Study of $\eta\rightarrow\pi^+\pi^-l^+l^-$

TL;DR

This study measures branching fractions, searches for $CP$ violation, and explores axion-like particles in $ ext{eta}$ decays using a large $J/ ext{psi}$ dataset, providing new limits and form factor insights.

Contribution

It provides the first precise measurement of $ ext{eta} ightarrow ext{pi}^+ ext{pi}^- e^+ e^-$ branching fraction and sets upper limits on $ ext{eta} ightarrow ext{pi}^+ ext{pi}^- ext{mu}^+ ext{mu}^-$, also investigating $CP$ violation and axion-like particles.

Findings

Measured $ ext{eta} ightarrow ext{pi}^+ ext{pi}^- e^+ e^-$ branching fraction as $(3.07 extpm0.12_{ m stat} extpm0.19_{ m syst}) imes 10^{-4}$.

No signal observed for $ ext{eta} ightarrow ext{pi}^+ ext{pi}^- ext{mu}^+ ext{mu}^-$, setting an upper limit of $4.0 imes 10^{-7}$.

Found no evidence of $CP$ violation in the decay, with asymmetry parameter $ ext{A}_{CP} = (-4.04 extpm4.69_{ m stat} extpm0.14_{ m syst}) ext{ extperthousand}$.

Abstract

Using a sample of $(10087\pm44)\times10^{6}$ $J/\psi$ events accumulated with the BESIII detector, we analyze the decays $\eta\rightarrow\pi^+\pi^-l^+l^-$ ($l=e$ or $\mu$) via the process $J/\psi\rightarrow\gamma\eta$. The branching fraction of $\eta\rightarrow\pi^+\pi^-e^+e^-$ is measured to be $\mathcal{B}(\eta\rightarrow\pi^+\pi^-e^+e^-)=(3.07\pm0.12_{\rm{stat.}}\pm0.19_{\rm{syst.}}) \times10^{-4}$. No signal events are observed for the $\eta\rightarrow\pi^{+}\pi^{-}\mu^{+}\mu^{-}$ decay, leading to an upper limit on the branching fraction of $\mathcal{B}(\eta\rightarrow\pi^{+}\pi^{-}\mu^{+}\mu^{-})<4.0\times10^{-7}$ at the 90\% confidence level. Furthermore, the $CP$-violation asymmetry parameter is found to be $\mathcal{A}_{CP}(\eta\rightarrow\pi^{+}\pi^{-}e^{+}e^{-})=(-4.04\pm4.69_{\rm{stat.}}\pm0.14_{\rm{syst.}})\%$, showing no evidence of $CP$-violation with current statistics. Additionally, we extract the transition form factor from the decay amplitude of $\eta\rightarrow\pi^+\pi^-e^+e^-$. Finally, axion-like particles are searched for via the decay $\eta\rightarrow\pi^+\pi^-a, a\rightarrow e^+e^-$, and upper limits on this branching fraction relative to that of $\eta\rightarrow\pi^+\pi^-e^+e^-$ are presented as a function of the axion-like particle mass in the range $5-200\ \mathrm{MeV}/c^{2}$.