Observation of the Decay $B \to K μ^{+} μ^{-}$

We report a search for the flavor-changing neutral current decay $B \to K^{(*)} \ell^{+} \ell^{-}$ using a 29.1 fb${}^{-1}$ data sample accumulated at the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB $e^{+}e^{-}$ storage ring. The decay process $B\to K\mu^+\mu^-$ is observed, for the first time, with a branching fraction of ${\cal B}(B \to K \mu^{+} \mu^{-}) = (0.99^{+0.40}_{-0.32}{}^{+0.13}_{-0.14}) \times 10^{-6}$. In addition, we set 90% confidence level upper limits ${\cal B}(B \to K e^{+} e^{-}) < 1.3 \times 10^{-6}$, ${\cal B}(B \to K^* e^{+} e^{-}) < 5.6 \times 10^{-6}$ and ${\cal B}(B \to K^* \mu^{+} \mu^{-}) < 3.1 \times 10^{-6}$.