Hard exclusive photoproduction of $\Phi$ and $J/\Psi$ mesons

Abstract

We consider the reaction $\gamma p \to M p$, with M being either a $\Phi$ or a $J/\Psi$ meson, within perturbative QCD treating the proton as a quark-diquark system. The phenomenological couplings of gauge bosons to (spatially extended) diquarks and the quark-diquark distribution amplitude of the proton are adopted from previous investigations of baryon form factors and two-photon processes. Going beyond leading order, we take into account hadron-mass effects by a systematic expansion in the small parameter (hadron mass/ photon energy). With the meson distribution amplitudes taken from the literature our predictions for the differential cross section at $| t | \agt 3 \hbox{GeV}^2$ seem to provide a reasonable extrapolation of the low-$t$ data in case of the $\Phi$. They fail, however, completely for the $J/\Psi$. A closer inspection reveals that `` hadron-mass corrections\rq\rq become dominant for t values of only a few GeV in case of the $J/\Psi$. This indicates that one has to go to much larger energies and momentum transfers in order to achieve a reliable description of $J/\Psi$ production within the hard-scattering approach.