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Enabled by the addition of a TOF detector system in 2010, the first phase ofthe Beam Energy Scan (BES-I) at RHIC allows STAR to conduct an unprecedentedenergy-dependent study of dielectron production within a homogeneousexperimental environment, and hence close the wide gap in the QCD phase diagrambetween SPS and top RHIC energies. This thesis concentrates on theunderstanding of the LMR enhancement regarding its $M_{ee}$, $p_{T}$ and energydependence. It studies dielectron production in Au+Au collisions at beamenergies of 19.6, 27, 39, and 62.4 GeV with sufficient statistics. Inconjunction with the published STAR results at top RHIC energy, this thesispresents results on the first comprehensive energy-dependent study ofdielectron production.  The comparison of $M_{ee}$- and $p_{T}$-spectra with cocktail simulations ofhadronic sources reveals a sizeable and steadily increasing excess yield in theLMR at all beam energies. The scenario of broadened in-medium $\rho$ spectralfunctions proves to not only serve well as dominating underlying source butalso to be universal in nature since it quantitatively and qualitativelyexplains the LMR enhancements from SPS to top RHIC energies. It shows that mostof the enhancement is governed by interactions of the $\rho$ meson with thermalresonance excitations in the late(r)-stage hot and dense hadronic phase. Thisconclusion is supported by the energy-dependent measurement of integrated LMRexcess yields and enhancement factors. The former do not exhibit a strongdependence on beam energy as expected from the approximately constant totalbaryon density above 20 GeV, and the latter show agreement with the CERESmeasurement at SPS energy. The consistency in excess yields and agreement withmodel calculations over the wide RHIC energy regime makes a strong case for LMRenhancements on the order of a factor 2-3.

Beam Energy Dependence of Dielectron Production in Au+Au Collisions from STAR at RHIC