Diffractive hard photoproduction at HERA and evidence for the gluon content of the pomeron

Abstract

Inclusive jet cross sections for events with a large rapidity gap with\nrespect to the proton direction from the reaction $ep \\rightarrow jet \\; + \\;\nX$ with quasi-real photons have been measured with the ZEUS detector. The cross\nsections refer to jets with transverse energies $E_T^{jet}>8$~GeV. The data\nshow the characteristics of a diffractive process mediated by pomeron exchange.\nAssuming that the events are due to the exchange of a pomeron with partonic\nstructure, the quark and gluon content of the pomeron is probed at a scale\n$\\sim (E_T^{jet})^2$. A comparison of the measurements with model predictions\nbased on QCD plus Regge phenomenology requires a contribution of partons with a\nhard momentum density in the pomeron. A combined analysis of the jet cross\nsections and recent ZEUS measurements of the diffractive structure function in\ndeep inelastic scattering gives the first experimental evidence for the gluon\ncontent of the pomeron in diffractive hard scattering processes. The data\nindicate that between 30\\% and 80\\% of the momentum of the pomeron carried by\npartons is due to hard gluons.\n