Enhanced Production of Low-Mass Electron-Positron Pairs in 40-AGeV Pb-Au Collisions at the CERN SPS

We report on first measurements of low-mass electron-positron pairs in Pb-Au collisions at the CERN SPS beam energy of 40 AGeV. The observed pair yield integrated over the range of invariant masses 0.2 < m ≤ 1 GeV/c 2 is enhanced over the expectation from neutral meson decays by a factor of 5.9 ′ 1.5(stat) ′ 1.2(syst data) ′ 1.8(syst meson decays), somewhat larger than previously observed at the higher energy of 158 AGeV. The results are discussed with reference to model calculations based on π + π - → e + e - annihilation with a modified p propagator. They may be linked to chiral symmetry restoration and support the notion that the in-medium modifications of the p are more driven by baryon density than by temperature.