We present evidence for the flavor-changing neutral current decay $B\to K^{*}{\ell }^{+}{\ell }^{-}$ and a measurement of the branching fraction for the related process $B\to K{\ell }^{+}{\ell }^{-}$, where ${\ell }^{+}{\ell }^{-}$ is either an $e^{+}{e}^{-}$ or a ${\mu }^{+}{\mu }^{-}$ pair. These decays are highly suppressed in the standard model, and they are sensitive to contributions from new particles in the intermediate state. The data sample comprises $123\times {10}^6$ ${\rm Y}(4S)\to B\overline{B}$ decays collected with the BABAR detector at the SLAC PEP-II $e^{+}{e}^{-}$ storage ring. Averaging over $K^{(*)}$ isospin and lepton flavor, we obtain the branching fractions $\mathcal{B}(B\to K{\ell }^{+}{\ell }^{-})=({0.65}_{-0.13}^{+0.14}\pm 0.04)\times {10}^{-6}$ and $\mathcal{B}(B\to K^{*}{\ell }^{+}{\ell }^{-})=({0.88}_{-0.29}^{+0.33}\pm 0.10)\times {10}^{-6}$, where the uncertainties are statistical and systematic, respectively. The significance of the $B\to K{\ell }^{+}{\ell }^{-}$ signal is over $8\sigma$, while for $B\to K^{*}{\ell }^{+}{\ell }^{-}$ it is $3.3\sigma$.

• Received 15 August 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.221802