Open charm photoproduction at HERA

Abstract

Results on charm photoproduction at HERA are presented. The inclusive photoproduction of D ∗ mesons has been measured using 1996/97 data samples with an integrated luminosity of 37 pb −1. Differential cross sections as function of the D ∗ transverse momentum and pseudorapidity are compared with next-to-leading order (NLO) perturbative QCD calculations using the “massive charm” and “massless charm” schemes. The data are generally above the NLO calculations, in particular in the forward (proton) direction. The study of dijet photoproduction associated with charm demonstrates that a significant resolved — in addition to the direct — proton component contributes to the cross section. Leading order QCD Monte Carlo calculations indicate that the resolved contribution arises mostly from a charm component in the photon. A massive charm NLO parton level calculation yields a lower cross section compared to the measured results in a kinematic region where the resolved photon contribution is significant. Additional results on inclusive D ∗ production are used to determine the gluon density in the proton.