Determination of the average lifetime of b baryons

Abreu, P. [--] Cortina Gil, Eduardo [UCL] DELPHI Collaboration [--] et al.

The average lifetime of 6-baryons has been studied using 3 X 10^ hadronic Z° 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 A, a lepton and an oppositely charged pion; the impact parameter distribution of 441 muons with high transverse momentum accompanied by a /I in the same jet; and the proper decay time distribution of 125 ylc-lepton decay vertices with the Ac exclusively reconstructed through its pKir, pK° and /ISTT decay modes. The combined result is: T(6-baryon) = (1.254 l^o\llistat) ±0.04isyst)^oil(syst)) ps where the first systematic error is due to experimental uncertainties and the second to the uncertainties in the modelling of the 6-baryon production and semi-leptonic decay. Including the measurement recently published by DELPHI based on a sample of proton-muon vertices, the average 6-baryon lifetime is : T(b-baryon) = {1.255H^QYolistat) ± 0.05) ps.