Observation ofB+→K¯0K+andB0→K0K¯0

We report observations of the $b\to d$ penguin-dominated decays $B^{+}\to {\overline{K}}^0{K}^{+}$ and $B^0\to K^0{\overline{K}}^0$ in $316{\mathrm{fb}}^{-1}$ of $e^{+}{e}^{-}$ collision data collected with the BABAR detector. We measure the branching fractions $\mathcal{B}(B^{+}\to {\overline{K}}^0{K}^{+})=(1.61\pm 0.44\pm 0.09)\times {10}^{-6}$ and $\mathcal{B}(B^0\to K^0{\overline{K}}^0)=(1.08\pm 0.28\pm 0.11)\times {10}^{-6}$ and the $CP$-violating charge asymmetry ${\mathcal{A}}_{CP}({\overline{K}}^0{K}^{+})=0.10\pm 0.26\pm 0.03$. Using a vertexing technique previously employed in several analyses of all-neutral final states containing kaons, we report the first measurement of time-dependent $CP$-violating asymmetries in $B^0\to K_S^0{K}_S^0$, obtaining $S=-{1.28}_{-0.73-0.16}^{+0.80+0.11}$ and $C=-0.40\pm 0.41\pm 0.06$. We also report improved measurements of the branching fraction $\mathcal{B}(B^{+}\to K^0{\pi }^{+})=(23.9\pm 1.1\pm 1.0)\times {10}^{-6}$ and $CP$-violating charge asymmetry ${\mathcal{A}}_{CP}(K^0{\pi }^{+})=-0.029\pm 0.039\pm 0.010$.