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

Using approximately 4 million hadronic Z 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 Lambda l(-) combinations, the average b baryon lifetime is measured to be 1.20 +/- 0.08 +/- 0.06 ps. From a sample of 193 fully reconstructed Lambda(c)(+) candidates correlated with a lepton and a sample of 46 Lambda l(+)l(-) combinations the Lambda(b) lifetime is measured to be 1.21+/-0.11 ps. The product branching fractions to these final states are Br(b-->Lambda(b)) . Br(Lambda(b) --> Lambda l(-)(v) over bar X) = 0.326 +/- 0.016 +/- 0.039% for the first sample and Br(b --> Lambda(b)) . Br(Lambda(b) --> Lambda(c)(+)l(-)(v) over bar X) = 0.86 +/- 0.07 +/- 0.14% for the second and third samples combined.