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 RK=1.08−0.09+0.11stat−0.04+0.04syst, 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 of its kind at a hadron collider.