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Rapidity Gaps between Jets in Photoproduction at HERA

Photoproduction events which have two or more jets have been studied in the $W_{γp}$ range 135~GeV $< W_{γp} <$ 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 ($Δη$) 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 $Δη$. It decreases exponentially as expected for processes in which colour is exchanged between the jets, up to a value of $Δη\sim 3$, 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.