Diffractive Photoproduction of ${\rm Y}$ at HERA

Abstract

Predictions for the diffractive photoproduction of the ${\rm Y}$-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 $J/\psi$-production.

Comments: 13 pages, latex2e, 5 figures, 5 tables. Revised version corrects a bug in the original code, the effect is to reduce the normalization by about 40%. Figures 3-5 updated. Minor modifications and clarifications included in the text in response to referees report. Qualitative conclusions remain unchanged