Spin-thermal shear coupling in relativistic nuclear collisions

TL;DR

This paper introduces a new non-dissipative contribution to spin polarization in relativistic fluids, proportional to thermal shear, resolving discrepancies between theory and experimental data in heavy-ion collisions.

Contribution

It reveals an overlooked thermal shear term in the covariant theory of relativistic quantum fluids, improving agreement with experimental spin polarization measurements.

Findings

The additional shear-related term explains local polarization discrepancies.

Inclusion of the shear term aligns theoretical predictions with experimental data.

The improved model enhances understanding of spin physics in relativistic fluids.

Abstract

The spin polarization measurements of particles emitted in heavy-ion collisions have opened the possibility for new phenomenological investigations of spin physics in relativistic fluids. The theoretical predictions of global polarization are in agreement with the data, but consistent discrepancies stand out for the local polarization. We show that the covariant theory of relativistic quantum fluids at local equilibrium implies an additional, non-dissipative contribution to the spin polarization vector, which is proportional to the thermal shear, which has been previously overlooked. This additional contribution, together with an improved approximation in the expansion of the local equilibrium density operator, restores the quantitative agreement between the theoretical predictions and the experimental data.