Diffractive Hard Photoproduction at HERA and Evidence for the Gluon Content of the Pomeron

TL;DR

This paper reports measurements of diffractive jet production at HERA, providing the first experimental evidence that the pomeron, a theoretical exchange object, contains a significant gluon component, influencing models of diffraction.

Contribution

It presents the first combined analysis of jet cross sections and diffractive structure functions, revealing the gluon content of the pomeron in hard scattering processes.

Findings

Gluons carry 30-80% of the pomeron's momentum.

Diffractive processes are mediated by pomeron exchange with partonic structure.

Data supports models with a hard gluon component in the pomeron.

Abstract

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