Dilepton Enhancement by Thermal Pion Annihilation in the CERES Experiment

We compare the recent CERES data on dielectron production in 200 GeV/u S+Au collisions with the theoretical predictions due to pion annihilation in a thermal hadronization and a string fragmentation scenario. Both models yield similar results for the dilepton mass spectrum. A satisfactory description of the experimental spectrum requires an in-medium reduction of the rho-mass and a freeze-out temperature of about 150 MeV in the thermal model. We emphasize and discuss the influence of experimental acceptance and resolution corrections.