Measurement of the isospin asymmetry in B → K(*)μ+μ− decays

Abstract

The isospin asymmetries of B → K^(∗)μ⁺μ⁻ decays and the partial branching fractions of B⁰ → K⁰μ⁺μ⁻ and B⁺ → K^∗+μ⁺μ⁻ are measured as a function of the di-muon mass squared q² using an integrated luminosity of 1.0 fb⁻¹ collected with the LHCb detector. The B → Kμ⁺μ⁻ isospin asymmetry integrated over q² is negative, deviating from zero with over 4 σ significance. The B → K^∗μ⁺μ⁻ decay measurements are consistent with the Standard Model prediction of negligible isospin asymmetry. The observation of the decay B⁰ → K^Sμ⁺μ⁻ is reported with 5.7 σ significance. Assuming that the branching fraction of B⁰ → K⁰μ⁺μ⁻ is twice that of B⁰ → K^Sμ⁺μ⁻, the branching fractions of B⁰ → K⁰μ⁺μ⁻ and B → K^∗+μ⁺μ⁻ are found to be (0.31^−0.06) × 10⁻⁶ and (1.16 ± 0.19) × 10⁻⁶, respectively.