S ep 2 00 0 A fresh look at diffractive J / ψ photoproduction at HERA , with predictions for THERA

A fresh look at diffractive J/ψ photoproduction at HERA, with predictions for THERA. Abstract A new analysis of the photoproduction of the J/ψ-family is presented incorporating our recent model for the dipole cross section. The relative contributions from small and large transverse size fluctuations of the photon are clear and explicitly illustrate the mixing of pertur-bative and non-perturbative effects in this process. The HERA data on F 2p (x, Q 2) are used to constrain non-perturbative QCD effects in the dipole cross section. Predictions are made over a very wide energy range extending out to the THERA regime, where we expect unitar-ity effects to gradually tame the steep rise with energy observed at HERA. We also estimate an effective slope of α ′ I P ∼ 0.1 GeV −2 for J/ψ-photoproduction at the HERA energy range due to the interplay of short and long-distance contributions.