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

TL;DR

This paper reports measurements of directed flow ($v_1$) for various particles in Au+Au collisions across different energies, revealing energy-dependent sign changes and centrality effects, challenging existing models.

Contribution

It provides new experimental data on directed flow for multiple particle species over a range of energies, highlighting discrepancies with theoretical models.

Findings

Protons show a minimum in $dv_1/dy$ near 10 GeV.

Net-proton $dv_1/dy$ has a minimum at 14.5 GeV and changes sign below 10 GeV.

Model calculations poorly match the experimental results.

Abstract

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 $p$, $\bar{p}$, $\Lambda$, $\bar\Lambda$, $K^\pm$, $K^0_S$ and $\pi^\pm$. At intermediate centrality, protons show a minimum in directed flow slope, $dv_1/dy\,|_{y\leq0.8}$, as a function of beam energy. Proton $dv_1/dy$ changes sign near 10 GeV, and the directed flow for $\Lambda$ 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 $dv_1/dy$ shows a strong centrality dependence, especially for $p$ and $\Lambda$ at the lower beam energies. Available model calculations are in poor agreement.