Rapidity Gaps in Hard Photoproduction

TL;DR

This paper reports on the observation of rapidity gaps in hard photoproduction events at HERA, indicating diffractive processes involving color singlet exchange with significant gluon content.

Contribution

It provides experimental evidence for diffractive photoproduction with rapidity gaps and analyzes the structure of the exchanged color singlet object, highlighting the role of hard gluons.

Findings

7% of events show rapidity gaps inconsistent with non-diffractive production

Color singlet exchange involves a negative squared invariant mass around 40 GeV^2

Between 30% and 80% of the exchanged object's momentum is carried by hard gluons

Abstract

Recent results obtained from studies of diffractive processes in hard photoproduction performed by the ZEUS collaboration using data delivered by HERA in 1993 and 1994 are presented. In particular, we have found that $(7 \pm 3)$\% of events with two jets at a pseudorapidity interval of 3.5 to 4 are inconsistent with a non-diffractive production mechanism. These events may be interpreted as arising due to the exchange of a colour singlet object of negative squared invariant mass ($-t$) around 40~GeV$^2$. We have also probed the structure of the exchanged colour singlet object in low--$t$ diffractive scattering. By comparing the results from photoproduction and electroproduction processes we find that between 30\% and 80\% of the momentum of the exchanged colour singlet object which is carried by partons is due to hard gluons.