Directed Flow of Identified Particles in Au + Au Collisions at $\sqrtsNN = 200$ GeV at RHIC

TL;DR

This paper reports measurements of directed flow ($v_1$) for identified particles in Au+Au collisions at 200 GeV, revealing a negative $v_1(y)$ slope for most particles and discrepancies with several theoretical models.

Contribution

First detailed measurement of $v_1$ for multiple identified particles at RHIC energies, with comparisons to various models highlighting limitations in current theoretical descriptions.

Findings

Most particles show a negative $v_1(y)$ slope.

Protons and antiprotons exhibit different $v_1(y)$ slopes, especially in mid-central collisions.

Current models fail to simultaneously describe $v_1(y)$ for pions and protons.

Abstract

STAR's measurements of directed flow ($v_1$) around midrapidity for $\pi^{\pm}$, K$^{\pm}$, K$_S^0$, $p$ and $\bar{p}$ in Au + Au collisions at $\sqrtsNN = 200$ GeV are presented. A negative $v_1(y)$ slope is observed for most of produced particles ($\pi^{\pm}$, K$^{\pm}$, K$_{S}^{0}$ and $\bar{p}$). In 5-30% central collisions a sizable difference is present between the $v_1(y)$ slope of protons and antiprotons, with the former being consistent with zero within errors. The $v_1$ excitation function is presented. Comparisons to model calculations (RQMD, UrQMD, AMPT, QGSM with parton recombination, and a hydrodynamics model with a tilted source) are made. For those models which have calculations of $v_1$ for both pions and protons, none of them can describe $v_1(y)$ for pions and protons simultaneously. The hydrodynamics model with a tilted source as currently implemented cannot explain the centrality dependence of the difference between the $v_1(y)$ slopes of protons and antiprotons.