Observation of the decay $B_s^0rightarrow K^0overline{K}^0$

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