Diffractive Photoproduction of $Υ$ at HERA

TL;DR

This paper analyzes diffractive Υ photoproduction at HERA within a hard exclusive QCD framework, incorporating skewed (off-diagonal) gluon distributions and a large real-part amplitude. It develops a rescaling procedure to determine the effective scale Q^2_eff for each Υ state and employs hybrid light-cone wavefunctions to model the qq̄ bound states. By computing the skewed gluon densities and the real part via dispersion relations, the authors predict cross sections that rise steeply with energy and align with ZEUS/H1 data, highlighting a stronger energy dependence than for J/ψ production. The work underscores the importance of skewedness and real-part corrections for heavy vector meson production and points toward future refinements with higher Fock states and deeper connections to quarkonium wavefunctions.

Abstract

Predictions for the diffractive photoproduction of the $Υ$-family at HERA energies, within the framework of the analysis by Frankfurt, Koepf and Strikman, are presented. Two novel effects lead to a significant enhancement of the original calculation: the non-diagonal (or skewed) kinematics, calculated to leading-log($Q^2$) accuracy, and the large magnitude of the real part of the amplitude. The resultant cross sections are found to agree fairly well with recent preliminary data from ZEUS and H1. A strong correlation between the mass of the diffractively produced state and the energy dependence of the cross section is found. In particular, a considerably stronger rise in energy is predicted than that found in $J/ψ$-production.