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

TL;DR

This paper reports the first observation of the rare decay process $B o K \, \ell^{+} \ell^{-}$, measuring its branching fraction using data from the Belle detector, confirming a flavor-changing neutral current decay.

Contribution

First observation of the decay $B \to K \ell^{+} \ell^{-}$ with a measured branching fraction, using data from the Belle experiment.

Findings

Branching fraction measured as (0.75^{+0.25}_{-0.21} ± 0.09) × 10^{-6}

Decay observed for the first time in this channel

Supports the existence of flavor-changing neutral current processes

Abstract

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