Observation of Isolated High ET Photons 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 γp 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 γ , shows a strong peak near unity, consistent with LO QCD Monte Carlo predictions. In the transverse energy and pseudorapidity range 5 ≤ E T γ η γ E T jet ≥ 5 GeV, and −1.5 ≤ η jet ≤ 1.8, with x γ OBS > 0.8, the measured cross section is 15.3 ± 3.8 ± 1.8 pb, in good agreement with a recent NLO calculation.