Thermal dileptons from quart and hadron phases of an expanding fireball

A fireball model with time evolution based on transport calculations is use d to examine the dilepton emission rate of an ultra-relativistic heavy-ion collision. A transition from hadronic matter to a quark-gluon plasma at a c ritical temperature Te between 130-170 MeV is assumed. We also consider a p ossible mixed phase scenario. Vie include thermal corrections to the hadron ic spectra below Te and use perturbation theory above Tc. The sensitivity o f the spectra with respect to the freeze-out temperature, the initial fireb all temperature and the critical temperature is investigated.