- 045 June 1996 ISOLATED PROMPT PHOTON PRODUCTION AT HERA

We study the expectations for prompt photon production rates at HERA in a fully consistent next-to-leading order (NLO) QCD analysis, taking into account the effects of experimental isolation requirements. In particular we examine the sensitivity of the isolated cross section to the photon's gluon content. The utility of hadronic large-p T prompt photon production for providing constraints on the proton's gluon content has led to the suggestion that it may also prove useful in determining the photonic gluon distribution at HERA via the study of photoproduction of direct photons 1. The process was subsequently studied quite extensively, with NLO QCD corrections to it partly taken into account 1,2,3,4. The main shortcoming of all these studies was that only the fully inclusive cross section was considered. Since, however, HERA is a collider it will be necessary to perform isolation cuts in the experiments in order to unambiguously identify the photon signal from the huge hadronic background, just as it is necessary at existing hadron colliders 5. Indeed, first reports on HERA prompt photon results 6,7 confirm this view. In order to make reliable and meaningful predictions it is crucial that the theoretical calculation correctly includes the experimental isolation constraints. This was done in 8 where the first complete NLO analysis, at the same time fully taking into account the effects of isolation, was presented. In the following we provide an update of our study 8 , for which we believe it is time now since most sets of parton distributions used in 8 have been updated in the meantime 9,10,11,12. Furthermore, we will also more closely match cuts used in other HERA pho-toproduction experiments performed so far 13 , hereby making our predictions more realistic and more directly comparable to future data. As with all photoproduction processes at HERA, there are two types of contributions to the cross section, the so-called direct and resolved contributions. In the case of prompt photon production there are two further subclasses in each category, which we label the fragmentation and non-fragmentation processes according to whether the prompt photon is produced directly in the hard scattering process or via fragmentation of a final state parton.