INTERMEDIATE-MASS MUON PAIR CONTINUUM IN PB-PB COLLISIONS AT 158 GEV C/

The NA50 experiment measures, at CERN SPS, the dimuon production in Pb -Pb collisions at 158 GeV/c incident momentum. In this paper an analys is of the intermediate mass region (1.5 less than or equal to M(mu mu) 12.5 GeV/c(2)) is presented. Known dimuon sources in this interval of the spectrum are the e.m. annihilation of <q(q)over bar> pairs (i.e, D rell-Yan) and the semileptonic decay of charmed hadrons. The contribut ion of these processes is calculated by means of a fit to the measured invariant mass distribution (for Drell-Yan) and of a linear extrapola tion of p-A experimental results obtained by NA38 (for charm). It will he shown that known sources do not account for all of the dimuon yiel d measured by NA50, i.e, an excess is observed. The size of the excess measured in Pb-Pb is larger than the one seen by NA38 in S-U collisio ns. The centrality dependence of this effect, both in S-U and Pb-Pb, i s also discussed.