# Events at Rice

## Thesis Defense

Speaker: Joseph Butterworth
Doctoral Candidate

### Electron-positron pair production in Au+Au collisions at $\sqrt{s_{NN}}$ = 27 GeV as part of the Beam Energy Scan Program at STAR

Tuesday, April 11, 2017
9:00 AM  to 11:00 AM

223  Herman Brown Hall

This thesis reports the e$^{+}$e$^{-}$ pair production in the top 80\% most central Au on Au collisions at $\sqrt{s_{NN}}$ = 27 GeV from the STAR experiment. The production exhibits an excess yield, in the low invariant mass region ($M_{ee}$ $\lesssim$ 1.2 GeVc$^{-2}$), when compared to a hadronic cocktail without $\rho$ contributions. This excess yield is shown to be successfully described by model calculations that include a broadening of the $\rho$ spectral function in a hot, dense medium, and similar agreement to the excess yield, after applying an acceptance-correction, is also exhibited. These results are a part of a systematic study of the e$^{+}$e$^{-}$ pair production in Relativistic Heavy Ion Collider Beam Energy Scan Program and are presented with data from Au on Au collisions at $\sqrt{s_{NN}}$ = 19.6, 39, 62.4, and 200 GeV. Each collision energy e$^{+}$e$^{-}$ pair measurement has an excess with respect to a hadronic model without $\rho$ contributions, and each excess yield is described by model calculations that include a broadening of the $\rho$ spectral function in a hot, dense medium. This suggests that the $\rho$ meson is modified by the hot, dense medium. The acceptance-corrected excess yields are also reported as a function $\sqrt{s_{NN}}$, charged particle multiplicity, and predicted lifetimes. Given uncertainties, no statistically significant excess yield dependence on $\sqrt{s_{NN}}$, charged particle multiplicity, and predicted lifetimes are observed for Au on Au collisions with a 0$-$80\% centrality.

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