Observation of $\eta^\prime\rightarrow\pi^+\pi^-\mu^+\mu^-$

TL;DR

This paper reports the first observation of the rare decay $ ext{eta}^ ext{'} ightarrow ext{pi}^+ ext{pi}^- ext{mu}^+ ext{mu}^-$ using a large dataset of $J/ ext{psi}$ events, measuring its branching fraction with high significance.

Contribution

It provides the first experimental evidence and measurement of the branching fraction for the decay $ ext{eta}^ ext{'} ightarrow ext{pi}^+ ext{pi}^- ext{mu}^+ ext{mu}^-$.

Findings

First observation of the decay with 8σ significance.

Measured branching fraction: (1.97 ± 0.33(stat) ± 0.18(syst)) × 10^{-5}.

Utilized 1.31 billion $J/psi$ events at BESIII.

Abstract

Using $(1310.6 \pm 7.0) \times 10^{6}~J/\psi$ events acquired with the BESIII detector at the BEPCII storage rings, the decay $\eta^\prime\rightarrow\pi^+\pi^-\mu^+\mu^-$ is observed for the first time with a significance of 8$\sigma$ via the process $J/\psi\rightarrow\gamma\eta'$. We measure the branching fraction of $\eta^\prime\rightarrow\pi^+\pi^-\mu^+\mu^-$ to be $\mathcal{B}(\eta^\prime\rightarrow\pi^+\pi^-\mu^+\mu^-)$=(1.97$\pm$0.33(stat.)$\pm$0.18(syst.))$\times10^{-5}$, where the first and second uncertainties are statistical and systematic, respectively.