Measuremént of the b baryon lifetime and branching fractions in Z decays

Using approximately 4 million hadronic Ζ decays recorded with the ALEPH detector from 1991 through 1995, the lifetime of the b baryon is measured with three independent methods. From the impact parameter distribution of candidate leptons in 1063 events with Λℓ^- combinations, the average b baryon lifetime is measured to be 1.20 ± 0.08 ± 0.06 ps. From a sample of 193 fully reconstructed Λ⁺ _c candidates correlated with a lepton and a sample of 46 Λℓ⁺ℓ^- combinations, the Λ_b lifetime is measured to be 1.21 ± 0.11 ps. The product branching fractions to these final states are Br(b → Λ_bb) · Br(Λ_b → Λℓ^-v̄X) = 0.326 ± 0.016 ± 0.039% for the first sample and Br(b → Λ_b) · Br(Λ_b → Λ⁺ _cℓ^-v̄X) = 0.86 ± 0.07 ± 0.14% for the second and third samples combined.