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 ( = 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 for semi-central collisions (28 ) when yields are integrated over m > 200 MeV/c 2 in invariant mass. The measured yield, its stronger-than-linear scaling with , and the dominance of low pair p t strongly suggest an interpretation as thermal radiation from pion annihilation in the hadronic fireball. The shape of the excess centring at 500 MeV/c 2, however, cannot be described without strong medium modifications of the 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.
© Springer-Verlag, 2005