Partial wave analysis of $J/\psi\rightarrow\gamma\eta\eta'$

TL;DR

This paper reports the first observation of an exotic $J^{PC}=1^{-+}$ state, $ ext{eta}_1(1855)$, in $J/ heta ightarrow ext{gamma} ext{eta} ext{eta'}$ decays, with precise measurements of its properties and implications for glueball content.

Contribution

The study presents the first detection of the exotic $ ext{eta}_1(1855)$ state with detailed measurements, expanding understanding of exotic mesons and glueball candidates.

Findings

Observation of $ ext{eta}_1(1855)$ with >19$\sigma$ significance.

Measured mass and width of $ ext{eta}_1(1855)$ as 1855 MeV/$c^2$ and 188 MeV.

Upper limit on $f_0(1710)$ decay ratio supports large glueball component.

Abstract

Based on a sample of (10.09$\pm$0.04)$\times$10$^{9}$ $J/\psi$ events collected with the BESIII detector operating at the BEPCII storage ring, a partial wave analysis of the decay $J/\psi \rightarrow \gamma\eta\eta'$ is performed. An isoscalar state with exotic quantum numbers $J^{PC}=1^{-+}$, denoted as $\eta_1(1855)$, has been observed for the first time with statistical significance larger than 19$\sigma$. Its mass and width are measured to be (1855$\pm$9$_{-1}^{+6}$)~MeV/$c^{2}$ and (188$\pm$18$_{-8}^{+3}$)~MeV, respectively. The product branching fraction ${\cal B}(J/\psi$$\rightarrow$$ \gamma\eta_1(1855)$$\rightarrow$$\gamma\eta\eta')$ is measured to be (2.70$\pm 0.41 _{-0.35}^{+0.16}) \times$10$^{-6}$. The first uncertainties are statistical and the second are systematic. In addition, an upper limit on the branching ratio ${\cal B}(f_0(1710)$$\rightarrow$$\eta\eta')$/${\cal B}(f_0(1710)$$\rightarrow$$\pi\pi)$ is determined to be $1.61 \times 10^{-3}$ at 90\% confidence level, which lends support to the hypothesis that the $f_0(1710)$ has a large glueball component.