Directed flow in Au+Au collisions from the RHIC Beam Energy Scan at the STAR experiment

TL;DR

This paper presents measurements of directed flow ($v_1$) for various particles in Au+Au collisions across a wide energy range, revealing sign changes and testing quark coalescence models to understand the collision dynamics.

Contribution

First-time use of $v_1$ measurements to test quark coalescence hypothesis in heavy-ion collisions, providing new insights into quark-level collision processes.

Findings

Sign change in $dv_1/dy$ for protons and $ ext{Lambda}$ around 10-15 GeV.

$v_1$ measurements largely consistent with quark coalescence models.

Deviations from coalescence expectations suggest new collision process probes.

Abstract

We report results of $v_1(y)$ and $dv_1/dy$ near mid-rapidity for $\pi^{\pm}$, $K^{\pm}$, $K_s^0$, $p$, $\overline{p}$, $\Lambda$, $\overline{\Lambda}$ and $\phi$ from Beam Energy Scan Au+Au collisions at $\sqrt{s_{NN}} = $ 7.7 - 200 GeV using the STAR detector at RHIC. The $dv_{1}/dy$ of $\pi^{\pm}$, $K^{\pm}$ and $K_s^0$ mesons remains negative over all beam energies. The $dv_1/dy$ of $p$ and $\Lambda$ baryons shows a sign change around 10 - 15 GeV, while net baryons (net p and net $\Lambda$) indicate a double sign change. The $dv_1/dy$ of $\overline{p}$, $\overline{\Lambda}$ and $\phi$ show a similar trend for $\sqrt{s_{NN}}>$ 14.5 GeV. For the first time, $v_{1}$ measurements are used to test a quark coalescence hypothesis. Many measurements are found to be consistent with the particles being formed via coalescence of constituent quarks. The observed deviations from that consistency offer a new approach for probing the collision process at the quark level.