A measurement of

From approximately 3 million hadronic decays of Z bosons recorded with the aleph detector at lep, a sample of 410 ± 32 B0 → D*+l-νl candidates is selected, where l is either an electron or a muon. The differential decay rate dΓ (B0 → D*+l-νl) dω from this sample is fitted, yielding a value for the product of the CKM matrix element |Vcb| and the normalisation of the decay form factor at the point of zero recoil of the D*+ meson F(ω = 1)|Vcb| = (31.4 ± 2.3stat ± 2.5syst) × 10-3. A value for |Vcb| is extracted using theoretical calculations of the form factor normalisation, with the result |Vcb| = (34.5 ± 2.5stat ± 2.7syst ± 1.5theory) × 10-3. From the integrated s the measured branching fraction is Br(B0 → D*+l-νl) = (5.18 ± 0.30stat ± 0.62syst)%. © 1995.