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

TL;DR

This study uses the UrQMD model to analyze how impact parameter, mean field potential, and collision dynamics influence the elliptic flow of nucleons in Au+Au collisions at intermediate energy, revealing the evolution and density dependence of $v_2$.

Contribution

It provides a detailed time evolution analysis of elliptic flow in heavy-ion collisions, highlighting the roles of mean field potential and collision effects at intermediate energies.

Findings

Initial $v_2$ is positive and increases with impact parameter.

Nucleon-nucleon collisions reduce $v_2$, promoting out-of-plane emission.

The density probed by $v_2$ is about 60% of the maximum during collisions.

Abstract

Within the ultrarelativistic quantum molecular dynamics (UrQMD) model, by reverse tracing nucleons that are finally emitted at mid-rapidity (|$y_0$| < 0.1) in the entire reaction process, the time evolution of elliptic flow ($v_2$) 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 $v_2$ 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 $v_2$ (enhance the out-of-plane emission), while the nuclear mean field potential may slightly raise the value of $v_2$ during the expansion stage in peripheral reactions. The related density mostly probed by $v_2$ of nucleons at mid-rapidity is found to be $\sim$ 60% of the maximum density reached during the collisions.