Measurement of Prompt Photon Cross Sections in Photoproduction at HERA

TL;DR

This paper reports measurements of prompt photon production cross sections in photoproduction at HERA, comparing experimental data with perturbative QCD predictions and event generator models to understand underlying dynamics.

Contribution

It provides the first detailed differential measurements of isolated prompt photons with associated jets in gamma-proton collisions at HERA, including angular correlations and momentum fractions.

Findings

Perturbative QCD predictions are about 30% below the data for inclusive photons.

Good agreement between data and QCD for photons with jets.

Event generators PYTHIA and HERWIG are also compared.

Abstract

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^jet, the pseudorapidities eta^gamma and eta^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.