Search for the rare decay $D^0\to \gamma \gamma$ at Belle

We search for the rare radiative decay $D^0\to \gamma \gamma$ using a data sample with an integrated luminosity of $832{\mathrm{fb}}^{-1}$ recorded by the Belle detector at the KEKB $e^{+}{e}^{-}$ asymmetric-energy collider. We find no statistically significant signal and set an upper limit on the branching fraction of $\mathcal{B}(D^0\to \gamma \gamma )<8.5\times 10^{-7}$ at 90% confidence level. This is the most restrictive limit on the decay channel to date.

Figure 1

Projections of candidate events onto the $M(\gamma \gamma )$ (left) and $\mathrm{\Delta }M$ (right) distributions, applying a signal-region criterion on the other variable. Points with error bars are the data, blue solid curves are the results of the fit, blue dotted curves represent the combinatorial background, magenta dashed curves are the peaking background, and red filled histograms show the signal component.

Figure 2

Ranges of the $c\to u\gamma$ branching fraction predicted in the SM and MSSM [5] are compared with our obtained upper limit on $\mathcal{B}(D^0\to \gamma \gamma )$, shown by the purple solid line. The limits from BABAR [9], BESIII [10], and CLEO [8] are indicated by the green dotted, red long-dashed, and black dashed lines, respectively.