Statistical $J/\psi$ production and open charm enhancement in Pb+Pb collisions at CERN SPS

The production of open and hidden charm hadrons in heavy ion collisions is considered within the statistical coalescence model. Charmed quarks and antiquarks are assumed to be created at the initial stage of the reaction and their number is conserved during the evolution of the system. They are distributed among open and hidden charm hadrons at the hadronization stage in accordance with the laws of statistical mechanics. The model is in excellent agreement with the experimental data on J/psi production in lead-lead collisions at CERN SPS and predicts a strong enhancement of the open charm multiplicity over the standard extrapolation from nucleon-nucleon to nucleus-nucleus collisions. A possible mechanism of the charm enhancement is proposed.