PHYSICS WITH CHARM QUARKS AT HERA
Measurements of charm production in deep inelastic scattering (DIS) and photo-production have been carried out by the ZEUS and H1 collaborations at HERA. Protons at 920 GeV are collided with positrons of 27.5 GeV. Results using integrated luminosities up to 120 pb−1 for HERA I and 35 pb−1 for HERA II are presented. Single and double differential cross sections are compared to perturbative QCD (pQCD) predictions. Charm DIS and photoproduction cross sections are in reasonable agreement with pQCD calculations. Charm and jet angular correlations show regions of phase space where NLO is not sufficient to describe these data sets, indicating the need for improved calculations such as MC@NLO or NNLO predictions. Charm fragmentation ratios and fractions generally support the hypothesis that fragmentation proceeds independently from the hard sub-process in e+e− and ep collisions. This test of charm universality is confirmed by the measured D meson ground states at HERA and LEP. Stringent tests of QCD have been performed; theoretical errors dominate or are equivalent to experimental errors in the comparisons.
