Rapidity Gaps between Jets in Photoproduction at HERA

TL;DR

This study investigates rapidity gaps between jets in photoproduction events at HERA, revealing evidence for diffractive scattering via a color singlet exchange, with the gap fraction decreasing exponentially then plateauing.

Contribution

It provides the first measurement of rapidity gap fractions in photoproduction at HERA, indicating evidence for hard diffractive scattering involving a color singlet exchange.

Findings

Rapidity gap fraction decreases exponentially with increasing pseudorapidity interval.

A plateau in gap fraction suggests the presence of diffractive processes.

Evidence supports the existence of hard diffractive scattering via a color singlet exchange.

Abstract

Photoproduction events which have two or more jets have been studied in the $W_{\gamma p}$ range 135~GeV $< W_{\gamma p} <$ 280~GeV with the ZEUS detector at HERA. A class of events is observed with little hadronic activity between the jets. The jets are separated by pseudorapidity intervals ($\Delta\eta$) of up to four units and have transverse energies greater than 6~GeV. A gap is defined as the absence between the jets of particles with transverse energy greater than 300~MeV. The fraction of events containing a gap is measured as a function of $\Delta\eta$. It decreases exponentially as expected for processes in which colour is exchanged between the jets, up to a value of $\Delta\eta \sim 3$, then reaches a constant value of about 0.1. The excess above the exponential fall-off can be interpreted as evidence for hard diffractive scattering via a strongly interacting colour singlet object.