Séminaire spécialisé

vendredi 26 février 2010 à 15:00

Evidence for the Production of Thermal Dimuons on the SPS

Ruben Shahoyan


All dilepton experiments at SPS have observed an excess in dilepton production in heavy ion collisions both at low (M<1GeV) and intermediate (1<M<3 GeV) masses. NA60 experiment has recently performed a precise and high-statistics measurement of dimuon production in indium-indium collisions at 158A GeV. The analysis of these data confirms the excess and shows that at masses above 1 GeV it is dominated by the prompt dimuons. Furthermore, the transverse mass distribution of the intermediate mass excess is well described by an exponential function, with inverse slope, T_eff~190 MeV independent of mass. In contrary, at lower masses significantly higher T_eff is observed, rising there with mass up to 250 MeV due to the radial flow. These observations are consistent with emission of thermal dimuons: at low masses it is caused mostly by the pi-pi annihilation via medium-modified rho meson while above 1 GeV/c2 it is dominated by the partonic source.