Test of Lepton Flavor Universality with B⁺ →k⁺π⁺π^-ℓ⁺ℓ^- Decays

The first test of lepton flavor universality between muons and electrons using B⁺→K⁺π⁺π^-ℓ⁺ℓ^- (ℓ=e, μ) decays is presented. The measurement is performed with data from proton-proton collisions collected by the LHCb experiment at center-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9 fb^-1. The ratio of branching fractions between B⁺→K⁺π⁺π^-e⁺e^- and B⁺→K⁺π⁺π^-μ⁺μ^- decays is measured in the dilepton invariant-mass-squared range 1.1<q²<7.0 GeV²/c⁴ and is found to be R_Kππ^-1=1.31^-0.17_+0.18(stat) _-0.09^+0.12(syst), in agreement with the standard model prediction. The first observation of the B⁺→K⁺π⁺π^-e⁺e^- decay is also reported.