Gluon GPDs and Exclusive Photoproduction of a Quarkonium in Forward Region

TL;DR

This paper investigates the role of twist-3 gluon GPDs in the forward photoproduction of quarkonia, revealing significant power corrections and polarization effects that impact cross-section calculations.

Contribution

It provides a detailed analysis of twist-3 contributions to gluon GPDs and their effects on quarkonium production, including polarization and power suppression insights.

Findings

Twist-3 contributions lead to longitudinal polarization of J/ψ.

Power corrections at twist-3 can significantly affect unpolarized cross-sections.

The amplitude for h_c production at twist-3 contains end-point singularities.

Abstract

Forward photoproduction of $J/\psi$ can be used to extract Generalized Parton Distributions(GPD's) of gluons. We analyze the process at twist-3 level and study relevant classifications of twist-3 gluon GPD's. At leading power or twist-2 level the produced $J/\psi$ is transversely polarized. We find that at twist-3 the produced $J/\psi$ is longitudinally polarized. Our study shows that in high energy limit the twist-3 amplitude is only suppressed by the inverse power of the heavy quark mass relatively to the twist-2 amplitude. This indicates that the power correction to the cross-section of unpolarized $J/\psi$ can have a sizeable effect. We have also derived the amplitude of the production of $h_c$ at twist-3, but the result contains end-point singularities. The production of other quarkonia has been briefly discussed.