Systematic study of low-mass electron pair production in p--Be and p--Au collisions at 450

. In a joint effort the CERES/NA45 and TAPS collaborations have measured low-mass electron pairs in p--Be and p--Au collisions at 450 GeV/c at the CERN SPS. In the range covered up to ß 1.5 GeV/c 2 the mass spectra from p--Be and p--Au collisions are well explained by electron pairs from decays of neutral mesons. For p--Au our result is new. For p--Be, the simultaneously measured electron pairs and photons served as a direct measure of the j Dalitz decay contribution to the inclusive pair spectrum in which instrumental uncertainties are highly reduced. We confirm the earlier finding of HELIOS-1 with significantly reduced systematic uncertainties of 23% in the mass range below 450 MeV/c 2 , and of 28% in the mass range above 750 MeV/c 2 at 90% confidence limit. Any unconventional source of electron pairs is limited by these error margins as the percentage fraction of the hadronic contribution. 1 Introduction The investigation of low-mass lepton pair production (m pair ! 1 GeV/c ..