Recent Photoproduction Results From ZEUS

Recent results for inclusive jet cross sections, dijet cross sections and dijet angular distributions are compared with NLO perturbative QCD calculations. The observation of isolated high PT photons (prompt photons) is also reported. INTRODUCTION Collisions of 27.5 GeV positrons with 820 GeV protons at HERA are studied using the ZEUS detector. The processes of interest here are those where an almost real photon (virtually P 2 ≈ 0) is radiated from the positron which then interacts with a parton in the proton to produce two high transverse energy jets. The hard scale provided by the large transverse momentum allows us to describe these processes using perturbative QCD (pQCD), and therefore they can be used to test it. The photon can either interact directly with the parton from the proton (direct processes), or it can first fluctuate into an hadronic state which then acts as source of partons for the hard scatter (resolved processes). In the resolved case, in addition to the two jets, there is also a photon remnant. In direct process since all the photon takes part in the interaction xγ , the fraction of the photon momentum entering into the hard scatter, is 1, and conversely for resolved processes xγ <1. In order to use this to identify and separate direct and resolved event samples it is necessary to define an experimentally measurable xγ that is valid to all orders. This is done in terms of the transverse energy E t and pseudorapidity η jet = −ln(tan jet 2 )) of the two highest transverse energy jets in an event [1]: x γ = 1 2yEe ( E t e −ηjet1 + E t e −ηjet2 ) (1) Higher order and hadronisation effects cause the average x γ of direct events to be slightly less than 1, and the distribution peaks at ∼0.85. A cut at c © 1997 American Institute of Physics 1