Search for the decay $J/\psi \to \gamma +\mathrm{\text{invisible}}$

A search for $J/\psi$ radiative decay to weakly interacting neutral final states was performed using the CLEO-c detector at the Cornell Electron Storage Ring. $J/\psi$ events were selected by observing the hadronic decay $\psi (2S)\to {\pi }^{+}{\pi }^{-}J/\psi$. A total of $3.7\times {10}^6$ $J/\psi$ events were used to study the decay $J/\psi \to \gamma +X$, where $X$ is a narrow state that is invisible to the detector. No significant signal was observed, and upper limits on the branching fraction were set for masses $m_X$ up to $960\mathrm{MeV}/c^2$. The upper limit corresponding to $m_X=0$ is $4.3\times {10}^{-6}$ at the 90% confidence level.

Figure 1

Recoil mass calculated from the di-pion four-momentum, after pion selection. A fit to the unshaded region of the spectrum was used to extract the tagged $J/\psi$ yield. The solid line shows the results of the fit and the dotted line shows the background contribution. Data in the shaded regions were used to study background.

Figure 2

(a) Invisible-decay events as a function of photon energy in the $J/\psi$ rest frame, and (b) the antineutron-enhanced spectrum obtained by selecting calorimeter showers with large lateral extent. The solid points are data and the unshaded histograms show the results of Monte Carlo simulations. The shaded histograms show the small contributions from events that have di-pion recoil mass adjacent to the $J/\psi$ mass window.

Figure 3

Fit to determine the branching fraction at $E_{\gamma }^{*}=1.548\mathrm{GeV}$, corresponding to $m_X=0$. The data are the same as in Fig. 2 but with finer binning. The solid line is the total fitted spectrum, and the dashed line shows the background contribution to the fit.

Figure 4

The 90% confidence-level upper limits for $J/\psi \to \gamma X$, where $X$ is invisible to the detector. The dashed line shows the results for statistical uncertainties alone, and the solid line includes systematic and statistical uncertainties.