Hard Photoproduction by Colour Singlet Exchange at HERA

TL;DR

This paper investigates rapidity gap events in high-energy photoproduction at HERA, providing evidence for colour singlet exchange mechanisms through jet analysis.

Contribution

It presents the first measurement of gap-fraction as a function of gap-width in photoproduction, indicating a plateau consistent with colour singlet exchange.

Findings

Observed a plateau in gap-fraction at higher levels than electroweak expectations.

Measured gap-fraction decreases exponentially with gap-width before plateauing.

Evidence supports the presence of colour singlet exchange in photoproduction events.

Abstract

A search for photoproduction events which contain a rapidity gap between the two highest transverse energy jets has been conducted at HERA using the ZEUS detector. The jets have transverse energies greater than 6~GeV, and are separated by pseudorapidity intervals of up to four units. The fraction of events containing a gap is measured as a function of the gap-width. It is expected that this gap-fraction will fall exponentially with the gap-width, until the dominant gap-production mechanism becomes colour singlet exchange, at which point it will plateau. An indication of a plateau in the measured gap-fraction has been found, at a higher level than that expected from electroweak exchange.