# Global $\Lambda$ polarization in heavy-ion collisions from a transport model

## TL;DR

This study uses a transport model to analyze the energy dependence of $ ext{Lambda}$ hyperon polarization in heavy-ion collisions, aligning with experimental data and revealing how angular momentum deposition varies with collision energy.

## Contribution

It provides a detailed transport model calculation of $ ext{Lambda}$ polarization across a range of energies, connecting polarization to fluid vorticity and angular momentum deposition.

## Key findings

- Polarization decreases with increasing collision energy.
- Model results agree with STAR experimental measurements.
- Smaller tilt of $ ext{Lambda}$ distribution at higher energies indicates less angular momentum deposition.

## Abstract

The polarizations of $\Lambda$ and $\bar{\Lambda}$ hyperons are important quantities in extracting the fluid vorticity of the strongly coupled quark gluon plasma and the magnitude of the magnetic field created in off-central heavy-ion collisions, through the spin-vorticity and spin-magnetic coupling. We computed the energy dependence of the global $\Lambda$ polarization in off-central Au+Au collisions in the energy range $\sqrt{s_{NN}}=7.7-200$ GeV using a multiphase transport model. The observed polarizations with two different impact parameters agree quantitatively with recent STAR measurements. The energy dependence of the global $\Lambda$ polarization is decomposed as energy dependence of the $\Lambda$ distribution at hadronization and the space-time distribution of the fluid-vorticity field. The visualization of both the $\Lambda$ distribution and the fluid-vorticity field show a smaller tilt at higher collisional energies, which indicates that the smaller global polarization at higher collisional energies is caused by a smaller angular momentum deposition at midrapidity.