Some aspects of global Lambda polarization in heavy-ion collisions

TL;DR

This paper investigates how large orbital angular momentum in non-central heavy-ion collisions leads to particle polarization, showing that polarization decreases with energy and aligns with experimental data, explained by fireball expansion dynamics.

Contribution

The study uses the AMPT model to analyze vorticity-induced polarization of Lambda hyperons across various energies, providing insights into energy dependence and angular momentum distribution.

Findings

Global polarization decreases with collision energy.

Results are consistent with STAR experimental measurements.

Angular momentum deposition varies with rapidity region.

Abstract

Large orbital angular momentum can be generated in non-central heavy-ion collisions, and part of it is expected to be converted into final particle's polarization due to the spin-orbit coupling. Within the framework of A Multi-Phase Transport (AMPT) model, we studied the vorticity-induced polarization of $\Lambda$ hyperons at the midrapidity region $|\eta|<1$ in Au-Au collisions at energies $\sqrt{s_{NN}}=7.7\sim200$ GeV. Our results show that the global polarization decreases with the collisional energies and is consistent with the recent STAR measurements. This behavior can be understood by less asymmetry of participant matter in the midrapidity region due to faster expansion of fireball at higher energies. As another evidence, we discuss how much the angular momentum is deposited in different rapidity region. The result supports our asymmetry argument.