not quite clear yet.Within the ultra-relativistic quantum molecular dynamics(UrQMD)model,we introduced a phenomenological pion-nucleon potential and studied its influence on the collective flow of ?+ and ?-charged mesons.By comparing the model simulations with the corresponding FOPI experimental data,we found that both the rapidity and transverse momentum dependences of the directed and elliptic flows of pion mesons can be semi-quantitatively reproduced by considering a relatively weak pion-nucleon potential,while the calculated results with a strong pion-nucleon potential and without the pion-nucleon potential are far from the experimental data.