Dijet Cross Sections and Rapidity Gaps between Jets in Photoproduction at Hera

In ep scattering at HERA, an almost real photon (P 2 ≈ 0, where P 2 is the negative four momentum squared of the photon) can be emitted from the positron with momentum fraction y and collide with the proton. At leading order (LO) there are two distinct types of process: (a) where all of the photon interacts directly with the proton (direct process) and (b) where the photon resolves into partons which interact with the proton (resolved process). Both processes can produce two high ET jets which provide a hard scale, meaning that they should be calculable in perturbative QCD. In this contribution we report on two different dijet cross section measurements. Another possibility is resolved hard scattering with the exchange of a colour singlet object. A measurement of the fraction of rapidity gap events compared to normal dijet events is presented, which probes these processes. The present results are based on 2.6 pb of the data taken during 1994 HERA running and on 5.7 pb from the 1995 data. In both years HERA was running with 820 GeV protons and 27.5 GeV positrons. Photoproduction processes are separated from other processes by the requirement that no positron is found in the main detector. This corresponds to a cut of P 2 < 4GeV and gives a median P 2 ≈ 10GeV. Jets are located using the energy deposit pattern in the uranium-scintillator calorimeter. Results presented here use both the cone type and the clustering type of jet finding algorithms. Two types of cone algorithm (EUCELL and PUCELL) are used in ZEUS. Both involve moving a cone around in η − φ a space. Both comply with the snowmass convention. The clustering algorithm (KTCLUS ) uses an ET recombination scheme to build jets from ‘clusters’ of nearby particles.