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

Abstract.

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 $\Lambda\ell^-$ combinations, the average b baryon lifetime is measured to be $1.20\pm{0.08}\pm{0.06}$ ps. From a sample of 193 fully reconstructed $\Lambda^+_{\rm c}$ candidates correlated with a lepton and a sample of 46 $\Lambda\ell^+\ell^-$ combinations, the $\Lambda_{\rm b}$ lifetime is measured to be $1.21\pm{0.11}$ ps. The product branching fractions to these final states are ${\rm Br(b\to \Lambda_{\rm b}}\cdot{\rm BR (\Lambda_{\rm b} \to \Lambda\ell^-\overline\nu X}) = 0.326\pm{0.016}\pm{0.039}$% for the first sample and ${\rm Br(b\to \Lambda_{\rm b}}\cdot{\rm BR(\Lambda_{ \rm b} \to \Lambda^+_{\rm c}\ell^-\overline\nu X}) =0.86\pm{0.07}\pm{0.14}$% for the second and third samples combined.