First results from CERES/NA45 on low-mass electron pair production in Pb-Au collisions

We report preliminary results of the first measurement of low-mass electron pairs in 160 GeV/nucleon Pb-Au collisions at central rapidities in a wide multiplicity range. This work is the continuation of our systematic studies on pair production in p-Be, p-Au and S-Au interactions. The motivation for this effort derives from a new source of low-mass dileptons recently observed in S-Au and S-W collisions by CERES and HELIOS/3, respectively. The results obtained in the analysis of the Pb-Au data confirm our previous finding that pair production in the mass range 0.2 < mee < 1.5 GeV/c2 is enhanced over the contributions from hadron decays while no significant excess could be observed for mee <~ 2mπ. A comparison of the results for different multiplicity bins favours a non-linear dependence of the e+e--yield on the accompanying charged particle densities