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

The low-mass dielectron signal from heavy-ion collisions at the CERN endash SPS reported by the CERES Collaboration is in excess of estimated hadronic decays, suggesting a possible contribution from two-pion annihilation or other hadronic reactions. In the absence of dramatic medium modifications, annihilation alone is unable to account for the data. We explore the role of pion plus resonance scattering [πρ→a1(1260)→πe+e-], which has favorable kinematics to populate masses between 2mπ and mρ. While it seems to account for some of the remaining excess beyond annihilation, it fails to allow quantitative interpretation of data. copyright 1996 The American Physical Society