Étude des dimuons de la région des masses intermédiaires produits dans les collisions d'ions lourds auprès du SPS du CERN

In this thesis, we study the dimuon production in the mass range between the $\phi$ and the J$\psi$ resonances (the Intermediate Mass Region, IMR) in proton-nucleus and nucleus-nucleus collisions. The NA50 experiment, at the CERN-SPS, detects muons pairs emitted in these interactions by means of a spectrometer. The centrality measurement of the collisions rests on three independent detectors: an electromagnetic calorimeter, a multiplicity detector and a zero degree hadron calorimeter. The proton-nucleus analysis results provide a reference value for the nucleus-nucleus interaction studies. The IMR experimental distributions, corrected for acceptance and resolution effects of the apparatus using a four dimensional deconvolution method, are correctly reproduced by the superposition of the conventional processes, namely Drell-Yan (DY) and semi-leptonic decay of D and \=D mesons (D\=D). Linear extrapolation with the nucleus mass number of p-A results to S-U and Pb-Pb collisions systematically underestimates the measured spectra in the intermediate mass region. This excess of IMR dimuons increases with the centrality of the collisions. Several models are presented and compared with the nucleus-nucleus data. We show that a model based on D-mesons final state rescattering cannot reproduce the experimental distributions. Finally, this study propose two possible explanations of the observed excess. The nucleus-nucleus data can be reproduced either by an increase of the charm production or by the radiation of thermal dimuons in a dense and hot medium.