Global polarization measurement in Au+Au collisions

TL;DR

This paper reports measurements of global polarization of Lambda hyperons in Au+Au collisions at RHIC energies, finding results consistent with zero and setting upper limits that inform theoretical models of spin-orbit coupling effects.

Contribution

First measurement of Lambda hyperon global polarization at two RHIC energies, providing constraints on theoretical predictions of spin alignment in heavy-ion collisions.

Findings

Global polarization consistent with zero within measurement precision

Upper limit on polarization set at |P_{Lambda,anti-Lambda}| <= 0.02

Results constrain theoretical models of spin-orbit coupling in quark-gluon plasma

Abstract

The system created in non-central relativistic nucleus-nucleus collisions possesses large orbital angular momentum. Due to spin-orbit coupling, particles produced in such a system could become globally polarized along the direction of the system angular momentum. We present the results of Lambda and anti-Lambda hyperon global polarization measurements in Au+Au collisions at sqrt{s_NN}=62.4 GeV and 200 GeV performed with the STAR detector at RHIC. The observed global polarization of Lambda and anti-Lambda hyperons in the STAR acceptance is consistent with zero within the precision of the measurements. The obtained upper limit, |P_{Lambda,anti-Lambda}| <= 0.02, is compared to the theoretical values discussed recently in the literature.