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

Abstract

NA50Muon pair production in \mbox{p-A}, \mbox{S-U} and \mbox{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 invariantmass region between the $\phi$ and the \jpsi. After subtraction ofthe combinatorial background due to $\pi$ and K decays, we find thatthe superposition of dimuons from semi-leptonic decays of D mesons andfrom the Drell-Yan process reproduces well the mass and transversemomentum spectra measured in \mbox{p-A} collisions. From the yield ofopen charm dimuons required to fit our data we extract a charmproduction cross section in good agreement with previous measurements.A linear extrapolation of the \mbox{p-A} sources with the product ofthe mass numbers of the projectile and target nuclei, $A\times B$, underestimates the yield measured in \mbox{S-U} and \mbox{Pb-Pb}collisions. The excess increases with the number of participantnucleons, and the ratio between the observed dimuon yield and theexpected sources reaches a factor 2 for central \mbox{Pb-Pb}interactions. The kinematical distributions of the measured dimuonexcess are compatible with those expected from the open charmcontribution