Measurements of |$V_{cb}$| and form factors and branching ratios of $B^{0} \to D^{*+}l^{-}\nu$ and $B^{0} \to D^{+}l^{-}\nu$

Two samples of exclusive semileptonic decays, 579 +- 30 B0->D*+ l- nu events and 266 +- 24 B0->D+ l- nu events, are selected from approximately 3.9 million hadronic Z decays collected by the ALEPH detector at LEP from 1991 to 1995. The reconstructed differential decay rate d Gamma/d omega of each sample is fitted, yielding a value of the product of the hadronic form factor F(omega=1) at zero recoil of the D(*)+ meson and the CKM matrix element |V_cb|, F_D*+ |V_cb| = (30.6 +- 1.8_stat +- 2.0_syst) 10^-3 and F_D+ |V_cb| = (28.8 +- 6.7_stat +- 6.9_sys) 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) = 0.94 +- 0.23_stat +- 0.23_syst. A value of |V_cb| is extracted from the simultaneous fit of the d Gamma/d omega distributions of the two samples, using theoretical constraints on the slope and curvature of the hadronic form factors and their calculated values at omega=1, with the result |V_cb| = (33.4 +- 2.0_stat +- 2.3_syst +- 1.3_th) 10^-3. The branching fractions Br(B0->D*+ l- nu) = (5.14 +- 0.24_stat +- 0.48_syst)and Br(B0->D+ l- nu) = (2.41 +- 0.21_stat +- 0.44_syst)are measured from the two integrated spectra.