Open charm production at HERA

Open charm production at HERA has developed as a precision testing ground for fixed order (FO) NLO pQCD calculations as well as for other QCD inspired models. The bulk of the available data is on inclusive charmed particle production cross sections for several species. I will focus on reviewing the status of the comparison of the data with FO NLO pQCD calculations trying to identify the open issues for HERA Run II. The description of the kinematics for these observables requires two variables for the charmed particle momentum (an integration on the azimuthal angle of the charmed particle is generally assumed) and one (W ) or two (Q and y, for instance) additional variables needed for the description of the inclusive photoproduction (PHP) or deep inelastic (DIS) cross section, respectively. The pseudorapidity, η = −log(tan(θ/2)), where θ is the polar angle with respect to the proton (p) direction, and the transverse momentum with respect to the beam line, pT, are typically chosen as the charmed particle variables. Open charm production is intimately related to the gluon parton density in the p because the Photon-Gluon Fusion (PGF) process, sketched in Figure 1 (a), is expected to be one of the dominant production mechanisms. Since the invariant mass of the partonic final state, M hard, is significant due to the presence of two heavy quarks (M hard > 4m 2 c), the kinematics are such that the fraction of the p momentum carried by the gluon, xg = xBj(M 2 hard+Q )/Q rather than equal to xBj , as in the quark parton model (QPM) type of process. The sensitivity to the gluon density, fg, and to αs can be read from the factorization formula for the PGF process written in equation (1), where σ̂ is the calculable partonic cross section.