Model Study For The Effects Of Equation Of State And Spectators On Elliptic Flow In Heavy Ion Collision

In the nature,quarks and gluons are confined in hadrons because of strong interactions.Only when extreme conditions of high temperature and high density are reached,hadrons can be de-confined and form a new material form—the quark-gluon plasma(QGP).Quantum chromodynamics(QCD)is considered to be the basic theory to describe the strong interaction of matter,while the phase structure of the nuclear matter described by strong interactions can be represented by a two-dimensional QCD phase diagram.The calculation from lattice QCD have shown that the phase transition from the hadron to the QGP is a crossover at vanishing baryon chemical potentials,while a first-order phase transition is expected for finite baryon chemical potentials.Thus,there must be a transition point between the boundary of the smooth crossing and the first-order phase transition,which is the critical point of QCD transformation.In the current relativistic heavy ion collision,one of the greatest interests of people is to determine the location of the critical point and explore the structure of the QCD phase diagram.In the relativistic heavy-ion collision experiment,in order to understand the properties of the material produced in the early stage of the collision,we need to use the dynamic information of the final particle to construct some observables.The elliptic flow(v2)describes the anisotropy of the azimuthal distribution of the final particles in the momentum space,which is sensitive to the early evolution of the collision and is one of the important observables to study the structure of the QCD phase diagram.If the phase change is a first-order phase change,the system pressure will hardly change during the phase transition,and the ratio of the pressure to the energy density ratio p/e will gradually decrease until it reaches a minimum value.This process is called the softening of the equation of state(EOS).Therefore,in the relativistic heavy-ion collision,it is of great significance to study the influence of the state equation on the elliptic flow for us to determine the QCD critical point.In this paper,we systematically study the various characteristics of the elliptic flow in Au+Au collisions at 3.29GeV ??SNN? 7GeV by using a microscopic transport model,including the centrality,rapidity,transverse momentum and beam energy dependence of v2.We focus on the effect of EOS,weak decay,hadronic scattering and spectator on the elliptic flow.In JAM model,we simulate the two different effects of the equation of state by imposing the mean-field and attracting orbits,respectively.When we employ the mean-field potential,the pressure of the system will increase and the equation of state will harden,leading to a decrease in the elliptic flow.However,when we select attractive orbits in two-body scattering,the system pressure decrease and the equation of state will soften,causing the elliptic flow to increase.As the collision energy increases,the effects of the mean field and the softening of EOS on the elliptic flow will decrease.It is seen that the elliptic flow is highly sensitive to EOS,the enhancement of v2 may be used as a signal of the first-order phase transition at high baryon density regions.On the other hand,we find that weak decay has no obvious influence on the elliptic flow,and hadron scatterings increase the elliptic flow while the squeeze-out effect of spectators can squeeze elliptic flow.And as the beam energy increases,the role of spectators will be smaller.It is worth noting that the impact of the spectators on the elliptic flow will increase with imposing mean-field potential,which means that the mean-field will enhance the squeeze-out effect of spectators.