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

Abstract

A test of lepton universality, performed by measuring the ratio of the branching fractions of the B ⁰ → K ^*0 μ ⁺ μ ⁻ and B ⁰ → K ^*0 e ⁺ e ⁻ decays, ${R}_{K^{*0}}$, 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/c ² 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, q ², to be

$$ {R}_{K^{*0}}=\left\{\begin{array}{l}{0.66_{-}^{+}}_{0.07}^{0.11}\left(\mathrm{stat}\right)\pm 0.03\left(\mathrm{syst}\right)\kern1em \mathrm{f}\mathrm{o}\mathrm{r}\kern1em 0.045<{q}^2<1.1\kern0.5em {\mathrm{GeV}}^2/{c}^4,\hfill \\ {}{0.69_{-}^{+}}_{0.07}^{0.11}\left(\mathrm{stat}\right)\pm 0.05\left(\mathrm{syst}\right)\kern1em \mathrm{f}\mathrm{o}\mathrm{r}\kern1em 1.1<{q}^2<6.0\kern0.5em {\mathrm{GeV}}^2/{c}^4.\hfill \end{array}\right. $$

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 ${R}_{K^{*0}}$ 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 q ² regions, respectively.