Elastic vector meson production and DVCS at HERA

Abstract

Recent results from the H1 and ZEUS experiments are reviewed. The cross sections of elastic photoand electroproduction of the vector mesons φ and J/ψ are measured as functions of Q2,W and t. The large statistics and the large coverage in t and W allow the determination of the effective Regge exchange trajectories, which are found to be different from the soft pomeron trajectory. Shrinkage is observed in the J/ψ photoproduction. Helicity analyses are used to determine the spin density matrix element r04 00 and the Q 2 dependence of the ratio of the cross sections for longitudinally and transversely polarised photons. S-channel helicity conservation (SCHC) is confirmed for J/ψ production While the properties of the φ production put it in a position between “soft” and “hard” diffraction, the J/ψ production shows “hard” features already in photoproduction. All observed properties are consistent with the view of the production process as a photon induced qq pair scattering off the proton, thereby forming a vector meson. pQCD-based models are able to give a fair description of the data when at least one of the involved scales reaches a large value. The J/ψ data show some sensitivity to the parameterisation of the gluon distribution in the proton. Finally, an analysis of Deep Virtual Compton Scattering is reported. The t dependence of the cross section is measured for the first time. Measurements of the W and Q2 dependences of the DVCS cross section are also presented. A pQCD-based model with calculations in next-to-leading order describes the data rather well.