Observation of $J/\psi \to 3\gamma$

We report the first observation of the decay $J/\psi \to 3\gamma$. The signal has a statistical significance of $6\sigma$ and corresponds to a branching fraction of $\mathcal{B}(J/\psi \to 3\gamma )=(1.2\pm 0.3\pm 0.2)\times {10}^{-5}$, in which the errors are statistical and systematic, respectively. The measurement uses $\psi (2S)\to {\pi }^{+}{\pi }^{-}J/\psi$ events acquired with the CLEO-$c$ detector operating at the CESR $e^{+}{e}^{-}$ collider.

Figure 1

Top four plots: in $3\gamma$ data (lower left) and MC events for different $J/\psi$ decays (top row and lower right), the largest vs the smallest two-photon mass combination per event. In the MC plots, darker shading of each bin signifies higher event density than lighter shading; in the data plot, each dot represents an event. The solid lines demarcate regions excluded from the $J/\psi \to 3\gamma$ selection. Bottom plot: distribution of $M({\pi }^{+}{\pi }^{-}-\mathrm{recoil})$ for the data events (points with error bars) overlaid with the $J/\psi \to 3\gamma$ signal MC prediction (dotted line histogram) and MC background plus signal (solid line histogram) normalized to the data population. The arrows indicate the region of accepted recoil mass.

Figure 2

As in Fig. 1, except zoomed in on the ${\eta }_c$ region, and the overlaid parallelogram indicates the signal region.

Figure 3

The distribution of ${\chi }^2/\mathrm{d}.\mathrm{o}.\mathrm{f}.$ for $J/\psi \to 3\gamma$ (lower right) and several sources of $\gamma {\pi }^0{\pi }^0$ background.

Figure 4

The distribution of ${\chi }^2/\mathrm{d}.\mathrm{o}.\mathrm{f}.$ for $J/\psi \to 3\gamma$, in the top plot showing data (points with error bars) overlaid with the sum (dotted line histogram) of three components: non-$J/\psi$ background from scaled data sidebands (shaded histogram) and MC predictions for signal (solid) and $J/\psi \to \gamma {\pi }^0{\pi }^0$ background (dashed). The bottom plot shows the same distribution, but with the MC and non-$J/\psi$ background subtracted from the data. The arrows indicate the values for signal selection and background normalization.