Intermediate mass dimuons in NA38/NA50

The NA38/NA50 experiments have measured, at the CERN SPS, the dimuon produc tion in proton-nucleus and nucleus-nucleus collisions. In this paper it is shown that the mass continuum between the phi and the J/psi can be satisfac torily described, after having removed the combinatorial background due to uncorrelated pi and K decays, as a sum of two contributions, namely the Dre ll-Yan process and the semi-leptonic decay of pairs of charmed mesons, whos e mass shape in the acceptance of the experiment has been evaluated using P YTHIA. However, in order to describe the A-B (namely S-U and Pb-Pb) mass sp ectra, the dimuon yield from open charm decays, which in p-A collisions is found to be consistent with direct open charm measurements from other CERN and FNAL experiments, has to be enhanced with respect to a linear extrapola tion of p-A results. The size of the enhancement smoothly increases from pe ripheral S-U to central Pb-Pb interactions, reaching a factor similar to 3 in central Pb-Pb collisions. The pr distributions of the events in the mass continuum are also compatible with the hypothesis of open charm enhancemen t in A-B collisions.