Anti-flow of Mesons in the High Baryon Density Region

TL;DR

This study investigates the anti-flow of mesons in high baryon density collisions, demonstrating that low transverse momentum mesons exhibit negative directed flow, with the JAM model reproducing experimental observations and highlighting the importance of spectator shadowing.

Contribution

The paper introduces a transport model analysis of meson flow, revealing the significance of spectator shadowing in anti-flow phenomena at high baryon densities.

Findings

JAM model qualitatively reproduces anti-flow of $K^0_S$

Negative $v_1$ slopes at low $p_T$

Spectator shadowing is crucial for anti-flow

Abstract

The E895 and STAR experiments demonstrate that the slopes of directed flow with respect to rapidity ($dv_1/dy|_{y=0}$) 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 $K^0_S$ 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.