Measurement of the branching fraction ratio $R_K$ at large dilepton invariant mass

A test of lepton universality between muons and electrons is performed using $B^{+}\to K^{+}{\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ decays (where $\mathrm{\ell }$ = $e$, $\mu$), in the dilepton invariant-mass-squared region above 14.3 GeV${}^2\mathrm{/}{c}^4$. The data used for the measurement consists of beauty meson decays produced in proton-proton collisions, corresponding to an integrated luminosity of 9 ${\text{fb}}^{-1}$, collected by the LHCb experiment between 2011 and 2018. The ratio of branching fractions for $B^{+}\to K^{+}{\mu }^{+}{\mu }^{-}$ and $B^{+}\to K^{+}{e}^{+}{e}^{-}$ decays is measured to be $R_K=1.08_{-0.09}^{+0.11}(\text{stat}){}_{-0.04}^{+0.04}(\text{syst})$, which is consistent with the Standard Model prediction of unity. This constitutes the most precise test of lepton flavour universality using $B^{+}\to K^{+}{\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ decays with dilepton invariant-mass-squared above the $\psi (2S)$ mass, whilst being the first of its kind at a hadron collider.