Is Hyperon Polarization in Relativistic Heavy Ion Collisions Connected to Axial U(1) Symmetry Breaking at High Temperature?

TL;DR

This paper investigates whether the polarization of hyperons in heavy ion collisions is linked to axial U(1) symmetry breaking at high temperatures, using a Nambu--Jona-Lasinio model with instanton-inspired temperature dependence.

Contribution

It introduces a model incorporating axial U(1) symmetry breaking to explain hyperon polarization and shows constituent strange quarks can reach spin equilibrium near hadronization temperatures.

Findings

Strange quarks reach spin equilibrium below 170 MeV.

Axial U(1) symmetry breaking influences hyperon polarization.

Equilibration times are consistent with experimental observations.

Abstract

Experiments at the Relativistic Heavy Ion Collider (RHIC) have measured the net polarization of $\Lambda$ and $\bar{\Lambda}$ hyperons and attributed it to a coupling between their spin and the vorticity of the fluid created in heavy ion collisions, but how the spin comes to equilibrium with vorticity is an open problem. Recently we found that vorticity fluctuations and helicity flip of strange quarks in quark-gluon plasma through perturbative QCD processes resulted in equilibration times far too long to be relevant. Here we consider the Nambu--Jona-Lasinio model with the inclusion of the six-quark Kobayashi--Maskawa--'t Hooft interaction which breaks axial U(1). Using instanton inspired models for the temperature dependence of the axial symmetry breaking, we find that constituent strange quarks can reach spin equilibrium at temperatures below about 170 MeV, just before they hadronize to form hyperons.