Measurements of |Vcb|, form factors and branching fractions in the decays B0 → D∗+ℓ−νℓ and B0 → D+ℓ−νℓ

Abstract Two samples of exclusive semileptonic decays, 579 B 0 → D ∗+ l − ν l events and 261 B 0 → D + l − ν l 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 | V cb | are measured to be F D ∗+ (1)|V cb | = (31.9 ± 1.8 stat ± 1.9 syst ) × 10 −3 , F D + (1)| V cb | = (27.8 ± 6.8 stat ± 6.5 syst ) × 10 −3 . The ratio of the form factors F D + (1) and F D ∗+ (1) is measured to be F D + (1) F D ∗+ (1) = 0.87 ± 0.22 stat ± 0.21 syst . A value of | V cb | 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 | V cb | = (34.4 ± 1.6 stat ± 2.3 syst ± 1.4 th ) × 10 −3 . The branching fractions are measured from the two integrated spectra to be Br ( B 0 → D ∗+ l − ν l ) = (5.53 ± 0.26 stat ±0.52 syst ) %, Br ( B 0 → D ∗+ l − ν l ) = (2.35 ± 0.20 stat ± 0.44 syst ) %.