Direct Observation of the Strange $b$ Baryon $Ξ_{b}^{-}$

Open Access

We report the first direct observation of the strange

b

baryon

Ξ_{b}^{-} ({\bar{Ξ}}_{b}^{+})

. We reconstruct the decay

Ξ_{b}^{-} \to J / ψ Ξ^{-}

, with

J / ψ \to μ^{+} μ^{-}

, and

Ξ^{-} \to Λ π^{-} \to p π^{-} π^{-}

p \bar{p}

collisions at

\sqrt{s} = 1.96 TeV

. Using

1.3 {fb}^{- 1}

of data collected by the D0 detector, we observe

15.2 \pm 4.4 (stat)_{- 0.4}^{+ 1.9} (syst)

Ξ_{b}^{-}

candidates at a mass of

5.774 \pm 0.011 (stat) \pm 0.015 (syst) GeV

. The significance of the observed signal is

5.5 σ

, equivalent to a probability of

3.3 \times 10^{- 8}

of it arising from a background fluctuation. Normalizing to the decay

Λ_{b} \to J / ψ Λ

, we measure the relative rate

\frac{σ (Ξ_{b}^{-}) \times B (Ξ_{b}^{-} \to J / ψ Ξ^{-})}{σ (Λ_{b}) \times B (Λ_{b} \to J / ψ Λ)} = 0.28 \pm 0.09 (stat)_{- 0.08}^{+ 0.09} (syst)

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Direct Observation of the StrangebBaryonΞb−