We report systematic measurements of dielectron ($e^{+}{e}^{-}$) 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 $\rho$-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 ($|y_{ee}|<$ 1) in the 0–80% centrality bin for each collision energy. The integrated excess yields for $0.4<M_{ee}<0.75\text{GeV}/c^2$, 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 $\rho$-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.

• Received 27 February 2023
• Accepted 4 May 2023

DOI:https://doi.org/10.1103/PhysRevC.107.L061901