Evidence for the Electroweak Penguin Decay $B\to X_s{\ell }^{+}{\ell }^{-}$}

Abstract

We report a search for the flavor-changing neutral current decay $B\to X_s{\ell }^{+}{\ell }^{-}$ using a 29.5 fb${}^{-1}$ data sample accumulated at the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ storage ring. We observe evidence for $B\to K{\mu }^{+}{\mu }^{-}$ and report the preliminary branching fraction of ${\cal B}(B \to K μ^{+} μ^{-}) = (0.99^{+0.39}_{-0.32}^{+0.13}_{-0.15}) \times 10^{-6}$. We also set the following 90% confidence level upper limits on the branching fractions for exclusive and inclusive decays, $\mathcal{B}(B\to K{e}^{+}{e}^{-})<1.2\times 10^{-6}$, $\mathcal{B}(B\to K^{\ast }(892)e^{+}{e}^{-})<5.1\times 10^{-6}$, $\mathcal{B}(B\to K^{\ast }(892){\mu }^{+}{\mu }^{-})<3.0\times 10^{-6}$, $\mathcal{B}(B\to X_s{e}^{+}{e}^{-})<10.1\times 10^{-6}$, $\mathcal{B}(B\to X_s{\mu }^{+}{\mu }^{-})<19.1\times 10^{-6}$.

Paper submitted to the 2001 Lepton Photon Conference