Enhanced production of low mass electron pairs in 200-GeV/u S - Au collisions at the CERN SPS

We report on measurements of low-mass electron pairs in 450 GeV {ital p}-Be, {ital 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{plus_minus}0.7(stat){plus_minus}2.0(syst) in the invariant mass range 0.2{lt}{ital m}{lt}1.5 GeV/{ital c}{sup 2} is observed. The properties of the excess suggest that it arises from two-pion annihilation {pi}{pi}{r_arrow}{ital e}{sup +}{ital e}{sup {minus}}.