Observation of $B^{+}\to {\overline{K}}^0{K}^{+}$ and $B^0\to K^0{\overline{K}}^0$

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 ${\mathcal{A}}_{CP}(K^0{\pi }^{+})=-0.029\pm 0.039\pm 0.010$.

Figure 1

Distributions of (left) $m_{\mathrm{ES}}$ and (right) $\Delta E$ for signal (main plot) and background (inset) (a),(b) $K_S^0{\pi }^{+}$, (c),(d) $K_S^0{K}^{+}$, and (e),(f) $K_S^0{K}_S^0$ candidates (points with error bars) in data obtained with the weighting technique described in the text. The solid curves represent the assumed shapes used in the fits.

Figure 2

Left: Distributions of $\Delta t$ for $B^0\to K_S^0{K}_S^0$ decays in data tagged as $B^0$ (top) or ${\overline{B}}^0$(middle) and the asymmetry (bottom). The data are enhanced in signal decays using requirements on probability ratios. The solid curve represents the PDF projection for the sum of signal and background, while the dotted curve shows the contribution from background only. Right: Likelihood contours in the $S$ vs $C$ plane, where $n\sigma$ corresponds to a change in $-2\log \mathcal{L}$ of 2.3 for $n=1$, 6.2 for $n=2$, and 11.8 for $n=3$. The circle indicates the physically allowed region, while the point with error bars denotes the result of the fit to the data.