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⁺→ K⁺ℓ⁺ℓ⁻ decays (where ℓ = e, µ), in the dilepton invariant-mass-squared region above 14.3 GeV²/c⁴. The data used for the measurement consists of beauty meson decays produced in proton-proton collisions, corresponding to an integrated luminosity of 9 fb⁻¹, collected by the LHCb experiment between 2011 and 2018. The ratio of branching fractions for B⁺→ K⁺µ⁺µ⁻ and B+→ K⁺e⁺e⁻ decays is measured to be R_K = 1.08^+0.11_−0.09 (stat)^+0.04_−0.04 (syst), which is consistent with the Standard Model prediction of unity. This constitutes the most precise test of lepton flavour universality using B⁺→ K⁺ℓ⁺ℓ⁻ decays with dilepton invariant-mass-squared above the ψ(2S) mass, whilst being the first