OPEN CHARM PRODUCTION IN pp AND HEAVY ION COLLISIONS IN QCD

The RHIC data on charm production are compared with the kT -factorization approach predictions, both standard NLO QCD and FONLL. The calculated results underestimate the STAR Collaboration data. The role of possible nuclear effects is discussed. PACS numbers: 25.75.Dw, 13.87.Ce, 24.85.+p The investigation of heavy quark production in high energy collisions is an important method for studing the quark-gluon structure of hadrons and the possible nuclear effects at early stages of secondary production. The description of hard interactions in hadron collisions within the framework of QCD is possible only with the help of some phenomenological assumptions which reduce the hadron–hadron interaction to the parton–parton one via the formalism of the hadron structure functions. The cross sections of hard processes in hadron–hadron interactions can be written as the convolutions of squared matrix elements of the subprocess calculated within the framework of QCD with the parton distributions in the colliding hadrons. The most popular phenomenological approach is the NLO QCD collinear approximation [1-4], where the cross sections of QCD subprocesses are calculated in the Next-toLeading Order (NLO) of αs series. The Fixed Order plus Next-to-Leading-Log (FONLL) [5] also resumes large perturbative terms proportional to α s log (pT/m) with k = n, n−1, where m is the heavy quark mass. In these calculations all particles involved are assumed to be on mass shell, carrying only longitudinal momenta, and the cross section is averaged over two transverse polarizations of the incident gluons. The virtualities q of the initial partons are taken into account only through their structure functions. The standard QCD expression for heavy quark production cross section in a hadron 1 hadron 2 collision has the form σ = ∫ 1 xa0 dxa ∫ 1 xb0 dxb ·Ga/1(xa, μF ) ·Gb/2(xb, μF ) · σ̂(ŝ, mQ, μR) , (1)