Excess electron pairs from heavy-ion collisions at CERN and more complete picture of thermal production

The low mass dielectron signal from heavy-ion collisions at the CERN-SPS reported by the CERES collaboration is in excess of estimated contributions from hadronic decays and has been suggested as coming simply from two-pion annihilation. Yet the largest discrepancy between observation and estimated decay occurs in mass where the annihilation is not expected to be strong. However, Compton-like scattering (πp →πe+e-) provides favorable kinematics to enhance theoretical prediction for masses between 2mπ and mρ. Choosing conditions reasonable for a fire cylinder produced in central collisions at SPS energies, the resulting mass distribution is consistent with the one observed from S+Au reactions at 200 GeV/u