Hard exclusive photoproduction of $\Phi$ mesons

We calculate the differential cross section and single-polarization observables for the reaction $\gamma p \to \Phi p$ 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 means of a systematic expansion in the small parameter (hadron mass/ photon energy). With the $\Phi$-meson distribution amplitude taken from the literature our predictions for the differential cross section at $| t | \agt 4 \text{GeV}^2$ seem to provide a reasonable extrapolation of the low-t data and are also comparable in magnitude with the results of a two-gluon exchange model in which the gluons are considered as a remnant of the pomeron. For momentum transfers of a few GeV hadron-mass effects appear still to be sizeable.