Measurement of the average b-baryon lifetime and the product branching ratio

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 aΛ baryon are identified through the correlation of the baryon number of theΛ and the electric charge of the lepton. To measure the lifetime, the decay point of the b baryon is estimated by theΛ-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$$\begin{gathered} \hfill f(b \to \Lambda _b ) \cdot BR(\Lambda _b \to \Lambda \ell ^ - \bar vX) = (2.91 \pm 0.23(stat.) \\ \hfill \pm 0.25(stat.)) \cdot 10^{ - 3} \\ \end{gathered} $$, where the symbolf(b→Λb) is the fraction of b quarks from Z0 decays forming b baryons,Λb represents all b-flavoured baryons and ℓ is either an electron or a muon.