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

The study of dilepton production plays a very important role in the understanding of ultrarelativistic heavy-ion physics. In particular, dileptons have the very interesting property of not undergoing strong interactions with the surrounding hadronic medium they traverse after being produced. The dilepton invariant mass spectrum appears as a wide continuum dominated by the resonances ( ρ,ω, φ, J/Ψ, Ψ ' ) decay peaks. While the contribution of the latter can be rather easily single out, the deconvolution of the processes in the continuum is not trivial. This problem plays a crucial role in our analysis dedicated to the study of the dimuon mass spectrum in the region between the φ and the J/Ψ masses, known as the Intermediate Mass Region (IMR). This zone of the mass spectrum is particularly interesting since it may contain dileptons created in the thermalized Quark Gluon Plasma or hadron gas produced in nucleus-nucleus collisions ( the so-called thermal dimuons ). We use for the analysis, data measured in p-A ( A = Al, Cu, Ag, W ), S-U and Pb-Pb collisions, in the NA38/NA50 experiments at the CERN SPS. Previous investigations, performed by the NA38, HELIOS and NA50 collaborations, have shown that the dimuon production in nucleus-nucleus collisions is abnormally enhanced with respect to proton-nucleus interactions. The aim of our analysis, besides confirming the previous NA38/NA50 observations is to investigate in more detail the centrality dependence of the dimuon yield and to confront the data with some explanations of the observed excess. The improvements are due to the much larger data sample available now and to an upgraded analysis technique. The data have been collected with two experimental set-ups: the NA38 apparatus and the NA50 apparatus. The p-A interactions have been studied with an incident beam intensity of about 3 x 109 p/burst with target thicknesses ranging from 0.3 to 0.5 λI. For S-U the experiment has been run at about 8 x 107 incident ions/burst with twelve U subtargets and the total thickness was 0.2 λI. In the study of Pb-Pb collisions, the average beam intensity was 5 x 107 ions/burst and 7 Pb subtargets with a total thickness of 0.3 λI. By subtracting the combinatorial background due to π and K decays, we find the superposition of dimuons from semi-leptonic decays of mesons and from 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. (authors)