Observation of $B_s^0\to \phi \gamma$ and Search for $B_s^0\to \gamma \gamma$ Decays at Belle

We search for the radiative penguin decays $B_s^0\to \varphi \gamma$ and $B_s^0\to \gamma \gamma$ in a $23.6{\mathrm{fb}}^{-1}$ data sample collected at the ${\rm Y}(5S)$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ asymmetric-energy collider. We observe for the first time a radiative penguin decay of the $B_s^0$ meson in the $B_s^0\to \varphi \gamma$ mode and we measure $\mathcal{B}(B_s^0\to \varphi \gamma )=\mathbf{(}{57}_{-15}^{+18}(\mathrm{stat}{)}_{-11}^{+12}(\mathrm{syst})\mathbf{)}\times {10}^{-6}$. No significant $B_s^0\to \gamma \gamma$ signal is observed and we set a 90% confidence level upper limit of $\mathcal{B}(B_s^0\to \gamma \gamma )<8.7\times {10}^{-6}$.

Figure 1

Diagrams describing the dominant processes for the $B_s^0\to \varphi \gamma$ (left) and $B_s^0\to \gamma \gamma$ (right) decays.

Figure 2

$M_{\mathrm{bc}}$, $\Delta E$, and $\cos {\theta }_{\mathrm{hel}}$ projections together with fit results for the $B_s^0\to \varphi \gamma$ mode. The points with error bars represent data, the thick solid curves are the fit functions, the thin solid curves are the signal functions, and the dashed curves show the continuum contribution. On the $M_{\mathrm{bc}}$ figure, signals from $B_s^0{\overline{B}}_s^0$, $B_s^{*}{\overline{B}}_s^0$, and $B_s^{*}{\overline{B}}_s^{*}$ appear from left to right. On the $\Delta E$ and $\cos {\theta }_{\mathrm{hel}}$ figures, due to the requirement $M_{\mathrm{bc}}>5.4\mathrm{GeV}/c^2$ only the $B_s^{*}{\overline{B}}_s^{*}$ signal contributes. The bottom right figure shows $\Delta E$ versus $M_{\mathrm{bc}}$ for selected data events. The dashed lines show the signal window.

Figure 3

$M_{\mathrm{bc}}$ and $\Delta E$ projections together with fit results for the $B_s^0\to \gamma \gamma$ mode. The points with error bars represent data, the thick solid curves are the fit functions, the thin solid curves are the signal functions, and the dashed curves show the continuum contribution. On the $M_{\mathrm{bc}}$ figure, signals from $B_s^0{\overline{B}}_s^0$, $B_s^{*}{\overline{B}}_s^0$, and $B_s^{*}{\overline{B}}_s^{*}$ appear from left to right. On the $\Delta E$ figure, due to the requirement $M_{\mathrm{bc}}>5.4\mathrm{GeV}/c^2$ only the $B_s^{*}{\overline{B}}_s^{*}$ signal contributes.