Measurement of global polarization of $\Lambda$ hyperons in Au+Au $\sqrt{s_{\mathrm{NN}}}$ = 7.2 GeV fixed target collisions at RHIC-STAR experiment

TL;DR

This paper reports the measurement of the global polarization of Lambda hyperons in Au+Au fixed-target collisions at 7.2 GeV, revealing a non-zero polarization that increases at lower energies and varies with collision centrality.

Contribution

First measurement of Lambda hyperon global polarization at 7.2 GeV in fixed-target mode at RHIC-STAR, extending the energy range of polarization studies.

Findings

Non-zero global polarization observed

Polarization increases at lower collision energies

Larger polarization in more peripheral collisions

Abstract

In non-central nuclear collisions, the created matter should exhibit strong vorticity, which induces spin polarization. Global polarization is a good tool to investigate the dynamics of the system. Global polarization of $\Lambda$ and $\bar{\Lambda}$ hyperons has been measured at $\sqrt{s_{\mathrm{NN}}}$ = 2.4 GeV - 5.02 TeV in various experiments. We have measured the global polarization of $\Lambda$ hyperons in Au+Au collisions with fixed-target configuration at $\sqrt{s_{\mathrm{NN}}}$ = 7.2 GeV at RHIC-STAR experiment. The non-zero signal is observed and it follows the global trend of $\Lambda$ global polarization that increases towards lower collision energies. The global polarization is larger in more peripheral collisions but it does not show any significant rapidity dependence as well as transverse momentum dependence within uncertainties.