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

Abstract

Enabled by the addition of a TOF detector system in 2010, the first phase of the Beam Energy Scan (BES-I) at RHIC allows STAR to conduct an unprecedented energy-dependent study of dielectron production within a homogeneous experimental environment, and hence close the wide gap in the QCD phase diagram between SPS and top RHIC energies. This thesis concentrates on the understanding of the LMR enhancement regarding its $M_{ee}$, $p_{T}$ and energy dependence. It studies dielectron production in Au+Au collisions at beam energies of 19.6, 27, 39, and 62.4 GeV with sufficient statistics. In conjunction with the published STAR results at top RHIC energy, this thesis presents results on the first comprehensive energy-dependent study of dielectron production. The comparison of $M_{ee}$- and $p_{T}$-spectra with cocktail simulations of hadronic sources reveals a sizeable and steadily increasing excess yield in the LMR at all beam energies. The scenario of broadened in-medium $\rho$ spectral functions proves to not only serve well as dominating underlying source but also to be universal in nature since it quantitatively and qualitatively explains the LMR enhancements from SPS to top RHIC energies. It shows that most of the enhancement is governed by interactions of the $\rho$ meson with thermal resonance excitations in the late(r)-stage hot and dense hadronic phase. This conclusion is supported by the energy-dependent measurement of integrated LMR excess yields and enhancement factors. The former do not exhibit a strong dependence on beam energy as expected from the approximately constant total baryon density above 20 GeV, and the latter show agreement with the CERES measurement at SPS energy. The consistency in excess yields and agreement with model calculations over the wide RHIC energy regime makes a strong case for LMR enhancements on the order of a factor 2-3.