Measurements of Dielectron Production in Au+Au Collisions at Snn=27 , 39, and 62.4 GeV from the STAR Experiment

We report systematic measurements of dielectron (${e}^{+}{e}^{\ensuremath{-}}$) invariant-mass ${M}_{ee}$ spectra at midrapidity in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{NN}}$ = 27, 39, and 62.4 GeV taken with the STAR detector at the Relativistic Heavy Ion Collider. For all energies studied, a significant excess yield of dielectrons is observed in the low-mass region ($0.40<{M}_{ee}<0.75 \mathrm{MeV}/{c}^{2}$) compared to hadronic cocktail simulations at freeze-out. Models that include an in-medium broadening of the $\ensuremath{\rho}$-meson spectral function consistently describe the observed excess. In addition, we report acceptance-corrected dielectron-excess spectra for $\mathrm{Au}+\mathrm{Au}$ collisions at midrapidity ($|{y}_{ee}|\phantom{\rule{4pt}{0ex}}<$ 1) in the 0--80% centrality bin for each collision energy. The integrated excess yields for $0.4<{M}_{ee}<0.75\phantom{\rule{4pt}{0ex}}\text{GeV}/{c}^{2}$, normalized by the charged particle multiplicity at midrapidity, are compared with previously published measurements for $\mathrm{Au}+\mathrm{Au}$ at $\sqrt{{s}_{NN}\phantom{{}^{1}}}$ = 19.6 and 200 GeV. Models that include an in-medium broadening of the $\ensuremath{\rho}$-meson spectral function consistently describe the observed excess. The normalized excess yields in the low-mass region show no significant collision energy dependence. The data, however, are consistent with model calculations that demonstrate a modest energy dependence.