yields in Pb + Au collisions at 158 AGeV / c : Assessment of baryonic contributions

Using a hydrodynamic approach to describe Pb+Au collisions at 158 AGeV/c, we analyze ee yields from matter containing baryons in addition to mesons. We employ ee production rates from two independent calculations, which differ both in their input physics and in their absolute magnitudes, especially in the mass range where significant enhancements over expected backgrounds exist in the CERES data. Although the presence of baryons leads to significant enhancement of ee emission relative to that from mesons-only matter, the calculated results fall below the CERES data in the range 400 < Me+e−/MeV < 600, by a factor of 2-3. Since the calculated ee spectra are relatively insensitive to the equation of state for initial conditions that fit the observed hadronic spectra, either in-medium modifications of the ee sources more significant than so far considered, or the presence of hitherto unidentified additional sources of ee is indicated. PACS number(s): 25.70.np, 12.38.Mh, 12.40.Ew, 47.75.+f