The NA50 experiment measures, at CERN SPS, the dimuon production in PbPb collisions at 158 GeV/c incident momentum. In this paper an analysis of the intermediate mass region (1.5 <Mμμ <2.5 GeV/c^2) is presented. Known dimuon sources in this interval of the spectrum are the e.m. annihilation of q qbar source pairs (i.e. Drell-Yan) and the semileptonic decay of charmed hadrons. The contribution of these processes is calculated by means of a fit to the measured invariant mass distribution (for Drell-Yan) and of a linear extrapolation of p-A experimental results obtained by NA38 (for charm). It will be shown that known sources do not account for all of the dimuon yield measured by NA50, i.e. an excess is observed. The size of the excess measured in PbPb is larger than the one seen by NA38 in SU collisions. The centrality dependence of this effect, both in SU and PbPb, is also discussed.
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