Search for ${\mathit{B}}_{\mathit{s}}^{\mathit{0}}\to \mathit{\gamma }\mathit{\gamma }$ and a measurement of the branching fraction for ${\mathit{B}}_{\mathit{s}}^{\mathit{0}}\to \mathit{\varphi }\mathit{\gamma }$

We search for the decay $B_s^0\to \gamma \gamma$ and measure the branching fraction for $B_s^0\to \varphi \gamma$ using $121.4{\mathrm{fb}}^{-1}$ of data collected at the $\mathrm{\Upsilon }(5S)$ 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 $(3.6\pm 0.5(\text{stat})\pm 0.3(\text{syst})\pm 0.6(f_s))\times 10^{-5}$, where $f_s$ is the fraction of $B_s^{(*)}{\overline{B}}_s^{(*)}$ in $b\overline{b}$ 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.

Figure 1

Leading-order Feynman diagrams for the decays (a) $B_s^0\to \varphi \gamma$ and (b) $B_s^0\to \gamma \gamma$.

Figure 2

Data fits for the $B_s^0\to \varphi \gamma$ analysis. The projections are shown only for events inside the $B_s^{*}{\overline{B}}_s^{*}$ signal region except for the plotted variable. The $B_s^{*}{\overline{B}}_s^{*}$ signal region is defined as $M_{\text{bc}}>5.4\mathrm{GeV}/c^2$, $-0.2<\mathrm{\Delta }E<0.02\mathrm{GeV}$, $|\cos {\theta }_{\text{hel}}|<0.8$ and $0.0<{\mathcal{C}}_{\mathrm{NB}}^{\prime }<10.0$. The points with error bars represent the data, the solid black curve represents the total fit function and the red dashed (blue dotted) curve represents the signal (continuum background) contribution.

Figure 3

Data fits for the $B_s^0\to \gamma \gamma$ analysis. The projections are shown only for events inside the $B_s^{*}{\overline{B}}_s^{*}$ signal region except for the plotted variable. The $B_s^{*}{\overline{B}}_s^{*}$ signal region is defined as $M_{\text{bc}}>5.4\mathrm{GeV}/c^2$ and $-0.3<\mathrm{\Delta }E<0.05\mathrm{GeV}$. The points with error bars represent the data, the solid black curve represents the total fit function, the dotted red curve represents the fit with the signal yield constrained to its 90% C.L. upper limit.