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 equations of state. A systematic study of the beam energy dependence is performed for Au + Au collisions at $\sqrt{{s}_{\mathit{NN}}}=2.3\ensuremath{-}62.4$ 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 ${v}_{1}$ for nucleons at midrapidity due to the presence of spectator matter, and this negative nucleon ${v}_{1}$ is turned to be positive by the meson-baryon interactions at the beam energy region of $\sqrt{{s}_{NN}}<30$ 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 ${v}_{1}$ 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 $\sqrt{{s}_{NN}}=5\ensuremath{-}7$ GeV. The elliptic flow at midrapidity above 10 GeV is not influenced by the squeeze-out due to spectator matter, while its effect is seen at the forward rapidity range of $y/{y}_{\mathrm{c}.\mathrm{m}.}>0.5$, which decreases as beam energy increases.