Predicting the Total Charm Cross Section

We discuss the energy dependence of the total charm cross section and some of its theoretical uncertainties including the quark mass, scale choice and the parton densities. Extracting the total charm cross section from data is a non-trivial task. To go from a finite number of measured D mesons in a particular decay channel to the total c(bar c) cross section one must: divide by the branching ratio for that channel; correct for the luminosity, σD = ND/Lt; extrapolate to full phase space from the finite detector acceptance; divide by two to get the pair cross section from the single Ds; and multiply by a correction factor to account for unmeasured charm hadrons. Early fixed-target data were at rather low pT, making the charm quark mass the most relevant scale. At proton and ion colliders, although the RHIC experiments can access the full pT range and thus the total cross section, the data reach rather high pT, pT >> m, making pT (mT) the most relevant scale. Here we focus on the total cross section calculation where the quark mass is the only relevant scale