A hydrodynamic study of hyperon spin polarization in relativistic heavy ion collisions

TL;DR

This paper uses hydrodynamic modeling to analyze hyperon spin polarization in heavy-ion collisions, successfully describing some observed patterns and predicting others, while identifying limitations in current theoretical approaches.

Contribution

It introduces a comprehensive hydrodynamic model incorporating pre-equilibrium dynamics to study hyperon spin polarization and makes predictions for unmeasured polarization components.

Findings

Model reproduces collision-energy, centrality, rapidity, and $p_T$ dependence of $oldsymbol{ ext{Lambda}}$ polarization.

Predicts global spin polarization of $oldsymbol{ ext{Xi}^-}$ and $oldsymbol{ ext{Omega}^-}$ as a function of collision energy.

Identifies discrepancies in azimuthal-angle dependence, indicating need for model improvements.

Abstract

We perform a systematic study of the spin polarization of hyperons in heavy-ion collisions using the MUSIC hydrodynamic model with A Multi-Phase Transport (AMPT) pre-equilibrium dynamics. Our model calculations nicely describe the measured collision-energy, centrality, rapidity, and $p_T$ dependence of $\Lambda$ polarization. We also study and predict the global spin polarization of $\Xi^-$ and $\Omega^-$ as a function of collision energy, which provides a baseline for the studies of the magnetic moment, spin, and mass dependence of the spin polarization. For the local spin polarization, we calculate the radial and azimuthal components of the transverse $\Lambda$ polarization and find specific modulating behavior which could reflect the circular vortical structure. However, our model fails to describe the azimuthal-angle dependence of the longitudinal and transverse $\Lambda$ polarization, which indicates that the hydrodynamic framework with the spin Cooper-Frye formula under the assumption of thermal equilibrium of spin degree of freedom needs to be improved.