Dimuon production in ultrarelativistic nuclear collisions and QCD phase transitions

Dilepton production rates in nuclear collisions are calculated in order to study their sensitivity to the quark-gluon plasma and to the hadronic phases. This treatment differs from previous work on the subject in two respects: The width of the rho-meson, being exchanged in annihilation, is made temperature dependent, thus taking into account resonance melting as the critical deconfinement temperatureT _d is approached. Secondly, we study in addition to the standard scenario where chiral symmetry restoration and deconfinement occur at the same temperature (T _c =T _d ), an alternative possibility where deconfinement preceeds chiral symmetry restoration (T _d T _c ). Results differ substantially from those obtained assuming a temperature independent rho-meson width, andT _c =T _d .