The energy dependence of the cross sections for electromagnetic diffractive processes can be well described by a single power, $W^delta$. For $J/psi$ photoproduction this holds in the range from 20 GeV to 2 TeV. This feature is most easily explained
by a single pole in the angular momentum plane which depends on the scale of the process, at least in a certain range of values of the momentum transfer. It is shown that this assumption allows a unified description of all electromagnetic elastic diffractive processes. We also discuss an alternative model with an energy dependent dipole cross section, which is compatible with the data up to 2 TeV and which shows an energy behaviour typical for a cut in the angular momentum plane.
We test the hypothesis that diffractive scattering in the perturbative and non-perturbative domain is determined by the exchange of a single pomeron with a scale dependent trajectory. Present data on diffractive vector meson production are well compa
tible with this model and recent results for $J/psi$ photoproduction at LHC strongly support it. The model is inspired by concepts of gauge/string duality applied to the pomeron.
We discuss various aspects of vector meson production, first analysing the interplay between perturbative and nonperturbative aspects of the QCD calculation. Using a general method adapted to incorporate both perturbative and nonpertubative aspects,
we show that nonperturbative effects are important for all experimentally available values of the photon virtuality Q2. We compare the huge amount of experimental information now available with our theoretical results obtained using a specific nonperturbative model without free parameters, showing that quite simple features are able to explain the data.
