# Gluon helicity flip in a plane wave background

## TL;DR

This paper calculates the probability of gluon helicity flip in a plane wave background within pure Yang-Mills theory and QCD, providing exact one-loop results and novel formalism for such background interactions.

## Contribution

It introduces a background-dressed spinor helicity formalism and computes the all-orders-in-coupling flip amplitude for gluons in a plane wave gauge field.

## Key findings

- Derived simple formulae for gluon helicity flip amplitude.
- Provided explicit examples for SU(2) gauge group.
- Analyzed high- and low-energy limits of the flip probability.

## Abstract

We compute the leading probability for a gluon to flip helicity state upon traversing a background plane wave gauge field in pure Yang-Mills theory and QCD, with an arbitrary number of colours and flavours. This is a one-loop calculation in perturbative gauge theory around the gluonic plane wave background, which is treated without approximation (i.e., to all orders in the coupling). We introduce a background-dressed version of the spinor helicity formalism and use it to obtain simple formulae for the flip amplitude with pure external gluon polarizations. We also give in-depth examples for gauge group SU(2), and evaluate both the high- and low-energy limits. Throughout, we compare and contrast with the calculation of photon helicity flip in strong-field QED.