Observation of the Decay $B_s^0\to K^0{\overline{K}}^0$

We measure the decay $B_s^0\to K^0{\overline{K}}^0$ using data collected at the $\mathrm{\Upsilon }(5S)$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. The data sample used corresponds to an integrated luminosity of $121.4{\mathrm{fb}}^{-1}$. We measure a branching fraction $\mathcal{B}(B_s^0\to K^0{\overline{K}}^0)=[{19.6}_{-5.1}^{+5.8}(\text{stat})\pm 1.0(\text{syst})\pm 2.0(N_{B_s^0{\overline{B}}_s^0})]\times {10}^{-6}$ with a significance of 5.1 standard deviations. This measurement constitutes the first observation of this decay.

Figure 1

Loop diagram for $B_s^0\to K^0{\overline{K}}^0$ decays.

Figure 2

Projections of the 3D fit to the real data: (a) $M_{\mathrm{bc}}$ in $-0.11\mathrm{GeV}<\mathrm{\Delta }E<0.02\mathrm{GeV}$ and $C_{\mathrm{NN}}^{\prime }>0.5$; (b) $\mathrm{\Delta }E$ in $5.405\mathrm{GeV}/c^2<M_{\mathrm{bc}}<5.427\mathrm{GeV}/c^2$ and $C_{\mathrm{NN}}^{\prime }>0.5$; and (c) $C_{\mathrm{NN}}^{\prime }$ in $5.405\mathrm{GeV}/c^2<M_{\mathrm{bc}}<5.427\mathrm{GeV}/c^2$ and $-0.11\mathrm{GeV}<\mathrm{\Delta }E<0.02\mathrm{GeV}$. The points with error bars are data, the (green) dashed curves show the signal, (magenta) dotted curves show the continuum background, and (blue) solid curves show the total. The ${\chi }^2/(\text{number of bins})$ values of these fit projections are 0.30, 0.43, and 0.26, respectively, which indicate that the fit gives a good description of the data. The three peaks in $M_{\mathrm{bc}}$ arise from $\mathrm{\Upsilon }(5S)\to B_s^0{\overline{B}}_s^0,(B_s^{*0}{\overline{B}}_s^0+B_s^0{\overline{B}}_s^{*0})$, and $B_s^{*0}{\overline{B}}_s^{*0}$ decays.

Figure 3

Background subtracted sPlot distributions of $M({\pi }^{+}{\pi }^{-})$ for the (a) higher momentum and (b) lower momentum $K_S^0$ candidates.