Measurements of |Vcb|, form factors and branching fractions in the decays and

Two samples of exclusive semileptonic decays, 579 B0 -> D*+l-\u3bdl events and 261 B0 -> D+l-\u3bdl events, are selected from approximately 3.9 million hadronic Z decays collected by the ALEPH detector at LEP. From the reconstructed differential decay rate of each sample, the product of the hadronic form factor F(\u3c9) at zero recoil of the D(*)+ meson and the CKM matrix element Vcb are measured to be FD*+ (1) Vcb = (31.9 +/- 1.8stat +/- 1.9syst) 7 10-3,FD+ (1) Vcb = (27.8 +/- 6.8stat +/- 6.5syst) 7 10-3. The ratio of the form factorsFD+ (1) and FD*+ (1) is measured to beFD+ (1)/FD*+ (1) = 0.87 +/- 0.22stat +/- 0.21syst. A value of Vcb is extracted from the two samples, using theoretical constraints on the slope and curvature of the hadronic form factors and their normalization at zero recoil, with the result Vcb = (34.4 +/- 1.6stat +/- 2.3syst +/- 1.4th) 7 10-3. The branching fractions are measured from the two integrated spectra to be Br(B0 -> D*+l-\u3bdl) = (5.53 +/- 0.26stat +/- 0.52syst) %, Br(B0 -> D+l-\u3bdl) = (2.35 +/- 0.20stat +/- 0.44syst) %