Measurements of |Vcb|, form factors and branching fractions in the decays B̄0 → D*+ℓ-v̄ℓ and B̄0 → D+ℓ-v̄ℓ

wo samples of exclusive semileptonic decays, 579 B̄0 → D*+ℓ-v̄ℓ events and 261 B̄0 → D+ℓ-v̄ℓ 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(ω) 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) × 10-3, ℱD+(1)|Vcb| = (27.8 ± 6.8stat ± 6.5syst) × 10-3. The ratio of the form factors ℱD+(1) and ℱD*+ (1) is measured to be ℱD+(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) × 10-3. The branching fractions are measured from the two integrated spectra to be Br(B̄0 → D*+ℓ-v̄ℓ) = (5.53 ± 0.26stat ± 0.52syst)%,Br(B̄0 → D+ℓ-v̄ℓ) = (2.35 ± 0.20stat ± 0.445syst)%.