Observation of the decay $B_s^0\to K^0{\bar{K}}^0$

We measure the decay $B_s^0\to K^0{\bar{K}}^0$ using data collected at the $\mathrm{{\rm Y}}(5S)$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. The data sample used corresponds to an integrated luminosity of 121.4 $\mathrm{f}{\mathrm{b}}^{-1}$. We measure a branching fraction $$\mathcal{B}$(B_s^0\rightarrow K^0\overline{K}^0) = [19.6,^{+5.8}{-5.1}({\rm stat.}),\pm1.0({\rm sys.}),\pm 2.0(N^{}{B_s^0\overline{B}_s^0})]\times 10^{-6}$ with a significance of 5.1 standard deviations. This measurement constitutes the first observation of this decay.