Directed Flow of $\Lambda$, $^{3}_{\Lambda}{\rm H}$ and $^{4}_{\Lambda}{\rm H}$ in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 3.2, 3.5, 3.9 and 4.5 GeV at RHIC

TL;DR

This study investigates the directed flow of hyper-nuclei and hyperons in Au+Au collisions at various energies to understand their production mechanisms and hyperon-nucleon interactions at high baryon densities.

Contribution

It provides the first systematic measurement of hyper-nuclei directed flow across multiple energies in the RHIC BES-II program, comparing results with light nuclei and transport models.

Findings

Hyper-nuclei $v_1$ slopes are positive and decrease with increasing energy.

Hyper-nuclei $v_1$ behavior is consistent with coalescence models.

Results offer insights into hyperon-nucleon interactions at high baryon densities.

Abstract

Studying hyper-nuclei yields and their collectivity can shed light on their production mechanism as well as the hyperon-nucleon interactions. Heavy-ion collisions from the RHIC beam energy scan phase II (BES-II) provide an unique opportunity to understand these at high baryon densities. In these proceedings, we present a systematic study on energy dependence of the directed flow ($v_{1}$) for $\Lambda$ and hyper-nuclei ($^{3}_{\Lambda}{\rm H}$, $^{4}_{\Lambda}{\rm H}$) from mid-central Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 3.2, 3.5, 3.9 and 4.5 GeV, collected by the STAR experiment with the fixed-target mode during BES-II. The rapidity (y) dependence of the hyper-nuclei $v_{1}$ is studied in mid-central collisions. The extracted $v_{1}$ slopes ($\mathrm{d}v_{1}/\mathrm{d}y|_{y=0}$) of the hyper-nuclei are positive and decrease gradually as the collision energy increases. These hyper-nuclei results are compared to that of light-nuclei including p, d, t/$\rm ^{3}He$ and $\rm ^{4}He$. Finally, these results are compared with a hadronic transport model including coalescence after-burner.