Test of lepton flavour universality with B+→K+π+π−ℓ+ℓ−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−B+→K+π+π−ℓ+ℓ− (ℓ=e,μℓ=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 9fb−19fb−1. The ratio of branching fractions between B+→K+π+π−e+e−B+→K+π+π−e+e− and B+→K+π+π−μ+μ−B+→K+π+π−μ+μ−decays is measured in the dilepton invariant-mass-squared range 1.1<q2<7.0 GeV2/c41.1<q2<7.0 GeV2/c4 and is found to be RKππ−1=1.31−0.17+0.18  (stat)  −0.09+0.12  (syst)RKππ−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−B+→K+π+π−e+e− decay is also reported.