Determination of the B-bar→D*lν-bar decay width and vertical bar Vcb vertical bar

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 vertical bar Vcb vertical bar using a sample of 3x106 BB-bar events in the CLEO detector at the Cornell Electron Storage Ring. We determine the yield of reconstructed B-bar0→D*+lν-bar and B-→D*0lν-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 dΓ/dw. By extrapolating dΓ/dw to w=1, the kinematic end point at which the D* is at rest relative to the B, we extract the product vertical bar Vcb vertical bar F(1), where F(1) is the form factor at w=1. We find vertical bar Vcb vertical bar F(1)=0.0431±0.0013(stat)±0.0018(syst). We combine vertical bar Vcb vertical bar F(1) with theoretical results for F(1) to determine vertical bar Vcb vertical bar=0.0469±0.0014(stat)±0.0020(syst)±0.0018(theor). We also integrate the differential decay rate over w to obtain B(B-bar0→D*+lν-bar)=(6.09±0.19±0.40)% and B(B-→D*0lν-bar)=(6.50±0.20±0.43)%