e+e--pair production in Pb-Au collisions at 158 GeV per nucleon

We present the combined results on electron-pair production in 158 GeV/n {Pb-Au} (\(\sqrt{s}\)= 17.2 GeV) collisions taken at the CERN SPS in 1995 and 1996, and give a detailed account of the data analysis. The enhancement over the reference of neutral meson decays amounts to a factor of 2.31\(\pm0.19 (stat.)\pm0.55 (syst.)\pm0.69 (decays)\) for semi-central collisions (28% \(\sigma/\sigma_{geo}\)) when yields are integrated over \(m>\) 200 MeV/\(c^2\) in invariant mass. The measured yield, its stronger-than-linear scaling with \(N_{ch}\), and the dominance of low pair \(p_t\) strongly suggest an interpretation as {\it thermal radiation} from pion annihilation in the hadronic fireball. The shape of the excess centring at \(m\approx\) 500 MeV/\(c^2\), however, cannot be described without strong medium modifications of the \(\rho\) meson. The results are put into perspective by comparison to predictions from Brown-Rho scaling governed by chiral symmetry restoration, and from the spectral-function many-body treatment in which the approach to the phase boundary is less explicit.