B→Kl+l−and Other RareB-Meson Decays

We calculate the rate and the $\mathrm{CP}$-nonconservation asymmetry for the rare decay $B\to K{l}^{+}{l}^{-}$ ($l=e or \mu$). In the standard model with three generations we find the branching ratio to be of the order of ${10}^{-6\mathrm{}}$, and the $\mathrm{CP}$-nonconservation asymmetry at most about 1%. The decay proceeds via the flavor-changing loop structure of the standard model, and unlike $K\to \pi e^{+}{e}^{-}$ suffers few uncertainties and can therefore be used as a probe of new physics. We also comment on other rare decay modes of the $B$ meson.