Study of $\eta (1475)$ and $X(1835)$ in radiative $J/\psi$ decays to $\gamma \varphi$

Abstract

The decay $J/\psi \to \gamma \gamma \varphi$ is studied using a sample of $1.31\times 10^9$ $J/\psi$ events collected with the BESIII detector. Two structures around 1475 MeV/c${}^2$ and 1835 MeV/c${}^2$ are observed in the $\gamma \varphi$ invariant mass spectrum for the first time. With a fit on the $\gamma \varphi$ invariant mass, which takes into account the interference between the two structures, and a simple analysis of the angular distribution, the structure around 1475 MeV/c${}^2$ is found to favor an assignment as the $\eta (1475)$ and the mass and width for the structure around 1835 MeV/c${}^2$ are consistent with the $X(1835)$. The statistical significances of the two structures are $13.5\sigma$ and $6.3\sigma$, respectively. The results indicate that both $\eta (1475)$ and $X(1835)$ contain a sizeable $s\bar{s}$ component.