Thermal Dilepton Radiation at Intermediate Masses at the CERN-SpS

Abstract

We investigate the significance of thermal dilepton radiation in the intermediate-mass region in heavy-ion reactions at CERN-SpS energies. Within a thermal fireball model for the space-time evolution, the radiation from hot matter is found to dominate over hard 'background' processes (Drell-Yan and open charm) up to invariant masses of about 2 GeV, with a rather moderate fraction emerging from early stages with temperatures $T\simeq 175-200$ MeV associated with deconfined matter. Further including a schematic acceptance for the NA50 experiment we find good agreement with the observed enhancement in the region 1.5 GeV~$<M_{\mu\mu}<$~3 GeV. In particular, there is no need to invoke any anomalous open charm enhancement.