Hard diffractive scattering in photoproduction at HERA

Photoproduction events which have two or more jets have been studied in the $W sb{ gamma p}$ range 135 GeV $< W sb{ 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 define 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 ${ Delta eta} sim3$, then reaches a constant value of about 0.1. The excess above the exponential fall-off can be interpreted as evidence for hard scattering via a strongly interacting colour singlet object