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

TL;DR

This paper reports a search for the rare decay $B^{0} o \gamma\gamma$ using combined data from Belle and Belle II, finding a slight excess of events and setting an upper limit on the decay's branching fraction.

Contribution

First combined analysis of Belle and Belle II data for $B^{0} o \gamma\gamma$ decay, providing the most stringent upper limit to date.

Findings

Observed 11 signal events with 2.5$\sigma$ significance.

Measured branching fraction $(3.7^{+2.2}_{-1.8}) imes 10^{-8}$.

Set upper limit $ extless 6.4 imes 10^{-8}$ at 90 ext{% CL}.

Abstract

We report the result of a search for the rare decay $B^{0} \to \gamma \gamma$ using a combined dataset of $753\times10^{6}$ $B\bar{B}$ pairs collected by the Belle experiment and $387\times10^{6}$ $B\bar{B}$ pairs collected by the Belle II experiment from decays of the $\rm \Upsilon(4S)$ 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 2.5$\sigma$ significance. We determine the branching fraction $\mathcal{B}(B^{0} \to \gamma\gamma) = (3.7^{+2.2}_{-1.8}(\rm stat)\pm0.5(\rm syst))\times10^{-8}$ and set a 90% credibility level upper limit of $\mathcal{B}(B^{0} \to \gamma\gamma) < 6.4\times10^{-8}$.