Observation of $J/\psi \to \gamma \eta {\pi }^0$

We present the first study of the process $J/\psi \to \gamma \eta {\pi }^0$ using $(223.7\pm 1.4)\times 10^6$ $J/\psi$ events accumulated with the BESIII detector at the BEPCII facility. The branching fraction for $J/\psi \to \gamma \eta {\pi }^0$ is measured to be $\mathcal{B}(J/\psi \to \gamma \eta {\pi }^0)=(2.14\pm 0.18(\text{stat})\pm 0.25(\text{syst}))\times 10^{-5}$. With a Bayesian approach, the upper limits of the branching fractions $\mathcal{B}(J/\psi \to \gamma a_0(980),a_0(980)\to \eta {\pi }^0)$ and $\mathcal{B}(J/\psi \to \gamma a_2(1320),a_2(1320)\to \eta {\pi }^0)$ are determined to be $2.5\times 10^{-6}$ and $6.6\times 10^{-6}$ at the 95% confidence level, respectively. All of these measurements are given for the first time.

Figure 1

Distributions of the $\gamma \gamma$ invariant mass from the (a) $\eta$ and (b) ${\pi }^0$ candidate decays. The arrows with dotted lines indicate the signal region, and the solid arrows indicate the sidebands.

Figure 2

(a) Scatter plot of ${\gamma }_1{\gamma }_2$ versus $\gamma {\pi }^0$ masses after selecting event candidates with ${\chi }_{4C}^2<30$ and ${\mathrm{\Delta }}_{{\pi }^0}>0.05\mathrm{GeV}/c^2$. (b) The $\gamma {\pi }^0$ invariant mass spectrum after additional selection criteria are applied for photon-pair candidates in the $\eta$ and ${\pi }^0$ signal regions.

Figure 3

Result of the fit to the $\eta$ mass distributions in the ${\pi }^0$ (a) signal and (b) sideband regions. The circular dots with error bars show the distribution. The solid curve represents the fit result, while the short-dashed and dot-dashed curves represent the $\eta$ signals and backgrounds, respectively.

Figure 4

Invariant $\eta {\pi }^0$ mass spectrum after final events selection and $\eta$ and ${\pi }^0$ mass cuts (points with error bars). The solid curve shows the phase space of $J/\psi \to \gamma \eta {\pi }^0$.

Figure 5

Fit to the $\eta {\pi }^0$ mass spectrum in the $[0.8,2.0]\mathrm{GeV}/c^2$ region. The points with error bars are data; the solid curve shows the overall fit projection; the short-dashed curve represents the $a_0(980)$ signal; the dotted curve represents the $a_2(1320)$ signal; the dot-dashed curve corresponds to the two background channels $J/\psi \to \gamma {\eta }^{\prime }$, ${\eta }^{\prime }\to 2{\pi }^0\eta$ and $J/\psi \to \gamma {\eta }^{\prime }$, ${\eta }^{\prime }\to \gamma \omega$; and the long-dashed curve shows the remaining nonresonant $\eta {\pi }^0$ events.