Test of lepton flavour universality with $B^{+}\to K^{+}{\pi }^{+}{\pi }^{-}{\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ decays

The first test of lepton flavor universality between muons and electrons using $B^{+}\to K^{+}{\pi }^{+}{\pi }^{-}{\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ ($\mathrm{\ell }=e,\mu$) 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 $9{\mathrm{f}\mathrm{b}}^{-1}$. The ratio of branching fractions between $B^{+}\to K^{+}{\pi }^{+}{\pi }^{-}{e}^{+}{e}^{-}$ and $B^{+}\to K^{+}{\pi }^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}$decays is measured in the dilepton invariant-mass-squared range $1.1<q^2<7.0{\mathrm{G}\mathrm{e}\mathrm{V}}^2\mathrm{/}{c}^4$ and is found to be $R_{K\pi \pi }^{-1}=1.31_{-0.17}^{+0.18}(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}){}_{-0.09}^{+0.12}(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})$, in agreement with the standard model prediction. The first observation of the $B^{+}\to K^{+}{\pi }^{+}{\pi }^{-}{e}^{+}{e}^{-}$ decay is also reported.