| Abstract |
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We present evidence for the $b\rightarrow d$ penguin-dominated decays
$B^+\rightarrow \bar{K^0} K^+$ and $B^0\rightarrow K^0 \bar{K^0}$ with
significances of $3.5$ and $4.5$ standard deviations, respectively. The
results are based on a sample of $227$ million $\Y(4S)\rightarrow B\bar{B}$
decays collected with the BaBar detector at the PEP-II asymmetric-energy
$e^+e^-$ collider at SLAC. We measure the branching fractions
${\cal B}(B^+\rightarrow \bar{K^0}K^+) = (1.5\pm 0.5\pm 0.1)\times 10^{-6}\,(< 2.4\times 10^{-6})$
and ${\cal B}(B^0\rightarrow K^0\bar{K^0}) = (1.19^{+0.40}_{-0.35}\pm 0.13)\times 10^{-6}$, where
the uncertainties are statistical and systematic, respectively, and the upper
limit on the branching fraction for $\Kzb\Kp$ is at the $90\%$ confidence
level. We also present improved measurements of the charge-averaged
branching fraction ${\cal B}(B^+\rightarrow K^0 \pi^+) = (26.0\pm 1.3\pm 1.0)\times 10^{-6}$
and $CP$-violating charge asymmetry
${\cal A}_{\CP}\,(K^0\pi^+) = -0.09\pm 0.05\pm 0.01$, where the uncertainties
are statistical and systematic, respectively.
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