Production of J/psi Mesons at HERA

Abstract

Inelastic and diffractive production of J/psi mesons at HERA is reviewed. The data on inelastic photoproduction are described well within errors by the Colour Singlet Model in next-to-leading order. A search for colour octet processes predicted within the NRQCD/factorisation approach is conducted in many regions of phase space. No unambiguous evidence has been found to date. Diffractive elastic production of J/psi mesons has been measured in the limit of photoproduction up to the highest photon proton center of mass energies. The increase of the cross section is described by pQCD models. At larger Q^2, the W dependence is found to be similar to that observed in photoproduction. First analyses of data at high t yield a powerlike dependence on |t|. A LO BFKL calculation gives a good description of the data.