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

摘要

The CERES/NA45 experiment at the CERN SPS has previously measurede + e - pair production in 160 A.GeV Pb-Au collisions. In the mass regionm > 02 GeV/c2, an enhancement of 2.7±04(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 analysis gives an even larger value of 50 ±13(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. Keywords Heavy-ion collisions - lepton pairs PACS No. 25.75.-q - 12.38.Mh - 13.85.Qk