Measurement of the Isospin Asymmetry in B → K(∗) μ+ μ− Decays

The isospin asymmetries of B → K(∗) μ+ μ− decays and the partial branching fractions of B0 → K0 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 → K μ+ μ− isospin asymmetry integrated over q^2 is negative, deviating from zero with over 4σ 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 → KS0 μ+ μ− is reported with 5.7σ significance. Assuming that the branching fraction of B0 → K0 μ+ μ− is twice that of B0 → KS0 μ+ μ−, the branching fractions of B0 → K0 μ+ μ− and B → K∗+ μ+ μ− are found to be (0.31 +0.07 -0.06) x 10^−6 and (1.16 ± 0.19) x 10^−6, respectively.