Modification of the ρ meson detected by low-mass electron-positron pairs in central Pb-Au collisions at 158A GeV/c

We present a measurement of e+e- pair production in central Pb-Au collisions at 158A GeV/c. As reported earlier, a significant excess of the e+e- pair yield over the expectation from hadron decays is observed. The improved mass resolution of the present data set, recorded with the upgraded CERES experiment at the CERN-SPS, allows for a comparison of the data with different theoretical approaches. The data clearly favor a substantial in-medium broadening of the ρ spectral function over a density-dependent shift of the ρ pole mass. The in-medium broadening model implies that baryon induced interactions are the key mechanism to the observed modifications of the ρ meson at SPS energy