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

We report a search for the flavor-changing neutral current decay $B\to K^{(*)}{l}^{+}{l}^{-}$ using a $29.1{\mathrm{fb}}^{-1\mathrm{}}$ data sample accumulated at the $\Upsilon \left(4S\right)$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ storage ring. We observe the decay process $B\to {\mathrm{Kl}}^{+}{l}^{-}(l=e,\mu )$, for the first time, with a branching fraction of $B(B\to {\mathrm{Kl}}^{+}{l}^{-})=({0.75}_{-0.21\mathrm{}}^{+0.25\mathrm{}}\pm 0.09)\times {10}^{-6\mathrm{}}$.