Dimuon and charm production in nucleus-nucleus collisions at the CERN-SPS

Muon pair production in p-A, S-U and Pb-Pb collisions has been studied by t he NA38 and NA50 collaborations at the CERN SPS. In this paper we present a n analysis of the dimuon invariant mass region between the phi and the J/ps i. In p-A collisions we find that, after the subtraction of the combinatori al background due to pi and K decays, the superposition of dimuons from sem i-leptonic decays of D mesons and from the Drell-Yan process reproduces wel l the measured mass and transverse momentum spectra. The yield of open char m dimuons required to fit our data leads to a charm production cross sectio n in good agreement with previous measurements. A linear extrapolation of t he p-A sources with the product of the mass numbers of the projectile and t arget nuclei, A x B, underestimates the dimuon yield measured in S-U and Pb -Pb collisions. The excess increases with the number of participant nucleon s, and the ratio between the observed dimuon yield and the expected sources reaches a factor 2 for central Pb-Pb interactions. The kinematical distrib utions of the measured dimuon excess are compatible with those expected fro m the open charm contribution.