Evidence for the Rare Decay B -> K*ll and Measurement of the B -> Kll Branching Fraction

Abstract

We present evidence for the flavor-changing neutral current decay $B\to K^{\ast }{\ell }^{+}{\ell }^{-}$ 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 ${\mu }^{+}{\mu }^{-}$ 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 $123\times 10^6$ ${\rm Y}(4S)\to B\bar{B}$ decays collected with the Babar detector at the PEP-II $e^{+}{e}^{-}$ storage ring. Averaging over $K^{(\ast )}$ 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 $\mathcal{B}(B\to K^{\ast }{\ell }^{+}{\ell }^{-})=(0.88_{-0.29}^{+0.33}\pm 0.10)\times 10^{-6}$, where the uncertainties are statistical and systematic, respectively. The significance of the $B\to K{\ell }^{+}{\ell }^{-}$ signal is over $8\sigma$, while for $B\to K^{\ast }{\ell }^{+}{\ell }^{-}$ it is $3.3\sigma$.

7 pages, 2 postscript figues, submitted to Phys. Rev. Lett