Measurements of |Vcb| 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 |Vcb| are measured to be FD*+(1)|V-cb| = (31.9 +/- 1.8(stat) +/- 1.9(syst)) x 10(-3), FD+(1)|(Vcb| = (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 |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 +/- 1.6(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))%.