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

A bstract The isospin asymmetries of B → K (∗) μ + μ − decays and the partial branching fractions of B 0 → K 0 μ + μ − and B + → 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 → Kμ + μ − isospin asymmetry integrated over q 2 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 B 0 → K S μ + μ − is reported with 5.7 σ significance. Assuming that the branching fraction of B 0 → K 0 μ + μ − is twice that of B 0 → K S μ + μ − , the branching fractions of B 0 → K 0 μ + μ − and B → K ∗+ μ + μ − are found to be (0 . 31 −0.06 ) × 10 −6 and (1 . 16 ± 0 . 19) × 10 −6 , respectively.