Measurement of |Vcb| at LEP

Semileptonic decays B0->D* l nu were selected from the samples of hadronic Z0 decays collected by the ALEPH, DELPHI and OPAL detectors at LEP. The differential decay width of this process was measured as a function of the product w of the B and D* 4-velocities, using exclusive (ALEPH, DELPHI, OPAL) or inclusive (DELPHI) methods of D* reconstruction. The distributions were fitted over the full w range to extract the product of |Vcb| times the normalization of the decay form factor F(1). The results of the various analyses were corrected to take into account the most recent values for the external imputs Rb, Br(b->B0), TauB0 and Br(D->K- pi+) and then averaged. The value of |Vcb| was computed using theoretical calculations of F(1), giving: |Vcb|excusive=(37.8±2.4(exp)±1.3(th))X10^-3. A second |Vcb| determination was obtained in the Heavy Quark Expansion framework from the measurements of the inclusive b quark semileptonic branching ratio and lifetime at LEP: |Vcb|inclusve=(42.0±0.5(exp)±4.1(th))X10^-3