Shear induced polarization: Collisional contributions

TL;DR

This paper investigates how shear-induced particle redistribution contributes to spin polarization in heavy ion collisions, introducing gauge-invariant self-energy contributions and analyzing their effects in a massless QED plasma.

Contribution

It identifies a new collisional contribution to spin polarization from shear-induced redistribution, incorporating gauge links for gauge invariance and analyzing their impact.

Findings

Self-energy contributions are gauge dependent without gauge links.

Gauge-invariant contributions are introduced via double gauge links.

A modest suppression of polarization occurs at high probe momentum.

Abstract

It has been realized that thermal shear plays a similar role as thermal vorticity in polarizing spin of particles in heavy ion collisions. We point out that shear has a fundamental difference that it leads to particle redistribution in the medium. The redistribution gives rise to an additional contribution to spin polarization through the self-energy, which is parametrically the same order as the one considered so far in the literature. The self-energy contribution is in general gauge dependent. We introduce double gauge links stretching along the Schwinger-Keldysh contour to restore gauge invariance. We also generalize the straight path to adapt to the Schwinger-Keldysh contour. We find another contribution associated with the gauge link, which is also parametrically the same order. We illustrate the two contributions with a massive probe fermion in massless QED plasma with shear. A modest suppression of spin polarization is found from the combined contributions when the probe fermion has momentum much greater than the temperature.