Test of lepton universality with ${\Lambda}_b^0\to {pK}^{-}{\mathrm{\ell}}^{+}{\mathrm{\ell}}^{-}$ decays

Abstract

The ratio of branching fractions of the decays ${\Lambda}_b^0$ → pK⁻e⁺e⁻ and ${\Lambda}_b^0$ → pK⁻μ⁺μ⁻, ${R}_{pK}^{-1}$, is measured for the first time using proton-proton collision data corresponding to an integrated luminosity of 4.7 fb⁻¹ recorded with the LHCb experiment at center-of-mass energies of 7, 8 and 13 TeV. In the dilepton mass-squared range 0.1 < q² < 6.0 GeV²/c⁴ and the pK⁻ mass range m(pK⁻) < 2600 MeV/c², the ratio of branching fractions is measured to be ${R}_{pK}^{-1}={1.17}_{-0.16}^{+0.18}\pm 0.07$, where the first uncertainty is statistical and the second systematic. This is the first test of lepton universality with b baryons and the first observation of the decay ${\Lambda}_b^0$ → pK⁻e⁺e⁻.