Measurement of prompt photons with associated jets in photoproduction at HERA

TL;DR

This study measures prompt photon production with associated jets in ep collisions at HERA, comparing experimental cross sections to various QCD models, revealing better agreement with kT-factorisation at lower energies and NLO QCD at higher energies.

Contribution

First detailed measurement of prompt photon plus jet production at HERA, testing and comparing different QCD theoretical models against experimental data.

Findings

kT-factorisation model agrees better at lower photon energies

NLO QCD matches data well at higher photon energies

Cross sections are underestimated by models below 7 GeV photon energy

Abstract

The photoproduction of prompt photons, together with an accompanying jet, has been studied in ep collisions at a centre-of-mass energy of 318 GeV with the ZEUS detector at HERA using an integrated luminosity of 77 pb-1. Cross sections were measured for the transverse energy of the photon and the jet larger than 5 and 6 GeV, respectively. The differential gamma+jet cross sections were reconstructed as functions of the transverse energy, pseudorapidity and x_gamma^obs, the fraction of the incoming photon momentum taken by the photon-jet system. Predictions based on leading-logarithm parton-shower Monte Carlo models and next-to-leading-order (NLO) QCD generally underestimate the cross sections for the transverse energies of prompt photons below 7 GeV, while the kT-factorisation QCD calculation agrees with the data better. When the minimum transverse energy of prompt photons is increased to 7 GeV, both NLO QCD and the kT-factorisation calculations are in good agreement with the data.