Energy dependence of acceptance-corrected dielectron excess mass spectrum at mid-rapidity in Au+Au collisions at $\sqrt{s_{NN}} = 19.6$ and 200 GeV

TL;DR

This study measures dielectron excess mass spectra in Au+Au collisions at 19.6 and 200 GeV, revealing energy-dependent medium properties and longer-lived hot dense matter at higher energies.

Contribution

First measurement of acceptance-corrected dielectron excess spectra at mid-rapidity in Au+Au collisions at these energies, with comparison to models and previous experiments.

Findings

Excess spectra are well described by a broadened rho spectral function.

Normalized excess yield at 19.6 GeV is similar to In+In collisions at 17.3 GeV.

At 200 GeV, the excess yield increases with collision centrality, indicating longer medium lifetime.

Abstract

The acceptance-corrected dielectron excess mass spectra, where the known hadronic sources have been subtracted from the inclusive dielectron mass spectra, are reported for the first time at mid-rapidity $|y_{ee}|<1$ in minimum-bias Au+Au collisions at $\sqrt{s_{NN}}$ = 19.6 and 200 GeV. The excess mass spectra are consistently described by a model calculation with a broadened $\rho$ spectral function for $M_{ee}<1.1$ GeV/$c^{2}$. The integrated dielectron excess yield at $\sqrt{s_{NN}}$ = 19.6 GeV for $0.4<M_{ee}<0.75$ GeV/$c^2$, normalized to the charged particle multiplicity at mid-rapidity, has a value similar to that in In+In collisions at $\sqrt{s_{NN}}$ = 17.3 GeV. For $\sqrt{s_{NN}}$ = 200 GeV, the normalized excess yield in central collisions is higher than that at $\sqrt{s_{NN}}$ = 17.3 GeV and increases from peripheral to central collisions. These measurements indicate that the lifetime of the hot, dense medium created in central Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV is longer than those in peripheral collisions and at lower energies.