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

Abstract

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 $\mathrm{{\rm Y}}(4S)$ 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 $\mathcal{B}(B^0\to \gamma \gamma )=(3.7_{-1.8}^{+2.2}(\mathrm{stat})\pm 0.5(\mathrm{syst}))\times 10^{-8}$ and set a 90% credibility level upper limit of $\mathcal{B}(B^0\to \gamma \gamma )<6.4\times 10^{-8}$.

arXiv commentPublished in PRD(L)

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