Wavefunction corrections and off-forward gluon distributions in diffractive J/psi electroproduction

TL;DR

This work analyzes two key corrections to diffractive J/ψ electroproduction: Fermi-motion effects of the heavy quark pair and the probing of off-forward (asymmetric) gluon distributions. By employing a rest-frame treatment with NRQCD-like expansion and ensuring gauge invariance, the author derives a gauge-invariant correction factor (1 + 8/9 ∇^2 φ/(M^2 φ)) that modestly reduces the leading cross section. For off-forward gluons, the paper defines and analyzes the asymmetric distribution F_ξ(y), showing how it enters the cross section alongside a calculable Ω factor from the heavy-quark loop, and provides perturbative estimates comparing F_ξ to the forward gluon density g. Overall, the results quantify how Fermi motion barely alters normalization, while off-forwardness can cause measurable deviations from forward gluon densities in diffractive processes, with implications for extracting gluon densities from such reactions.

Abstract

Diffractive production of J/Psi particles by virtual photons on a proton target is studied with a view towards understanding two important corrections to the leading order result. First, the effect of Fermi motion of the heavy quarks is studied by performing a systematic expansion in the relative velocity, and a simple correction factor is derived. This is considerably less than estimated previously. Second, since the kinematics necessarily requires that non-zero momentum be transferred to the proton, off-forward gluon distributions are probed by the scattering process. To estimate the importance of the off-forwardness, we compute, in leading order perturbation theory, the extent of deviation from the usual forward gluon distribution in a quark.