New contributions to gluon poles in direct photon production

TL;DR

This paper investigates the role of new twist-3 gluon contributions in direct photon production with transversely polarized hadrons, highlighting their importance for gauge invariance and universality of parton distributions.

Contribution

It introduces new twist-3 terms in the hadron tensor using the contour gauge, emphasizing their significance for gauge invariance and universality breaking.

Findings

New twist-3 terms are essential for gauge invariance.

These terms contribute as much as standard ones to the hadron tensor.

They impact the universality of twist-3 parton distributions.

Abstract

We consider the direct photon production in two hadron collision with one of the hadrons being transversely polarized. By using the contour gauge for gluon fields, we find that there are new twist-$3$ terms present in the hadron tensor of the considering process in addition to the standard twist-$3$ terms. In this work, we demonstrate that the significance of these new terms are two-fold: first, they are crucial to both the QED and QCD gauge invariance and, second, their contributions to the hadron tensor are at least the same as those from the standard ones. We also study the resuting effects which are responsible for the universality breaking of the corresponding twist-$3$ parton distributions.