Accessing chiral-even quark generalised parton distributions in the exclusive photoproduction of a $ \gamma \pi ^{\pm} $ pair with large invariant mass in both fixed-target and collider experiments

TL;DR

This paper proposes a new method to access chiral-even quark GPDs through exclusive photoproduction of a gamma-pi pair at high invariant mass, applicable in various experimental settings including future colliders.

Contribution

It introduces a collinear factorisation approach to compute the process at leading twist and order, providing cross-section estimates for multiple experimental platforms.

Findings

Cross-sections are large enough for experimental detection.

The process is sensitive to GPDs at small skewness.

Comparison of asymptotic and holographic DAs shows notable differences.

Abstract

We compute the exclusive photoproduction of a $\gamma\,\pi^\pm$ pair using the collinear factorisation framework, in the kinematic regime where the pair has a large invariant mass. This exclusive channel presents a new avenue for the investigation of GPDs. It is particularly interesting as the high centre of mass energies available at future experiments will allow the study of GPDs at small skewness $ \xi $. We compute the scattering amplitude of the process, at leading twist and leading order in $\alpha_s$, which is used to estimate its cross-section and linear polarisation asymmetries with respect to the incoming photon, for JLab~12-GeV, COMPASS, future EIC and LHC (in ultra-peripheral collisions) kinematics. We find that the order of magnitude of estimates are sufficiently large for a dedicated experimental analysis to be performed, especially at JLab. We also compare the results from an asymptotic distribution amplitude (DA) to those using a recently proposed \textit{holographic} DA.