Measurement of charge-dependent directed flow in STAR Beam Energy Scan (BES-II) Au+Au and U+U collisions

TL;DR

This paper measures the rapidity dependence of directed flow for various particles in Au+Au and U+U collisions across a range of energies, providing insights into electromagnetic fields and baryon transport in heavy-ion collisions.

Contribution

It presents the first detailed measurement of charge-dependent directed flow in U+U collisions and compares it with Au+Au and isobar collisions, revealing system size and energy dependencies.

Findings

Charge-dependent $dv_1/dy$ varies with system size and energy.

Differences in directed flow suggest electromagnetic field effects.

Results enhance understanding of baryon transport mechanisms.

Abstract

We presented the rapidity dependence of directed flow ($v_1$) and its slope ($dv_1/dy$) for $\pi^\pm$, $K^\pm$ and $p(\bar{p}$) in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7 -- 19.6 GeV from the Beam Energy Scan Phase-II, as well as in U+U collisions at $\sqrt{s_{NN}}$ = 193 GeV measured by the STAR experiment. The $v_1$ values are reported as a function of rapidity and centrality. Additionally, the $dv_1/dy$ and the charge dependent difference, $\Delta(dv_1/dy)$, of the three identified particles in U+U collisions is compared to those in Au+Au and isobar (Ru+Ru and Zr+Zr) collisions. These findings offer insights into the initial electromagnetic field as well as baryon transport at various system sizes and beam energies.