The dependence of dijet production on photon virtuality in ep collisions at HERA

The dependence of dijet production on the virtuality of the exchanged photon, Q(2), has been studied by measuring dijet cross sections in the range 0 less than or similar to Q(2) < 2000 GeV(2) with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb(-1). Dijet cross sections were measured for jets with transverse energy E(T)(jet) > 7.5 and 6.5 GeV and pseudorapidities in the photon-proton centre-of-mass frame in the range -3 < eta(jet) < 0. The variable x(gamma)(obs), a measure of the photon momentum entering the hard process, was used to enhance the sensitivity of the measurement to the photon structure. The Q(2) dependence of the ratio of low- to high-x(gamma)(obs) events was measured. Next-to-leading-order QCD predictions were found to generally underestimate the low-x(gamma)(obs) contribution relative to that at high x(gamma)(obs). Monte Carlo models based on leading-logarithmic parton-showers, using a partonic structure for the photon which falls smoothly with increasing Q(2), provide a qualitative description of the data.