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.