Polarization of Direct Photons from Gluon Anisotropy in Ultrarelativistic Heavy Ion Collisions

TL;DR

This paper demonstrates that anisotropic gluon momentum distributions in ultrarelativistic heavy ion collisions can produce polarized direct photons, offering a new way to probe early-stage gluon pressure anisotropy.

Contribution

It introduces a mechanism linking gluon anisotropy to photon polarization and estimates the polarization asymmetry in a simplified model.

Findings

Polarization asymmetry could reach up to 10%.

Photon polarization measurements can reveal gluon pressure anisotropy.

Gluon anisotropy influences direct photon polarization in heavy ion collisions.

Abstract

We show how anisotropy in momentum of the gluon distribution in ultrarelativistic heavy ion collisions gives rise to polarization of direct photons produced via gluon-quark Compton scattering, as well as by quark-antiquark annihilation into a gluon-photon pair. We estimate the polarization asymmetry from the Compton process within a toy model where the polarized photons are produced from thermal gluons scattered by heavy-quark scattering centers moving with Bjorken boost-invariant flow, and find that it could be as large as 10%. We conclude that polarization measurements of directly produced photons can shed light on gluon pressure anisotropy in the early stages of collisions.