Observation of $B\to K^{\ast }{\ell }^{+}{\ell }^{-}$

Abstract

We report the first observation of the flavor-changing neutral current decay $B\to K^{\ast }{\ell }^{+}{\ell }^{-}$ and an improved measurement of the decay $B\to K{\ell }^{+}{\ell }^{-}$, where $\ell$ represents an electron or a muon, with a data sample of 140 fb${}^{-1}$ accumulated at the ${\rm Y}(4S)$ resonance with the Belle detector at KEKB. The results for the branching fractions are $\mathcal{B}(B\to K^{\ast }{\ell }^{+}{\ell }^{-})=(11.5_{-2.4}^{+2.6}\pm 0.8\pm 0.2)\times 10^{-7}$ and $\mathcal{B}(B\to K{\ell }^{+}{\ell }^{-})=(4.8_{-0.9}^{+1.0}\pm 0.3\pm 0.1)\times 10^{-7}$, where the first error is statistical, the second is systematic and the third is from model dependence.

11 pages, 2 figures, submitted to PRL