Measurement of |Vcb| using B̄0→D∗+ℓ−ν̄ decays

Abstract We report a measurement of the Cabibbo-Kobayashi-Maskawa matrix element | V cb |. From approximately 4.2 million hadronic Z 0 decays recorded with the OPAL deteector, a sample is selected containing 1251 ± 125 B 0 → D ∗+ l − ν l candidates, where l is either an electron or a muon. Using Heavy Quark Effective Theory calculations for the decay form factor at zero recoil of the D ∗+ meson in the B 0 rest frame, we derive | V cb | = [36.0 ± 2.1 (stat) ± 2.4 (syst) ± 1.2 (theory)] × 10 −3 .