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

The isospin asymmetries of $B\to K^{(\ast )}{\mu }^{+}{\mu }^{-}$ decays and the partial branching fractions of $B^0\to K^0{\mu }^{+}{\mu }^{-}$ and $B^{+}\to K^{\ast +}{\mu }^{+}{\mu }^{-}$ 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 $B \to K\mu^+\mu^-$isospinasymmetryintegratedover$q^2$ is negative, deviating from zero with over 4 $\sigma$ significance. The $B\to K^{\ast }{\mu }^{+}{\mu }^{-}$ decay measurements are consistent with the Standard Model prediction of negligible isospin asymmetry. The observation of the decay $B^0 \to K^0_{\rm\scriptscriptstyle S}\mu^+\mu^-$isreportedwith5.7$\sigma$significance\mathrm{.}Assumingthatthebranchingfractionof$B^0 \to K^0\mu^+\mu^-$istwicethatof$B^0 \to K^0_{\rm\scriptscriptstyle S}\mu^+\mu^-$, the branching fractions of $B^0\to K^0{\mu }^{+}{\mu }^{-}$ and $B\to K^{\ast +}{\mu }^{+}{\mu }^{-}$ are found to be ($0.31_{-0.06}^{+0.07})\times {10}^{-6}$ and ($1.16\pm0.19) \times 10^{-6}$, respectively.