Determination of the average lifetime of b-baryons

The average lifetime of b-baryons has been studied using 3 \times 10^6 hadronic Z^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 a \Lambda, a lepton and an oppositely charged pion; the impact parameter distribution of 441 muons with high transverse momentum accompanied by a \mLs in the same jet; and the proper decay time distribution of 125 \Lambda_c-lepton decay vertices with the \Lambda_c exclusively reconstructed through its pK\pi, pK^0 and \mLs3\pi decay modes. The combined result is~:\par \begin{center} \tau(b-baryon) = (1.25^{+0.13}_{-0.11}\ pm0.04(syst)^{+0.03}_{-0. 05}(syst)) ps\par \end{center} where the first systematic error is due to experimental uncertainties and the second to the uncertainties in the modelling of the b-baryon production and semi-leptonic decay. Including the measurement recently published by DELPHI based on a sample of proton-muon vertices, the average b-baryon lifetime is ~:\par \begin{center} \tau(b-baryon) = (1.25\pm0.11(stat)\pm0.05(syst)) ps.\par \end{center}