Observation of isolated high E(t) photons in photoproduction at HERA

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.Comment: 22 pages including 8 figure