## Abstract

A test of lepton universality, performed by measuring the ratio of the branching fractions of the $B \ ^0 \to K \ ^{*0} \mu^+ \mu^-$ and $B \ ^0 \to K \ ^{*0} e^+ e^-$ 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

$R_{K^{∗0}} = \begin{cases} 0.66_{-0.07}^{+0.11}(stat) \pm0.03(syst) \ for \ 0.045<q^2<1.1GeV^2/c^4, \\ 0.69_{−0.07}^{+0.11}(stat) \pm 0.05(syst) \ for \ 1.1<q^2<6.0GeV^2/c^4. \end{cases}$

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.