Search for the decay B⁰→γγ using Belle and Belle II data

We report the result of a search for the rare decay B⁰ → γ γ using a combined dataset of 753×10⁶ BB̄ pairs collected by the Belle experiment and 387×10⁶ BB̄ pairs collected by the Belle II experiment from decays of the Υ(4S) resonance produced in e⁺e⁻ collisions. A simultaneous fit to the Belle and Belle II data sets yields 11.0+6.5-5.5 signal events, corresponding to a 2.5σ significance. We determine the branching fraction B(B⁰ → γγ) = (3.7+2.2-1.8( stat)±0.5( syst))×10⁻⁸ and set a 90% credibility level upper limit of B(B⁰ → γγ) < 6.4×10⁻⁸.