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 $2.7\pm0.4(statist.)\pm0.5(syst.)$ 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 $4.5\pm1.2(statist.)$. The results are compared to theoretical model calculations based on $\pi^{+}\pi^{-}$ annihilation with a modified $\rho$-propagator.