Direct Observation of the Strange b Baryon Ξb-

We report the first direct observation of the strange b baryon Ξ_b^-(Ξ¯_b⁺). We reconstruct the decay Ξ_b^-→J/ψΞ^-, with J/ψ→μ⁺μ^-, and Ξ^-→Λπ^-→pπ^-π^- in pp¯ collisions at s=1.96TeV. Using 1.3fb^-1 of data collected by the D0 detector, we observe 15.2±4.4(stat)_-0.4^+1.9(syst) Ξ_b^- candidates at a mass of 5.774±0.011(stat)±0.015(syst)GeV. The significance of the observed signal is 5.5σ, equivalent to a probability of 3.3×10^-8 of it arising from a background fluctuation. Normalizing to the decay Λ_b→J/ψΛ, we measure the relative rate (σ(Ξ_b^-)×B(Ξ_b^-→J/ψΞ^-))/(σ(Λ_b)×B(Λ_b→J/ψΛ))=0.28±0.09(stat)_-0.08^+0.09(syst).