Photoproduction of Heavy Quarks at Hera

Heavy-flavoured hadrons are mainly produced at HERA in those events in which the virtuality of the photon exchanged between the electron and the proton is very small (see R. Thorne’s talk [1] for a review on heavy quark production in deep-inelastic scattering.) The electron, in this case, is equivalent to a beam of on-shell photons carrying a fraction of the electron energy distributed according to the WeizsäckerWilliams function [2]. The underlying production mechanism is therefore a photoproduction one, which has been studied extensively in fixed-target experiments (see ref. [3] for a complete bibliography); HERA offers the opportunity of testing the predictions of perturbative QCD for this process in an energy regime which is totally unexplored, the available centre-of-mass energy being about one order of magnitude larger than in fixed-target experiments. This is an interesting opportunity, since photoproduction is much less affected by nonperturbative and higher order effects than hadroproduction. A well-known feature of photoproduction processes is that the incoming photon may fluctuate into a hadronic state before undergoing a hard collision. The corresponding contribution to the cross section is referred to as hadronic (or resolved), to contrast with the case when the photon directly interacts with the hadron (point-like or direct.) Therefore, a differential photon–hadron cross section can be written as the sum of a point-like and a hadronic photon contribution: