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

TL;DR

This study investigates the directed flow of quarks in heavy-ion collisions across various energies, testing the coalescence model and revealing a deviation at the lowest energy, which provides insights into the collision dynamics and the equation of state.

Contribution

It introduces a method to decompose hadron directed flow into quark contributions and tests the coalescence sum rule across different energies, highlighting a deviation at 7.7 GeV.

Findings

Support for the coalescence picture at 11.5-200 GeV

Deviation from coalescence at 7.7 GeV

Extraction of directed flow slopes for produced and transported quarks

Abstract

Directed flow ($v_1$) is a good probe of the early-stage dynamics of collision systems, and the $v_1$ slope at midrapidity ($dv_1/dy|_{y=0}$) is sensitive to the system's equation of state. STAR has published $v_1(y)$ measurements for ten particle species ($\pi^\pm$, $p$, $\bar{p}$, $\Lambda$, $\bar{\Lambda}$, $\phi$, $K^\pm$ and $K^0_{S}$) 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 $dv_1/dy$ values of $\bar{p}$, $K^-$ and $\bar{\Lambda}$ 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 $dv_1/dy$ of transported quarks is studied via net particles (net $p$ and net $\Lambda$). In these proceedings, we further extract the $v_1$ slopes of produced and transported quarks, assuming that the coalescence sum rule is valid.