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 ${\hat{I}}^{3}p$ 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, $x_{\hat{I}}^3$, 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, $-0.7\le \hat{I}{\cdot }^3<0.8$, $\eTj\ge 5$ GeV, and $-1.5\le \hat{I}{\cdot }^{jet}\le 1.8$, with $\xgO > 0.8,$ the measured cross section is $15.3\pm 3.8\pm 1.8pb$, in good agreement with a recent NLO calculation.

22 pages including 8 figures