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±0.7(stat)±2.0(syst) in the invariant mass range 0.2<m<1.50ex0exGeV/c² is observed. The properties of the excess suggest that it arises from two-pion annihilation ππ→e⁺e^-.