Difference in elliptic flow between particles and antiparticles in Au + Au collisions at √^sNN = 7.7∼62.4 GeV in UrQMD

The elliptic flow for proton, π⁺, K⁺ and their corresponding antiparticles are investigated at mid-rapidity for Au + Au collisions at √^SNN = 7.7, 11.5, 19.6, 27, 39, 62.4 GeV, utilizing ultrarelativistic quantum molecular dynamics (UrQMD-3.3pl). We analyze the difference in V₂ (p_T) between the particles and the corresponding antiparticles as a function of the transverse momentum P_T. We find that in UrQMD the V₂ for K^- is larger than that for K⁺ at all BES energies. The V₂(p_T) values are almost identical for π⁺ and π^- at 7.7∼62.4 GeV. While in experiments, at lower energies, 7.7, 11.5 and 19.6 GeV, v₂(π^-) is larger than v₂(π⁺) for all P_T values. We can clearly see that in UrQMD the V₂ for antiproton is larger than that for proton at all BES energies, which is in contradiction to the experimental result. The opposite trend observed in experiment is therefore an indication that the strong coupling in heavy ion collision and the non equilibrium in transport process may need further understanding. (authors)