Observation of ${\Lambda }_b^0$→J/ψ Λ at the Fermilab proton-antiproton collider

The decay ${\Lambda }_b^0$→J/ψ Λ is observed in 110 ${\mathrm{pb}}^{\mathrm{-}1}$ of pp¯ collisions taken at $\sqrt{s}=1.8\mathrm{}$ TeV. These data are used to measure a Λ${}_b^0$ mass of 5621±4(stat)±3(syst) MeV/${\mathit{c}}^2$, and a mass difference between the Λ${}_b^0$ and the ${\mathit{B}}^0$ of 340±5(stat)±1(syst) MeV/${\mathit{c}}^2$. The production cross-section times branching fraction for the decay ${\Lambda }_b^0$→J/ψ Λ relative to that for the decay $B^0$→J/ψ $K_S^0$ has been measured to be 0.27±0.12(stat)±0.05(syst).