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 the NA38 and NA50 collaborations at theCERN SPS. In this paper we present an analysis of the dimuon invariant mass region between the $\phi$ and the J/$\psi$. After subtraction of the combinatorial background due to $\pi$ and K decays, we find that the superposition of dimuons from semi-leptonic decays of D mesons and from the Drell-Yan process reproduces well the mass and transverse momentum spectra measured in p-A collisions. From the yield of open charm dimuons required to fit our data we extract a charm production cross section in good agreement with previous measurements. A linear extrapolation of the p-A sources with the product of the mass numbers of the projectile and target nuclei, $A\times B$, underestimates the yield measured in S-U and Pb-Pb collisions. The excess increases with the number of participant nucleons, and the ratio between the observed dimuon yield and the expected sources reaches a factor 2 for central Pb-Pb interactions. The kinematical distributions of the measured dimuon excess are compatible with those expected from the open charm contribution.