Test of lepton universality with $Lambda^{0}_{b} to p K^- ell^+ ell^-$ decays

The ratio of branching fractions of the decays $Lambda^{0}_{b}to pK^{-}e^{+}e^{-}$ and $Lambda^{0}_{b}to pK^{-}mu^{+}mu^{-}$, $R^{-1}_{pK}$, 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 $0.1 < q^{2} < 6.0$ $GeV^{2}/c^{4}$ and the $pK^{-}$ mass range $m(pK^{-}) < 2600$ $MeV/c^{2}$, the ratio of branching fractions is measured to be $R^{-1}_{pK} = 1.17 ^{+0.18}_{-0.16} pm 0.07$, 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 $Lambda^{0}_{b}to pK^{-}e^{+}e^{-}$.