Systematic analysis of charmonium photoproduction

Abstract

The available data on J/ψ photoproduction are analysed in terms of pomeron exchange, two-gluon exchange and photon-gluon fusion models. Allowing the pomeron-quark interaction to be flavour-dependent and introducing both soft and hard pomerons it is possible to reproduce the data at √s > 10 GeV and small |t|. The two-gluon exchange calculations indicate strong sensitivity to the gluon distribution function. The results obtained with the most modern MRST2001 and DL PDF reproduce the forward J/ψ photoproduction cross section at √s > 10 GeV. The calculations with the photon-gluon fusion model and with MRST2001 and DL PDF are also in reasonable agreement with the data on the total J/ψ photoproduction cross section. However none of the models describe the data at √s < 10 GeV or |t| ≥ GeV2. We attribute the J/ψ photoproduction at low energies and large |t| to a mechanism different from pomeron or two-gluon exchange. We consider that this might be the exchange of an axial vector trajectory that couples with the axial form factor of the nucleon.