Dilepton emission in ultrarelativistic nuclear collisions at the CERN SPS

We report on the measurement of electron-positron pairs in ultrarelativistic heavy-ion reactions at the CERN SPS. The enhancement of dilepton emission first seen by the CERES and HELIOS/3 collaborations for 200 A GeV sulphur projectiles on heavy targets is corroborated by the results for 158 A GeV Pb projectiles on a Au target. The enhancement increases faster than linearly with charged- particle multiplicity and is strongest at low pair p/sub perpendicular to /. Explanations based on pi /sup +/ pi /sup -/ annihilation in the hot and dense hadronic medium are discussed. The upgrade of the CERES spectrometer and plans for running at a lower beam energy are briefly described. (13 refs).