Test of lepton universality with B ⁰ → K ^*0 ℓ ⁺ ℓ ⁻ decays

A test of lepton universality, performed by measuring the ratio of the branching fractions of the B⁰ → K^*0μ⁺μ⁻ and B⁰ → K^*0e⁺e⁻ decays, RK*0, is presented. The K^*0 meson is reconstructed in the final state K⁺π⁻, which is required to have an invariant mass within 100 MeV/c² of the known K^*(892)⁰ mass. The analysis is performed using proton-proton collision data, corresponding to an integrated luminosity of about 3 fb⁻¹, collected by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. The ratio is measured in two regions of the dilepton invariant mass squared, q², to be (Formula presented.) The corresponding 95.4% confidence level intervals are [0.52, 0.89] and [0.53, 0.94]. The results, which represent the most precise measurements of RK*0 to date, are compatible with the Standard Model expectations at the level of 2.1–2.3 and 2.4–2.5 standard deviations in the two q² regions, respectively.