Polarization in heavy ion collisions: a theoretical review

TL;DR

This paper reviews recent theoretical advances in understanding spin polarization phenomena in relativistic heavy ion collisions, focusing on the effects of shear tensor, pseudo-gauge transformations, and dissipative effects.

Contribution

It provides a derivation of the fermion spin polarization vector at local thermal equilibrium and discusses the implications of various effects on polarization predictions.

Findings

Shear tensor impacts local spin polarization

Pseudo-gauge transformations influence polarization calculations

Dissipative effects are significant in polarization models

Abstract

In these proceedings I discuss the recent progress in the theory of spin polarization in relativistic fluids. To date, a number of studies have begun to examine the impact of the shear tensor on the local spin polarization and whether this contribution can restore agreement between the measurements and the predictions obtained from a polarization induced by the gradients of the plasma. I present the derivation of the spin polarization vector of a fermion at local thermal equilibrium and I discuss the role of pseudo-gauge transformations and of dissipative effects. I list what we can learn from the polarization measured at lower energies. Finally, I discuss possible applications of spin polarization measurements in relativistic heavy ion collisions.