Measurement of prompt photon cross sections in photoproduction at HERA

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Results are presented on the photoproduction of isolated prompt photons, inclusively and associated with jets, in the \(\gamma p\) center of mass energy range 142 < W < 266 GeV. The cross sections are measured for the transverse momentum range of the photons \(5 < E_T^{\gamma} < 10\) GeV and for associated jets with E T jet > 4.5 GeV. They are measured differentially as a function of \(E_T^{\gamma}, E_T^{\mathrm{jet}}\) , the pseudorapidities \(\eta^{\gamma}\) and \(\eta^{\mathrm{jet}}\) and estimators of the momentum fractions \(x_{\gamma}\) and x p of the incident photon and proton carried by the constituents participating in the hard process. In order to further investigate the underlying dynamics, the angular correlation between the prompt photon and the jet in the transverse plane is studied. Predictions by perturbative QCD calculations in next to leading order are about \(30\%\) below the inclusive prompt photon data after corrections for hadronisation and multiple interactions, but are in reasonable agreement with the results for prompt photons associated with jets. Comparisons with the predictions of the event generators PYTHIA and HERWIG are also presented.

Received: 13 July 2004, Revised: 15 November 2004, Published online: 17 December 2004