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 $|y_{ee}|<1$ in minimum-bias Au+Au collisions a
t $sqrt{s_{NN}}$ = 19.6 and 200 GeV. The excess mass spectra are consistently described by a model calculation with a broadened $rho$ spectral function for $M_{ee}<1.1$ GeV/$c^{2}$. The integrated dielectron excess yield at $sqrt{s_{NN}}$ = 19.6 GeV for $0.4<M_{ee}<0.75$ GeV/$c^2$, normalized to the charged particle multiplicity at mid-rapidity, has a value similar to that in In+In collisions at $sqrt{s_{NN}}$ = 17.3 GeV. For $sqrt{s_{NN}}$ = 200 GeV, the normalized excess yield in central collisions is higher than that at $sqrt{s_{NN}}$ = 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 $sqrt{s_{NN}}$ = 200 GeV is longer than those in peripheral collisions and at lower energies.
We report on mid-rapidity mass spectrum of di-electrons and cross sections of pseudoscalar and vector mesons via $e^{+}e^{-}$ decays, from $sqrt{s} = 200$ GeV $p+p$ collisions, measured by the large acceptance experiment STAR at RHIC. The ratio of th
e di-electron continuum to the combinatorial background is larger than 10% over the entire mass range. Simulations of di-electrons from light-meson decays and heavy-flavor decays (charmonium and open charm correlation) are found to describe the data. The extracted $omegarightarrow e^{+}e^{-}$ invariant yields are consistent with previous measurements. The mid-rapidity yields ($dN/dy$) of $phi$ and $J/psi$ are extracted through their di-electron decay channels and are consistent with the previous measurements of $phirightarrow K^{+}K^{-}$ and $J/psirightarrow e^{+}e^{-}$. Our results suggest a new upper limit of the branching ratio of the $eta rightarrow e^{+}e^{-}$ of $1.7times10^{-5}$ at 90% confidence level.
