Charm Production in ep-Collisions.pdf

a r X i v : h e p - e x / 0 5 0 5 0 7 8 v 1 2 5 M a y 2 0 0 5 CHARM PRODUCTION IN EP-COLLISIONS J. Wagner, representing the ZEUS and H1 Collaborations DESY FH1, Notkestrasse 85 D-22603 Hamburg, Germany Recent results on the production of open charm in electron proton scattering at HERA are reviewed. Results on the fragmentation of charm are presented and compared to the measure- ments at e+e? colliders. Recent results on the charm contribution to the proton structure function F2 are shown. Furthermore, measurements of inclusive D ? meson, D? + jet and D? + muon production are presented. These results are compared with perturbative QCD calculations at next-to-leading order as well as with Monte Carlo predictions. 1 Introduction At HERA, electrons (or positrons) of energy 27.5 GeV are collided with 920 GeV protons, pro- viding a center-of-mass energy of √ s ≈ 318 GeV. During the HERA-I run, the experiments H1 and ZEUS each collected data corresponding to an integrated luminosity of about 110 pb?1, thus allowing significant tests of perturbative QCD for heavy quark production. In electron proton collisions, heavy quarks are predominantly produced via the photon gluon fusion (PGF) mechanism, in which a photon emitted by the incoming electron interacts with a gluon in the proton forming a quark anti-quark pair. The cross section is largest in photopro- duction, in which the exchanged photon is almost real Q2 ～ 0, and decreases towards the region of deep inelastic scattering (DIS) which is at larger values of Q2 (Q2 ? 2GeV2). Heavy quark production can be described by factorising the process into three parts: the proton structure, the hard partonic interaction and the fragmentation of the final state quarks into hadrons. The structure function F cc?2 denotes the contribution from events with charm to the inclusive proton structure function F2. The hard interaction process is calculable in perturbative QCD as the heavy quark mass provides a hard scale. The HERA data a