Measurement of the average lifetime of b-hadrons in Z decays

We present a measurement of the average b-hadron lifetime τb at the e+e−collider LEP. Using hadronic Z decays collected in the period from 1991 to 1994, two independent analyses have been performed. In the first one, the b-decay position is reconstructed as a secondary vertex of hadronic b-decay particles. The second analysis is an updated measurement of τb using the impact parameter of leptons with high momentum and high transverse momentum. The combined result isτb=[1549±9(stat)±15(syst)]fs.In addition, we measure the average charged b-decay multiplicity 〈nb〉 and the normalized average b-energy 〈xE〉b at LEP to be〈nb〉=4.90±0.04(stat)±0.11(syst),〈xE〉b=0.709±0.004(stat+syst).