Beam-Energy and Centrality Dependence of Directed Flow of Identified Particles

These proceedings present directed flow ($v_1$) measurements in Au+Au collisions from STAR’s Beam Energy Scan (BES) program at the Relativistic Heavy-Ion Collider, for $p$, $\bar{p}$, $\mathrm{\Lambda }$, $\bar{\mathrm{\Lambda }}$, $K^{\pm }$, $K_S^0$ and ${\pi }^{\pm }$. At intermediate centrality, protons show a minimum in directed flow slope, $d{v}_1\mathrm{/}dy{\mathrm{\mid }}_{y\le 0.8}$, as a function of beam energy. Proton $d{v}_1\mathrm{/}dy$ changes sign near 10 GeV, and the directed flow for $\mathrm{\Lambda }$ is consistent with the proton result. The directed flow slope for net protons shows a clear minimum at 14.5 GeV and becomes positive at beam energies below 10 GeV and above 30 GeV. New results for net-kaon directed flow slope resemble net protons from high energy down to 14.5 GeV, but remain negative at lower energies. The slope $d{v}_1\mathrm{/}dy$ shows a strong centrality dependence, especially for $p$ and $\mathrm{\Lambda }$ at the lower beam energies. Available model calculations are in poor agreement.