Directed flow of quarks from the RHIC Beam Energy Scan measured by STAR

Directed flow ($v_1$) is a good probe of the early-stage dynamics of collision systems, and the $v_1$ slope at midrapidity ($d{v}_1\mathrm{/}dy{\mathrm{\mid }}_{y=0}$) is sensitive to the system’s equation of state. STAR has published $v_1(y)$ measurements for ten particle species (${\pi }^{\pm }$, $p$, $\bar{p}$, $\mathrm{\Lambda }$, $\bar{\mathrm{\Lambda }}$, $\varphi$, $K^{\pm }$ and $K_S^0$) in Au+Au collisions at eight beam energies from $\sqrt{s_{NN}}=7.7$ GeV to 200 GeV. In this study, we employ a simple coalescence idea to decompose $v_1$ of hadrons into $v_1$ of constituent quarks. The $d{v}_1\mathrm{/}dy$ values of $\bar{p}$, $K^{-}$ and $\bar{\mathrm{\Lambda }}$ are used to test the coalescence sum rule for produced quarks. Data involving produced quarks support the coalescence picture at $\sqrt{s_{NN}}=11.5$ GeV to 200 GeV, and a sharp deviation from this picture is observed at 7.7 GeV. The $d{v}_1\mathrm{/}dy$ of transported quarks is studied via net particles (net $p$ and net $\mathrm{\Lambda }$). In these proceedings, we further extract the $v_1$ slopes of produced and transported quarks, assuming that the coalescence sum rule is valid.