Accessing chiral-even quark generalised parton distributions in the exclusive photoproduction of a γπ± pair with large invariant mass in both fixed-target and collider experiments

We compute the exclusive photoproduction of a gamma pi(+/-) pair using the collinear factorisation framework, in the kinematic regime where the pair has a large invariant mass and the meson has a sufficiently large transverse momentum to not resonate with the outgoing nucleon. 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 holographic DA.

Automated summary

We calculate the exclusive photoproduction of a gamma pi(+/-) pair using the collinear factorisation framework, in a regime where the pair has a large invariant mass and the meson has a sufficiently large transverse momentum to avoid resonance with the outgoing nucleon. This process provides a new avenue for studying Generalized Parton Distributions (GPDs), particularly at small skewness xi, due to the high center-of-mass energies available at future experiments. By computing the scattering amplitude at leading twist and leading order in alpha(s), we estimate the cross-section and linear polarisation asymmetries for various kinematics at JLab 12-GeV, COMPASS, future EIC, and LHC (in ultra-peripheral collisions). Our results suggest that the magnitudes of the estimates are large enough to warrant dedicated experimental analysis, especially at JLab. We also compare the results obtained using an asymptotic distribution amplitude (DA) with those obtained using a recently proposed holographic DA.