Constraining the Gluon in the Proton via Photoproduction Reactions

The theoretical predictions for prominent processes like gg → H or the t t̄ cross section still suffer from a rather large uncertainty stemming from the parton distributions functions (PDFs) in the proton, in particular from the gluon distribution. In view of the LHC with its large gluon luminosity, it is therefore important to analyse how present experiments can be used to constrain the gluon further. HERA has always played a major role in the task of PDF determinations, most importantly through DIS experiments. However, the large-x range (x>∼ 0.2) is still rather poorly constrained, being mainly probed only by fixed target experiments and recently by the high-ET Tevatron jet data 1. In photoproduction reactions at HERA, a quasi-real photon, emitted at small angle from the electron, interacts with a parton from the proton. The photon can either participate directly in the hard interaction or be resolved into a partonic system, in which case a parton stemming from the photon takes part in the hard interaction. Therefore the interest in photoproduction experiments has rather been focused on measuring the parton distributions in the photon than in the proton. However, as will be argued in the following, photoproduction reactions can also serve to probe the gluon in the proton, complementary to other measurements. Here I will consider in particular the following two reactions: γ p → γ + jet and γ p → h + jet, where h is a charged hadron. As compared to dijet photoproduction, these reactions have the experimental advantage that photons as well as charged hadrons are straightforward to measure. However, large-pT photons can also stem from the fragmentation of a hard parton or from light meson decay, such that isolation cuts have to be imposed. While isolation may introduce a source of systematic errors, it also reduces the uncertainty stemming from the photon fragmentation functions. In the case of charged hadron production, the dependence on