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

TL;DR

This paper reports evidence for the rare decay processes B -> K*ll and B -> Kll, measuring their branching fractions using data from the BaBar detector, which provides insights into potential new physics beyond the Standard Model.

Contribution

First evidence for B -> K*ll decay and precise measurement of B -> Kll branching fraction using a large data sample from BaBar, testing Standard Model predictions.

Findings

Branching fraction for B -> Kll: (0.65^{+0.14}_{-0.13}±0.04)×10^{-6}

Branching fraction for B -> K*ll: (0.88^{+0.33}_{-0.29}±0.10)×10^{-6}

Significance over 8σ for B -> Kll, 3.3σ for B -> K*ll

Abstract

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 $\mu^+\mu^-$ 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 $123\times 10^6$ $\Upsilon(4S)\to B\bar{B}$ decays collected with the Babar detector at the PEP-II $e^+e^-$ storage ring. Averaging over $K^{(*)}$ isospin and lepton flavor, we obtain the branching fractions ${\mathcal B}(B\to K\ell^+\ell^-)=(0.65^{+0.14}_{-0.13}\pm 0.04)\times 10^{-6}$ and ${\mathcal B}(B\to K^*\ell^+\ell^-)=(0.88^{+0.33}_{-0.29}\pm 0.10)\times 10^{-6}$, 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 $3.3\sigma$.