Direct observation of the strange b Baryon ≡ b^-

Abstract

We report the first direct observation of the strange b baryon ≡_b^-(≡ ^-_b⁺). We reconstruct the decay ≡ _b^-→J/ψ ≡ ^-,with J/ψ ≡ μ⁺μ^-, and ≡ →Λ π →pΛΛ in pp̅ 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)≡_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 3.3χ 10^-8 of it arising from a background fluctuation. Normalizing to the decay Λ_b→jiψΛ, we measure the relative rate σ(≡_b^-)×B(≡_b^-)× B(≡_b^-→ J/ψ≡=0.28±0.09(stat)_-0.08 ^+0.09(syst).