Search for the decay $B^0\to \gamma \gamma$

The rare decay $B^0\to \gamma \gamma$ is searched for in $104{\mathrm{fb}}^{-1}$ of data, corresponding to $111\times {10}^{6}B\overline{B}$ pairs, collected with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. No evidence for the signal is found, and an upper limit of $6.2\times {10}^{-7}$ at 90% confidence level is set for the corresponding branching fraction.

Figure 1

A possible diagram contributing to $B^0\to \gamma \gamma$ at the lowest order in the SM. The exchange of a charged Higgs boson instead of the $W$ boson could contribute to this process in some extensions of the SM.

Figure 2

$\Delta E$ versus $M_{\mathrm{bc}}$ for data events, selected as described in the text. Seven events are in the signal window (solid rectangle). The fit window is shown as a dashed rectangle.

Figure 3

Projections of the fit result on $\Delta E$ (with $M_{\mathrm{bc}}$ in its signal window) and on $M_{\mathrm{bc}}$ (with $\Delta E$ in its signal window). The fitting curve (solid line) is plotted with data (circles with error bars, drawn as asymmetric Poisson confidence intervals) and the $uds$ background (dashed line). The filled regions represent the signal (dark shading) and the $B^0\to {\pi }^0{\pi }^0$ and $B^0\to \eta {\pi }^0$ backgrounds (light shading).