An explanation of the elliptic flow difference between proton and   anti-proton from the UrQMD model with hadron potentials
  The time evolution of both proton and anti-proton $v_2$ flows from Au+Au collisions at $\sqrt{s_{NN}}$=7.7 GeV are examined by using both pure cascade and mean-field potential versions of the UrQMD model. Due to a stronger repulsion at the early stage introduced by the repulsive potentials and hence much less annihilation probabilities, anti-protons are frozen out earlier with smaller $v_2$ values. Therefore, the experimental data of anti-proton $v_2$ as well as the flow difference between proton and anti-proton can be reasonably described with the potential version of UrQMD. 
Qingfeng Li, Yongjia Wang, Xiaobao Wang, and Caiwan Shen
nucl-th