Photoproduction of vector mesons: From gamma-proton to nucleus-nucleus collisions

Experiments on vector meson photoproduction have been a testbed of ideas on the production mechanism, the QCD Pomeron exchange. In particular in varying the mass of the vector meson we can study the Pomeron exchange from the soft to the perturbatively hard regimes. The production mechanism also contains information on the quark-antiquark wave function of the produced meson. High energy protons or ions are the source of a flux of Weizsäcker-Williams photons, which can be utilized to study the photoproduction of vector mesons also at the Tevatron and LHC colliders. We discuss how information on the small-x gluon distribution in protons and nuclei can be obtained. We present our calculations based on a k_⊥-factorization approach which allows us to construct the unintegrated glue of a nucleus from the free-nucleon one. We also briefly discuss the production of J/ψ pairs in nucleus-nucleus collisions via photon-photon fusion.