Di-electron spectrum at mid-rapidity in $p+p$ collisions at $\sqrt{s} = 200$ GeV

TL;DR

This study measures the di-electron spectrum at mid-rapidity in proton-proton collisions at 200 GeV, comparing data with simulations, and provides new limits on rare meson decay branching ratios.

Contribution

First measurement of di-electron spectra at 200 GeV in p+p collisions with detailed comparison to models and extraction of meson yields.

Findings

Di-electron continuum ratio exceeds 10% over the mass range.

Simulations of meson decays match the observed data.

New upper limit on eta to e+e- decay branching ratio.

Abstract

We report on mid-rapidity mass spectrum of di-electrons and cross sections of pseudoscalar and vector mesons via $e^{+}e^{-}$ decays, from $\sqrt{s} = 200$ GeV $p+p$ collisions, measured by the large acceptance experiment STAR at RHIC. The ratio of the di-electron continuum to the combinatorial background is larger than 10% over the entire mass range. Simulations of di-electrons from light-meson decays and heavy-flavor decays (charmonium and open charm correlation) are found to describe the data. The extracted $\omega\rightarrow e^{+}e^{-}$ invariant yields are consistent with previous measurements. The mid-rapidity yields ($dN/dy$) of $\phi$ and $J/\psi$ are extracted through their di-electron decay channels and are consistent with the previous measurements of $\phi\rightarrow K^{+}K^{-}$ and $J/\psi\rightarrow e^{+}e^{-}$. Our results suggest a new upper limit of the branching ratio of the $\eta \rightarrow e^{+}e^{-}$ of $1.7\times10^{-5}$ at 90% confidence level.