Beam energy dependence of the squeeze-out effect on the directed and elliptic flow in Au + Au collisions in the high baryon density region

Abstract

We present a detailed analysis of the beam energy dependence of the mechanisms for the generation of directed and elliptic flows in Au+Au collisions focusing on the role of hadronic rescattering and spectator shadowing within a microscopic transport model JAM with different equation of state. A systematic study of the beam energy dependence is performed for Au+Au collisions at √ sNN = 2.3 − 62.4 GeV. The transition of the dynamical origin of the directed flow is observed. We find that the initial Glauber type nucleon-nucleon collisions generate negative v1 for nucleons at midrapidity due to the presence of spectator matter, and this negative nucleon v1 is turned to be positive by the meson-baryon interactions at the beam energy region of √ sNN < 30 GeV. In contrast, above 30 GeV there is no spectator shadowing at midrapidity, and initial nucleon-nucleon collisions do not generate directed flow, but subsequent rescatterings among produced particles generate negative v1 for nucleons. It is demonstrated that negative pion-directed flows are mostly generated by the interaction with the spectator matter. It is also shown that the squeeze-out effect is largely suppressed in the case of softening, which leads to the enhancement of elliptic flow around √ sNN = 5 − 7 GeV. The elliptic flow at midrapidity above 10 GeV is not influenced by the squeezeout due to spectator matter, while its effect is seen at the forward rapidity range of y/yc.m. > 0.5, which decreases as beam energy increases.

Anisotropic flow measurements from RHIC to SIS

“…4 summarizes the effect of spectator shadowing on the elliptic flow of nucleons at midrapidity as a function of beam energy. The results were obtained using the hadronic transport model JAM [30] for mean-field simulation as well as simulations with a first-order phase transition. For a broad range of beam energies ( √ s NN = 2 − 7 GeV), the elliptic flow results can be understood in terms of a delicate balance between (i) the ability of pressure developed early in the reaction zone, to effect a rapid transverse expansion of nuclear matter, and (ii) the passage time t pass for removal of the shadowing of participant hadrons by the projectile and target spectators [29,30,33].…”

“…The results were obtained using the hadronic transport model JAM [30] for mean-field simulation as well as simulations with a first-order phase transition. For a broad range of beam energies ( √ s NN = 2 − 7 GeV), the elliptic flow results can be understood in terms of a delicate balance between (i) the ability of pressure developed early in the reaction zone, to effect a rapid transverse expansion of nuclear matter, and (ii) the passage time t pass for removal of the shadowing of participant hadrons by the projectile and target spectators [29,30,33]. The characteristic time for the development of expansion perpendicular to the reaction plane is ∼ R/c s , where the speed of sound c s = √ ∂P/∂ε, R is the nuclear radius, P is the pressure and ε is the energy density [29,33].…”

“…Data are taken from References: FOPI [23],E895[33] and STAR[36]. Right: Excitation function of integral v 2 for nucleons in midcentral Au+Au collisions (4.6 < b < 9.4 fm) from hadronic transport model JAM[30] with first-order phase transition and mean-field simulations with (closed symbols) and without spectator interactions (open symbols). Figure is taken from[30]…”

JAM: an event generator for high energy nuclear collisions

“…The second approach recently implemented in JAM is to modify the scattering style in order to control the EoS [15][16][17][18][19][20]. In the standard implementation of the two-body scattering in the cascade type simulations, it is usually assumed that the azimuthal angle is randomly generated, while the scattering angle is selected according to the distribution which is consistent with the experimental data.…”

“…Another possible way to change the pressure of the system is to modify scattering style in the two-body collisions; selecting repulsive orbit enhances the pressure, while attractive orbit reduces the pressure [11][12][13]. We have demonstrated that by imposing an additional condition in the two-body scattering [14], it is possible to con-trol the pressure of the system and to simulate an equation of state (EoS) such as first-order phase transition or crossover transition [15][16][17][18][19][20].…”