Determination of the B̄→D*ℓν̄ decay width and |V cb|

In the Standard Model, the charged current of the weak interaction is governed by a unitary quark mixing matrix that also leads to CP violation. Measurement of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements is essential to searches for new physics, either through the structure of the CKM matrix, or a departure from unitarity. We determine the CKM matrix element |Vcb| using a sample of 3 x 10^6 B B-bar events in the CLEO detector at the Cornell Electron Storage Ring. We determine the yield of reconstructed B0-bar --> D*+ l nu-bar and B- --> D*0 l nu-bar decays as a function of w, the boost of the D* in the B rest frame, and from this we obtain the differential decay rate dGamma/dw. By extrapolating dGamma/dw to w=1, the kinematic end point at which the D* is at rest relative to the B, we extract the product |Vcb|F(1), where F(1) is the form factor at w=1. We find |Vcb| = 0.0431 +- 0.0013 (stat) +- 0.0018 (syst). We combined |Vcb|F(1) with theoretical results for F(1) to determine |Vcb| = 0.0469 +- 0.0014(stat) +- 0.0020(syst) +- 0.0018(theo). We also integrate the differential decay rate over w to obtain BF(B0-bar --> D*+ l nu-bar) = (6.09 +- 0.19 +- 0.40)% and BF(B- --> D*0 l nu-bar) = (6.50 +- 0.20 +- 0.43)%.