Diffractive Dijet Cross Sections in Photoproduction at HERA

TL;DR

This paper reports measurements of diffractive dijet cross sections in photoproduction at HERA, revealing contributions from both resolved and direct photon processes and supporting models with gluon-rich pomeron structure.

Contribution

It provides the first detailed measurements of diffractive dijet cross sections as functions of photon and pomeron momentum fractions, supporting QCD-based models with gluon-dominated pomeron structure.

Findings

Evidence for resolved and direct photon components.

Cross section increases with beta^OBS, indicating significant pomeron momentum participation.

Data consistent with gluon-rich pomeron models evolving via QCD equations.

Abstract

Differential dijet cross sections have been measured with the ZEUS detector for photoproduction events in which the hadronic final state containing the jets is separated with respect to the outgoing proton direction by a large rapidity gap. The cross section has been measured as a function of the fraction of the photon (x_gamma^OBS) and pomeron (beta^OBS) momentum participating in the production of the dijet system. The observed x_gamma^OBS dependence shows evidence for the presence of a resolved- as well as a direct-photon component. The measured cross section d(sigma)/d(beta^OBS) increases as beta^OBS increases indicating that there is a sizeable contribution to dijet production from those events in which a large fraction of the pomeron momentum participates in the hard scattering. These cross sections and the ZEUS measurements of the diffractive structure function can be described by calculations based on parton densities in the pomeron which evolve according to the QCD evolution equations and include a substantial hard momentum component of gluons in the pomeron.