Using $449\times {10}^6$ $B\overline{B}$ pairs collected with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider, we observe clear signals for $B^{+}\to {\overline{K}}^0{K}^{+}$ and $B^0\to {\overline{K}}^0{K}^0$ decays with $5.3\sigma$ and $6.0\sigma$ significance, respectively. We measure the branching fractions $\mathcal{B}(B^{+}\to {\overline{K}}^0{K}^{+})=({1.22}_{-0.28-0.16}^{+0.32+0.13})\times {10}^{-6}$ and $\mathcal{B}(B^0\to {\overline{K}}^0{K}^0)=({0.87}_{-0.20}^{+0.25}\pm 0.09)\times {10}^{-6}$, and partial-rate asymmetries $A_{CP}(B^{+}\to {\overline{K}}^0{K}^{+})={0.13}_{-0.24}^{+0.23}\pm 0.02$ and $A_{CP}(B^0\to {\overline{K}}^0{K}^0)=-{0.58}_{-0.66}^{+0.73}\pm 0.04$. From a simultaneous fit, we also obtain $\mathcal{B}(B^{+}\to K^0{\pi }^{+})=({22.8}_{-0.7}^{+0.8}\pm 1.3)\times {10}^{-6}$ and $A_{CP}(B^{+}\to K^0{\pi }^{+})=0.03\pm 0.03\pm 0.01$. The first and second error in the branching fractions and the partial-rate asymmetries are statistical and systematic, respectively. No signal is observed for $B^0\to K^{+}{K}^{-}$ decays, and for this branching fraction, we set an upper limit of $4.1\times {10}^{-7}$ at the 90% confidence level.

• Received 14 December 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.181804