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/c2 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 ρ propagator. They may be linked to chiral symmetry restoration and support the notion that the in-medium modifications of the ρ are more driven by baryon density than by temperature