Detalle de: Measurement of |V cb | using the semileptonic decay $\overline{\rm {B^{0}_{d}}} \rightarrow {\rm D}^* \ell^- \overline{\nu}_{\ell}$

Data from Z decays in DELPHI have been searched for anti-B0d -> D*+ l- anti-nu_l with the D*+ decaying to D0 pi+ and D0 -> K- pi+, K- pi+ pi+ pi- or K- pi+ (pi0). These events are used to measure the CKM matrix element |V_cb| and the form factor slope, rho_{A_1}^2: F_D*(1)|Vcb| = 0.0392 +/- 0.0018 +/- 0.0023; rho_{A_1}^2 = 1.32 +/- 0.15 +/- 0.33 corresponding to a branching fraction: BR(anti-B0d -> D*+ l- anti-nu_l) = (5.90 +/- 0.22 +/- 0.50)%. Combining these and previous DELPHI measurements gives: F_D*(1)|Vcb| = 0.0377 +/- 0.0011 +/- 0.0019, rho_{A_1}^2 = 1.39 +/- 0.10 +/- 0.33 and BR(anti-B0d -> D*+ l- anti-nu_l) = (5.39 +/- 0.11 +/- 0.34)%. Using F_{D*}(1) = 0.91 +/- 0.04, yields: |Vcb| = 0.0414 +/- 0.0012(stat.) +/- 0.0021(syst.) +/- 0.0018(theory). The b-quark semileptonic branching fraction into a D*+ emitted from higher mass charmed excited states has also been measured to be: BR(b -> D*+ X l- anti-nu_l) = (0.67 +/- 0.08 +/- 0.10)%.