The Q(2) dependence of dijet cross sections in gamma p interactions at HERA

The dependence of the photon structure on the photon virtuality, Q(2), is studied by measuring the reaction e(+) p --> e(+) + jet + jet + X at photon-proton centre-of-mass energies 134 < W < 223 GeV. Events have been selected in the Q(2) ranges approximate to 0 GeV(2), 0.1-0.55 GeV(2), and 1.5-4.5 GeV(2), having two jets with transverse energy E(f)(jet) > 5.5 GeV in Ihe Final state. The dijet cross section has been measured as a Function of the fractional momentum of the photon participating in the hard process, x(gamma)(OBS). The ratio of the dijet cross section with x(gamma)(OBS) < 0.75 to that with x(gamma)(OBS) > 0.75 decreases as Q(2) increases. The data are compared with the predictions of NLO pQCD and leading-order Monte Carlo programs using various parton distribution functions of the photon. The measurements can be interpreted in terms of a resolved photon component that falls with Q(2) but remains present at values of Q(2) up to 4.5 GeV(2). However, none of the models considered gives a good description of the data. (C) 2000 Elsevier Science B.V. All rights reserved.