Search for the decay $J/\psi \to \gamma +$ invisible

We search for $J/\psi$ radiative decays into a weakly interacting neutral particle, namely an invisible particle, using the $J/\psi$ produced through the process $\psi (3686)\to {\pi }^{+}{\pi }^{-}J/\psi$ in a data sample of $(448.1\pm 2.9)\times {10}^6\psi (3686)$ decays collected by the BESIII detector at BEPCII. No significant signal is observed. Using a modified frequentist method, upper limits on the branching fractions are set under different assumptions of invisible particle masses up to $1.2\mathrm{GeV}/c^2$. The upper limit corresponding to an invisible particle with zero mass is $7.0\times {10}^{-7}$ at the 90% confidence level.

Figure 1

Fit to the $M_{{\pi }^{+}{\pi }^{-}}^{\mathrm{rec}}$ distribution. The blue solid line is the sum of signal (red dashed line) and background (pink dashed line). The shaded region, (3.0625,3.0775) and $(3.1165,3.1315)\mathrm{GeV}/c^2$, is determined as sideband region for non-$J/\psi$ background study.

Figure 2

The $E_{\gamma }^{*}$ distribution. Data is shown with black dots. The total background from $\psi (3686)\to {\pi }^{+}{\pi }^{-}J/\psi$, estimated from MC simulation, is shown with the black solid line and includes contributions from the subsequent decays $J/\psi \to \gamma {\pi }^0$ (long dashed yellow line), $\gamma \eta$ (short dashed green line), and $\gamma K_L{K}_L$ (dotted pink line). Non-$J/\psi$ backgrounds are estimated using $J/\psi$ sideband events (hatched histogram). The red and blue solid lines show the signal shape with 0 and $1\mathrm{GeV}/c^2$ mass assumptions, respectively.

Figure 3

Upper limits at the 90% C.L. for the branching fractions (upper plot) and the signal significance (bottom plot) for the decay $J/\psi \to \gamma +\text{invisible}$. In the upper plot, the black line is for data, the black dashed line represents the expected values and the green (yellow) band represents the $1\sigma (2\sigma )$ region.

Figure 4

Upper limits at the 90% C.L. for (a) $g_c\times {\tan }^2\beta (g_b)\times \sqrt{\mathcal{B}({\mathrm{A}}^0\to invisible)}$ and (b) $\sqrt{g_b{g}_c}\times \sqrt{\mathcal{B}(A^0\to invisible)}$.