Elliptic flow in heavy-ion collisions at intermediate energy: the role of impact parameter, mean field potential, and collision term

Abstract

Within the ultrarelativistic quantum molecular dynamics (UrQMD) model, by reverse tracing nucleons that are finally emitted at mid-rapidity (|y0| < 0.1) in the entire reaction process, the time evolution of elliptic flow (v2) of these traced nucleons produced in Au+Au collisions at beam energy of 0.4 GeV/nucleon with different impact parameters (b) is studied. The initial value of v2 is positive and increases with b, then it decreases as time passes and tends to saturate at a negative value. It is found that nucleon-nucleon collisions always depress the value of v2 (enhance the outof-plane emission), while the nuclear mean field potential may slightly raise the value of v2 during the expansion stage in peripheral reactions. The related density mostly probed by v2 of nucleons at mid-rapidity is found to be ∼ 60% of the maximum density reached during the collisions.