Measurement of the isospin asymmetry in $B \to K^{(*)}\mu^+\mu^-$ decays

TL;DR

This paper measures isospin asymmetries and branching fractions in B meson decays to K and K* muon pairs, revealing a significant deviation from the Standard Model in one decay mode and confirming predictions in another.

Contribution

It provides the first measurement of isospin asymmetry in B to K muon decays and reports the observation of the B0 to K0S muon decay with high significance.

Findings

B to K muon asymmetry is negative with over 4 sigma significance.

B to K* muon asymmetry is consistent with Standard Model.

First observation of B0 to K0S muon decay at 5.7 sigma.

Abstract

The isospin asymmetries of $B \to K^{(*)}\mu^+\mu^-$ decays and the partial branching fractions of $B^0 \to K^0\mu^+\mu^-$ and $B^+ \to K^{*+}\mu^+\mu^-$ are measured as a function of the di-muon mass squared $q^2$ using an integrated luminosity of 1.0 fb$^{-1}$ collected with the LHCb detector. The $B \to K\mu^+\mu^-$ isospin asymmetry integrated over $q^2$ is negative, deviating from zero with over 4 $\sigma$ significance. The $B \to K^{*}\mu^+\mu^-$ decay measurements are consistent with the Standard Model prediction of negligible isospin asymmetry. The observation of the decay $B^0 \to K^0_{\rm\scriptscriptstyle S}\mu^+\mu^-$ is reported with 5.7 $\sigma$ significance. Assuming that the branching fraction of $B^0 \to K^0\mu^+\mu^-$ is twice that of $B^0 \to K^0_{\rm\scriptscriptstyle S}\mu^+\mu^-$, the branching fractions of $B^0 \to K^0\mu^+\mu^-$ and $B \to K^{*+}\mu^+\mu^-$ are found to be ($0.31^{+0.07}_{-0.06}) \times 10^{-6}$ and ($1.16\pm0.19) \times 10^{-6}$, respectively.