Search for the decay $B^0\to \gamma \gamma$ using Belle and Belle II data

We report the result of a search for the rare decay $B^0\to \gamma \gamma$ using a combined dataset of $753\times {10}^6$ $B\bar{B}$ pairs collected by the Belle experiment and $387\times {10}^6$ $B\bar{B}$ pairs collected by the Belle II experiment from decays of the $\mathrm{{\rm Y}}(4\mathrm{S})$ resonance produced in $e^{+}{e}^{-}$ collisions. A simultaneous fit to the Belle and Belle II data sets yields $11.0_{-5.5}^{+6.5}$ signal events, corresponding to a 2.5$\sigma$ significance. We determine the branching fraction $\mathcal{B}(B^0\to \gamma \gamma )=(3.7_{-1.8}^{+2.2}(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 0.5(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}))\times {10}^{-8}$ and set a 90% credibility level upper limit of $\mathcal{B}(B^0\to \gamma \gamma )<6.4\times {10}^{-8}$.