Enhanced Production of Low-Mass Electron Pairs in 200 GeV/Nucleon S-Au Collisions at the CERN Super Proton Synchrotron

We report on measurements of low-mass electron pairs in 450 GeV $p$-Be, $p$-Au, and 200 GeV/nucleon S-Au collisions at central rapidities. For the proton induced interactions, the low-mass spectra are, within the systematic errors, satisfactorily explained by electron pairs from hadron decays, whereas in the S-Au system an enhancement over the hadronic contributions by a factor of $5.0\ifmmode\pm\else\textpm\fi{}0.7(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}2.0(\mathrm{syst})$ in the invariant mass range $0.2<m<1.5\phantom{\rule{0ex}{0ex}}\mathrm{GeV}{/c}^{2}$ is observed. The properties of the excess suggest that it arises from two-pion annihilation $\ensuremath{\pi}\ensuremath{\pi}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}$.