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

Abstract

A test of lepton universality between muons and electrons is performed using $B^{+}\to K^{+}{\ell }^{+}{\ell }^{-}$ decays (where $\ell$ = $e$, $\hat{I}¼$), in the dilepton invariant-mass-squared region above 14.3 GeV${}^2/c^4$. 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 $B^{+}\to K^{+}\hat{I}{¼}^{+}\hat{I}{¼}^{-}$ 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 $B^{+}\to K^{+}{\ell }^{+}{\ell }^{-}$ decays with dilepton invariant-mass-squared above the $\ddot{I}ˆ(2S)$ mass, whilst being the first of its kind at a hadron collider.

All figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://lbfence.cern.ch/alcm/public/analysis/full-details/3164/ (LHCb public pages)