Measurement of the average b-baryon lifetime and the product branching ratio {Mathematical expression}

The average lifetime of b-flavoured baryons measured with the OPAL detector is updated to include data collected between 1990 and 1994 at LEP. Bottom-flavoured baryons that decay semileptonically and produce a Λ baryon are identified through the correlation of the baryon number of the Λ and the electric charge of the lepton. To measure the lifetime, the decay point of the b baryon is estimated by the Λ-lepton vertex, and the observed distribution of decay lengths is fitted simultaneously in 874 right-sign and 384 wrong-sign combinations. In a separate method, the impact parameter distribution of the leptons is fitted in a subset of these data. When the two results are combined, taking correlations into account, the average b-baryon lifetime is found to be τ=1.16±0.11(stat.)±0.06(syst.) ps. Using the same data, the product branching ratio is measured to be {Mathematical expression}, where the symbol f(b→Λ _b) is the fraction of b quarks from Z⁰ decays forming b baryons, Λ _b represents all b-flavoured baryons and ℓ is either an electron or a muon.