Measurements of vertical bar V-cb vertical bar form factors and branching fractions in the decays (B)over-bar(0)->D(*+)l(-)(nu)over-bar(l) and (B)over-bar(0)->D(+)l(-)(nu)over-bar(l)

Two samples of exclusive semileptonic decays, 579 (B) over bar(0) --> D*(+)l(-)(v) over bar(l) events and 261 (B) over bar(0) --> D(+)l(-)(v) over bar(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(omega) at zero recoil of the D-(*+) meson and the CKM matrix element V-cb are measured to be FD*+(1)V-cb = (31.9 +/- 1.8(stat) +/- 1.9(syst)) x 10(-3), FD+(1)(V-cb = (27.8 +/- 6.8(stat) +/- 6.5(syst)) x 10(-3). The ratio of the form factors FD+(1) and FD*+(1) is measured to be FD+(1)/FD*+(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)) x 10(-3). The branching fractions are measured from the two integrated spectra to be Br((B) over bar(0) --> D(*+)l(-)(v) over bar(l)) = (5.53 +/- 0.26(stat) +/- 0.52(syst))%, Br((B) over bar(0) --> D(+)l(-)(v) over bar(l)) = (2.35 +/- 0.20(stat) +/- 0.44(syst))%. (C) 1997 Elsevier Science B.V.