Charm in dijet photoproduction at HERA

The production of charm in dijet photoproduction events at HERA has been studied. Charm production from the interaction of low virtuality photons is sensitive to both the charm and gluon densities within the photon, while the observation of jets with high transverse energy enables the study of the hard sub-process kinematics directly. Jets are found using the KT algorithm, while charm is tagged by identifying tracks from the decay of D*±(2010) mesons via the D*+ → D0π+ → (K-π+)π+ and charge conjugate channels. The differential cross-section dσ(ep → D* + dijets + X)/dxγOBS where xγOBS is the fraction of the photon energy contributing to the two jets with highest transverse energy, is presented for events with ET(jet1,2) > 7,6GeV and pT(D*) > 3GeV. A significant contribution to the cross-section from resolved (xγOBS < 0.75) processes is observed. Comparison with Leading order QCD Monte Carlo indicate that greater than 90% of the resolved cross-section results from events where the charm is produced within the photon. A preliminary NLO calculation using the massive scheme of Frixione et al lies below the data. (author)