Determination of the average lifetime of b-baryons

The average lifetime ofb-baryons has been studied using 3×106 hadronicZ 0 decays collected by the DELPHI detector at LEP. Three methods have been used, based on the measurement of different observables: the proper decay time distribution of 206 vertices reconstructed with aΛ, a lepton and an oppositely charged pion; the impact parameter distribution of 441 muons with high transverse momentum accompanied by aΛ in the same jet; and the proper decay time distribution of 125Λ c-lepton decay vertices with theΛ c exclusively reconstructed through itspKπ,pK 0 andΛ3π decay modes. The combined result is: $$\begin{gathered} \tau (b - baryon) = (1.254_{ - 0.109}^{ + 0.121} (stat) \hfill \\ \pm 0.04(syst)_{ - 0.05}^{ + 0.03} (syst)) ps \hfill \\ \end{gathered} $$ where the first systematic error is due to experimental uncertainties and the second to the uncertainties in the modelling of theb-baryon production and semi-leptonic decay. Including the measurement recently published by DELPHI based on a sample of proton-muon vertices, the averageb-baryon lifetime is: $$\tau (b - baryon) = (1.255_{ - 0.102}^{ + 0.115} (stat) \pm 0.05) ps$$ .