Spin polarization in heavy-ion collisions induced by thermal vorticity and thermal shear

TL;DR

This paper explores how thermal vorticity and shear in relativistic fluids influence spin polarization of fermions, providing a theoretical framework to connect hydrodynamic quantities with spin observables in heavy-ion collisions.

Contribution

It introduces a quantum field theory approach linking fermion spin polarization to thermal vorticity and shear, advancing understanding of spin effects in QCD matter.

Findings

Spin polarization relates to thermal vorticity and shear.

Spin-rotation coupling connects to gravitational form factors.

Spin serves as a new probe for QCD properties.

Abstract

The vorticity is a quantity defined in a relativistic fluid that describes how much a fluid element is rotating and accelerating. By measuring the spin polarization of hadrons, it was found that the quark gluon plasma produced in heavy-ion collisions is the most "vorticous" fluid ever observed. More generally, this opens the possibility to study the physics of QCD matter using spin. Here I use statistical quantum field theory applied to a fluid in local thermodynamic equilibrium to show how to connect the average spin of a fermion with hydrodynamic quantities, and in particular with the thermal vorticity and the thermal shear. I show that the spin polarization of a Dirac particle induced by thermal vorticity is related to the gravitational (in medium) form factor related to spin-rotation coupling. For these reasons, as we are understanding the role of spin in hydrodynamics and in heavy-ion collisions, spin is becoming a promising tool to investigate the properties of QCD and whose applications are just begun to be explored.