Search for $B_s^0\to \gamma \gamma$ and a measurement of the branching fraction for $B_s^0\to \varphi \gamma$

We search for the decay $B_s^0\to \gamma \gamma$ and measure the branching fraction for $B_s^0\to \varphi \gamma$ using 121.4~${\text{fb}}^{-1}$ of data collected at the $\mathrm{{\rm Y}}(5S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. The $B_s^0\to \varphi \gamma$ branching fraction is measured to be $(3.6 \pm 0.5 ($\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}$) \pm 0.3 ($\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}$) \pm 0.6 (f_{s})) \times 10^{-5}$,where$f_{s}$isthefractionof$B_{s}^{()}\bar{B}_{s}^{()}$in$b\bar{b}$ events. Our result is in good agreement with the theoretical predictions as well as with a recent measurement from LHCb. We observe no statistically significant signal for the decay $B_s^0\to \gamma \gamma$ and set a $90\mathrm{\%}$ confidence-level upper limit on its branching fraction at $3.1\times {10}^{-6}$. This constitutes a significant improvement over the previous result.