The acceptance-corrected dielectron excess mass spectra, where the known
hadronic sources have been subtracted from the inclusive dielectron mass
spectra, are reported for the first time at mid-rapidity ∣yee∣<1 in
minimum-bias Au+Au collisions at sNN = 19.6 and 200 GeV. The excess
mass spectra are consistently described by a model calculation with a broadened
ρ spectral function for Mee<1.1 GeV/c2. The integrated
dielectron excess yield at sNN = 19.6 GeV for 0.4<Mee<0.75
GeV/c2, normalized to the charged particle multiplicity at mid-rapidity, has
a value similar to that in In+In collisions at sNN = 17.3 GeV. For
sNN = 200 GeV, the normalized excess yield in central collisions is
higher than that at sNN = 17.3 GeV and increases from peripheral to
central collisions. These measurements indicate that the lifetime of the hot,
dense medium created in central Au+Au collisions at sNN = 200 GeV
is longer than those in peripheral collisions and at lower energies.Comment: 9 pages, 6 figure