Anti-flow of Mesons in the High Baryon Density Region

Abstract

The E895 and STAR experiments demonstrate that the slopes of directed flow with respect to rapidity ($d{v}_1/dy{|}_{y=0}$) of mesons are negative in the low transverse momentum ($p_T$) region, $p_T<0.8$ GeV/$c$, in Au + Au collisions at $\sqrt{s_{NN}}=3.0-3.9$ GeV. Using the transport model JAM, we investigate the directed flow of ${\pi }^{\pm }$, $K^{\pm }$, and $K^0$ as functions of rapidity, $p_T$, and collision energy in Au + Au collisions at the same energies as those in the E895 and STAR experiments. We find that the JAM model can qualitatively reproduce the anti-flow of $K_S^0$ observed in the E895 experiment. The $v_1$ slopes of pions and kaons are analyzed as functions of the $p_T$ window, revealing a strong $p_T$ dependence of the $v_1$ slopes. Negative $v_1$ slopes are observed in the low $p_T$ region, $p_T<0.8$ GeV/$c$, while positive slopes are shown in the higher $p_T$ region. We find that the shadowing effect from spectators is crucial in generating the anti-flow of mesons at low $p_T$ in the high baryon density region.

Comments: 7 pages, 7 figures, accepted by Phys. Rev. C