Test of lepton universality with Λb0→pK−ℓ+ℓ− decays

Abstract

The ratio of branching fractions of the decays Λb0$$ {\Lambda}b^0 $$ → pK−e+e− and Λb0$$ {\Lambda}b^0 $$ → pK−μ+μ−,RpK−1$$ {R}{pK}^{-1} $$, 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 RpK−1=1.17−0.16+0.18±0.07$$ {R}{pK}^{-1}={1.17}_{-0.16}^{+0.18}\pm 0.07 $$, 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 Λb0$$ {\Lambda}_b^0 $$ → pK−e+e−.