Low-mass lepton pair production in Pb-Au collisions at 40 A.GeV

The CERES/NA45 experiment at the CERN SPS has previously measurede e pair production in 160 A GeV Pb-Au collisions. In the mass region m 0 2G eV/c 2 , an enhancement of 2 7 0 4(stat.) 0 5(syst.) compared to the expectation from known hadronic decay sources was observed. In the 40 A GeV data taken in 1999, an enhancement is again found; a preliminary anal- ysis gives an even larger value of 5 0 1 3(stat.). The results are compared to theoretical model calculations based on π π annihilation with a modified ρ-propagator; they may be related to chiral symmetry restoration.