Low-mass lepton pair production in Pb - Au collisions at 40 AGeV

The CERES/NA45 experiment at the CERN SPS has previously measured $e^{+}e^{-}$ pair production in 160 AGeV Pb-Au collisions. In the mass region m > 0.2 GeV/c$^{2}$, an enhancement of 2.7 $\pm$ 0.4(statist.) $\pm$ 0.5(syst.) compared to the expectation from known hadronic decay sources was observed. In the 40 AGeV data taken in 1999, an enhancement is again found; a preliminary analysis gives the even larger value of 4.5 $\pm$ 1.2(statist.). The results are compared to theoretical model calculations based on $\pi^{+}\pi^{-}$ annihilation with a modified $\rho$-propagator.