Observation of Isolated High-ET Photons in Photoproduction at HERA

TL;DR

This paper reports the first observation of isolated high-energy photons with associated jets in photoproduction at HERA, confirming predictions of QCD and providing new insights into photon-proton interactions.

Contribution

It presents the first experimental observation of isolated high-ET photons with jets in photoproduction at HERA, aligning with QCD predictions and NLO calculations.

Findings

Strong peak in photon energy fraction near unity

Measured cross section agrees with NLO QCD calculations

First observation of such events in photoproduction at HERA

Abstract

Events containing an isolated prompt photon with high transverse energy, together with a balancing jet, have been observed for the first time in photoproduction at HERA.The data were taken with the ZEUS detector, in a $\gamma p$ centre of mass energy range 120--250 GeV. The fraction of the incoming photon energy participating in the production of the prompt photon and the jet, $x_\gamma$, shows a strong peak near unity, consistent with LO QCD Monte Carlo predictions. In the transverse energy and pseudorapidity range $5\le \eTg < 10$ GeV, $-0.7 \le \eta^\gamma < 0.8$, $\eTj\ge 5$ GeV, and $-1.5\le \eta^{jet}\le 1.8$, with $\xgO > 0.8,$ the measured cross section is $15.3 \pm 3.8 \pm 1.8 pb$, in good agreement with a recent NLO calculation.