The splitting of directed flow for identified light hadrons ($K$ and $p$) and strange baryons ($\Xi$ and $\Omega$) in Au+Au collisions at STAR

TL;DR

This paper reports the first measurements of rapidity-odd directed flow of $\\Xi$ and $\\Omega$ in Au+Au collisions at 27 and 200 GeV, examining the coalescence sum rule and comparing results with transport models including electromagnetic effects.

Contribution

It provides new experimental data on directed flow of strange baryons and tests the coalescence sum rule at different energies, including the influence of electromagnetic fields in models.

Findings

Violation of the coalescence sum rule increases with charge and strangeness differences.

PHSD model with electromagnetic field matches the experimental data within uncertainties.

First measurement of directed flow for $\\Xi$ and $\\Omega$ at these energies.

Abstract

The first measurements for rapidity-odd directed flow of $\Xi$ and $\Omega$ in Au+Au collisions at $\sqrt{s_{\mathrm {NN}}}=$ 27 and 200 GeV are reported. The coalescence sum rule is examined with various combinations of hadrons where all constituent quarks are produced, such as $K^{\!^-}\!\!({ \bar{u}}{s})$, $\bar{p}({\bar{u}\bar{u}}{\bar{d}})$, $\bar{\Lambda}(\bar{u}\bar{d}\bar{s})$, $\phi({s}{\bar{s}})$, $\bar{\Xi}^{+}(\bar{d}\bar{s}\bar{s})$, ${\Omega}^{-}(sss)$, and $\bar{\Omega}^{+}(\bar{s}\bar{s}\bar{s})$. For such combinations, a systematic violation of the sum rule is observed with increasing difference in the electric charge and the strangeness content of the combinations. Measurements are compared with the calculations of A Multi-Phase Transport (AMPT) model and Parton-Hadron String Dynamics (PHSD) model with electromagnetic (EM) field. The PHSD model with EM field agrees with the measurements within uncertainties.