# Observation and study of the decay $J/\psi\rightarrow\phi\eta\eta'$

## TL;DR

This paper reports the observation of the decay $J/ar{psi}
ightarrowar{\phi}ar{\eta}ar{\eta'}$ and evidence for a new structure, $X$, in the $ar{\phi}ar{\eta'}$ mass spectrum, with detailed measurements of its properties.

## Contribution

First observation of the decay $J/ar{psi}
ightarrowar{\phi}ar{\eta}ar{\eta'}$ and evidence for a new resonance $X$ in the $ar{\phi}ar{\eta'}$ spectrum with measured mass, width, and quantum number hypotheses.

## Key findings

- Branching fraction of $J/ar{psi}
ightarrowar{\phi}ar{\eta}ar{\eta'}$ is $(2.32\pm0.06\pm0.16) imes10^{-4}$.
- Evidence for a structure $X$ with mass around 2.0-2.1 GeV/$c^2$ and width about 130-177 MeV.
- Significance of the $X$ structure is 4.4$\sigma$ for $J^P=1^-$ and 3.8$\sigma$ for $J^P=1^+$.

## Abstract

We report the observation and study of the decay $J/\psi\rightarrow\phi\eta\eta'$ using $1.3\times{10^9}$ $J/\psi$ events collected with the BESIII detector. Its branching fraction, including all possible intermediate states, is measured to be $(2.32\pm0.06\pm0.16)\times{10^{-4}}$. We also report evidence for a structure, denoted as $X$, in the $\phi\eta'$ mass spectrum in the $2.0-2.1$ GeV/$c^2$ region. Using two decay modes of the $\eta'$ meson ($\gamma\pi^+\pi^-$ and $\eta\pi^+\pi^-$), a simultaneous fit to the $\phi\eta'$ mass spectra is performed. Assuming the quantum numbers of the $X$ to be $J^P = 1^-$, its significance is found to be 4.4$\sigma$, with a mass and width of $(2002.1 \pm 27.5 \pm 21.4)$ MeV/$c^2$ and $(129 \pm 17 \pm 9)$ MeV, respectively, and a product branching fraction $\mathcal{B}(J/\psi\rightarrow\eta{}X)\times{}\mathcal{B}(X\rightarrow\phi\eta')=(9.8 \pm 1.2 \pm 1.7)\times10^{-5}$. Alternatively, assuming $J^P = 1^+$, the significance is 3.8$\sigma$, with a mass and width of $(2062.8 \pm 13.1 \pm 7.2)$ MeV/$c^2$ and $(177 \pm 36 \pm 35)$ MeV, respectively, and a product branching fraction $\mathcal{B}(J/\psi\rightarrow\eta{}X)\times{}\mathcal{B}(X\rightarrow\phi\eta')=(9.6 \pm 1.4 \pm 2.0)\times10^{-5}$. The angular distribution of $J/\psi\rightarrow\eta{}X$ is studied and the two $J^P$ assumptions of the $X$ cannot be clearly distinguished due to the limited statistics. In all measurements the first uncertainties are statistical and the second systematic.