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

We present the combined results on electron-pair production in 158GeV/n Pb-Au ( √ 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±0.19 (stat.) ± 0.55(syst.) ±0.69 (decays) for semi-central collisions (28% σ/σgeo) when yields are in- tegrated over m >200MeV/c 2 in invariant mass. The measured yield, its stronger-than-linear scaling with Nch, and the dominance of low pair pt strongly suggest an interpretation as thermal radiation from pion annihilation in the hadronic fireball. The shape of the excess centring at m ≈500MeV/c 2 , however, cannot be described without strong medium modifications of the ρ 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.