Research Papers in Physics and Astronomy


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Published in PHYSICAL REVIEW LETTERS 99, 052001 (2007). © 2007 The American Physical Society. Used by permission.


We report the first direct observation of the strange b baryon Ξb-( Ξb+). We reconstruct the decay Ξb-J/ψΞ-, withJ/ψ → μ+ μ-, and Ξ- → Λ π -p π-+ π+- in pp̅ collisions at √s = 1.96 TeV. Using 1.3 fb-1 of data collected by the D0 detector, we observe 15.2 ± 4.4(stat) -0.4+1.9(syst) Ξb- candidates at a mass of 5.774 ± 0.:011(stat) ± 0.015(syst) GeV. The significance of the observed signal is 5.5 σ, equivalent to a probability of 3.3 x 10-8 of it arising from a background fluctuation. Normalizing to the decay ΛbJ/ψΛ, we measure the relative rate [σ(Ξb-)x ℬ(Ξb-J/ψΞ-)]/[σ (Λb) x ℬ(ΛbJ/ψ Λ)] = 0.28 ± 0.09 (stat) -0.08+0.09 (syst).

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