Determination of |Vcb| from the semileptonic decay B0 → D*−ℓ+ν

Semileptonic decays B→ D*− l + νX were selected from a sample of 3.1 million hadronic Z decays collected by the DELPHI detector at LEP. A topological search for semileptonic B decays to resonant and non-resonant D*− π + states was performed and the ratio of the branching fractions: $$\frac{{Br(B \to D^* \ell ^ + X)}}{\begin{gathered} Br(B \to D^* \ell ^ + vX) + Br(B^0 \to D^{* - } \ell ^ + v) \hfill \\ = 0.19 \pm 0.10(stat) \pm 0.06(syst) \hfill \\ \end{gathered} }$$ was determined. Taking into account this contribution, the differential decay width of B0→D*−l+ l + ν was measured as a function of the momentum transfer from the B to the D*− in two separate analyses, using exclusive and inclusive methods of D*− reconstruction. The distributions were fitted over the full momentum transfer range to extract the product of |Vcb| times the normalization of the decay form factorF(q max 2 ): $$F(q_{max}^2 )|V_{cb} | = (35.4 + 1.9(stat) \pm 2.4(syst)) \cdot 10^{ - 3} $$ . The value of |Vcb| was computed using theoretical calculations ofF( q max 2 ), giving: $$|V_{cb} | = (39.9 \pm 2.0(stat) \pm 2.6(syst) \pm 1.7(theory)) \cdot 10^{ - 3} $$ . The total branching fractionBr(B 0→D*− l + gn) was determined to be: $$Br(B^0 \to D^{* - } \ell ^ + v) = (5.52 \pm 0.17(stat) \pm 0.68(syst))\% $$ .