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

The isospin asymmetries of $B \to K^{(*)}μ^+μ^-$ decays and the partial branching fractions of $B^0 \to K^0μ^+μ^-$ and $B^+ \to K^{*+}μ^+μ^-$ 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 $B \to Kμ^+μ^-$ isospin asymmetry integrated over $q^2$ is negative, deviating from zero with over 4 $σ$ significance. The $B \to K^{*}μ^+μ^-$ decay measurements are consistent with the Standard Model prediction of negligible isospin asymmetry. The observation of the decay $B^0 \to K^0_{\rm\scriptscriptstyle S}μ^+μ^-$ is reported with 5.7 $σ$ significance. Assuming that the branching fraction of $B^0 \to K^0μ^+μ^-$ is twice that of $B^0 \to K^0_{\rm\scriptscriptstyle S}μ^+μ^-$, the branching fractions of $B^0 \to K^0μ^+μ^-$ and $B \to K^{*+}μ^+μ^-$ are found to be ($0.31^{+0.07}_{-0.06}) \times 10^{-6}$ and ($1.16\pm0.19) \times 10^{-6}$, respectively.