Search for $B_s^0\to \gamma \gamma$ and a measurement of the branching fraction for $B_s^0\to \varphi \gamma$

Abstract

We search for the decay $B_s^0\to \gamma \gamma$ and measure the branching fraction for $B_s^0\to \varphi \gamma$ using 121.4~${\text{fb}}^{-1}$ of data collected at the ${\rm Y}(5S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. The $B_s^0\to \varphi \gamma$ branching fraction is measured to be $(3.6\pm 0.5(stat.)\pm 0.3(syst.)\pm 0.6(f_s))\times 10^{-5}$, where $f_s$ is the fraction of $B_s^{(\ast )}{\bar{B}}_s^{(\ast )}$ in $b\bar{b}$ events. Our result is in good agreement with the theoretical predictions as well as with a recent measurement from LHCb. We observe no statistically significant signal for the decay $B_s^0\to \gamma \gamma$ and set a $90\%$ confidence-level upper limit on its branching fraction at $3.1\times 10^{-6}$. This constitutes a significant improvement over the previous result.

6 pages, 3 figures

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