Study of the decay B° → γ γ at Belle and Belle II

Abstract

We report the result of a search for the rare decay B0 → γγ using a combined dataset of 753 × 106 B ¯B pairs collected by the Belle experiment and 387 × 106 B ¯B pairs collected by the Belle II experiment at the Υ(4S) resonance produced in electron-positron collisions. A simultaneous fit to the Belle and Belle II data sets yields 11.0+6.5 −5.5 signal events. We determine the branching fraction B(B0 → γγ) = (3.7+2.2 −1.8(stat) ± 0.5(syst)) × 10−8 with a signal significance of 2.5σ and set a 90% confidence level upper limit of B(B0 → γγ) < 6.4 × 10−8. This result improves on the previously published 90% confidence level upper limit by BABAR by a factor of about five and provides the most stringent limit to date.