Measurement of Vcb from the decay process B̄0 → D*+ℓ-v̄

Abstract A new precise measurement of | V cb | and of the branching ratio BR ( B 0 →D ∗+ l − ν l ) has been performed using a sample of about 5000 semileptonic decays B 0 →D ∗+ l − ν l , selected by the DELPHI detector at LEP I by tagging the soft pion from D ∗+ →D 0 π + . The results are: V cb =(39.0±1.5 (stat.) +2.5 −2.6 (syst. exp.) ±1.3 (syst. th.) )×10 −3 , BR ( B 0 →D ∗+ l − ν l )=(4.70±0.13 (stat.) +0.36 −0.31 (syst. exp.) )% . The analytic dependencies of the differential cross-section and of the Isgur–Wise form factor as functions of the variable w=v B 0 ·v D ∗ have also been obtained by unfolding the experimental resolution.