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

Abstract

The low-mass dielectron signal from heavy-ion collisions at the CERN--SPS reported by the {\em CERES} collaboration is in excess of estimated hadronic decays suggestive of 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 [$\pi\rho\to a_{1}(1260) \to \pi e^+e^-$] which has favorable kinematics to populate masses between $2m_{\pi}$ and $m_{\rho}$. While it seems to account for some of the remaining excess beyond annihilation, it fails to allow quantitative interpretation of data.