Measurements of prompt photon photoproduction at HERA

Measurements of inclusive prompt photon and prompt photons, together with an accompanying jet, in photoproduction at HERA have been made with the ZEUS detector, using an integrated luminosity of 38.4 pb-1. We have performed two analyses in the study of prompt photon production. First inclusive cross section measurements for prompt photon production have been presented as a function of the pseudorapidity and the transverse energy (ηγ, ETγ) of the photon, for ETγ > 5 GeV in the γp centre-of-mass energy range 134-285 GeV. Comparisons are made with predictions from Monte Carlo models having leading-logarithm parton showers, and with next-to-leading order QCD calculations, using currently available parameterisations of the photon structure. For positive ηγ (proton direction) there is good agreement, but for negative ηγ all predictions fall below the data. None of the available variations of the model p rameters was found to be capable of removing the discrepancy with the data. The results indicated a need to review the present theoretical modelling of the parton structure of the photon at high xγ regions. A study of the intrinsic parton transverse momentum, kT, of the quarks in the proton, as modelled within the framework of the PYTHIA Monte Carlo, has been performed using the kinematical properties of events with a measured jet as well as a prompt photon. A fit to the data gives a value of = 1.39 ± 0.36-0.23+0.12 GeV. This result is compared with earlier high-energy proton-scattering measurements. A rising trend of with interaction energy is confirmed. (author)