A measurement of the ratio of branching fractions of the decays $B^{+}\to K^{+}{\mu }^{+}{\mu }^{-}$ and $B^{+}\to K^{+}{e}^{+}{e}^{-}$ is presented. The proton-proton collision data used correspond to an integrated luminosity of $5.0{\mathrm{fb}}^{-1}$ recorded with the LHCb experiment at center-of-mass energies of 7, 8, and 13 TeV. For the dilepton mass-squared range $1.1<q^2<6.0{\mathrm{GeV}}^2/c^4$ the ratio of branching fractions is measured to be $R_K=0.84{6_{-0.054}^{+0.060}}_{-0.014}^{+0.016}$, where the first uncertainty is statistical and the second systematic. This is the most precise measurement of $R_K$ to date and is compatible with the standard model at the level of 2.5 standard deviations.

• Received 25 March 2019

DOI:https://doi.org/10.1103/PhysRevLett.122.191801

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© 2019 CERN, for the LHCb Collaboration