Measurement of dijet photoproduction events with rapidity gaps with ZEUS at HERA

Photoproduction dijet events with a large rapidity gap between the jets 
were studied with the ZEUS detector at HERA using an integrated luminosity 
of 39 pb-1. Rapidity gaps are defined in terms of the energy flow between 
the jets, the requirement being that the total sum of transverse energy in 
this region is less than E_T^{CUT}. The data show a clear excess over the 
predictions of standard photoproduction models. This is interpreted as 
evidence for the exchange of a strongly interacting color-singlet object. 
Monte Carlo models including such a color-singlet exchange are able to 
describe the data.