Measurement of the weak charged current structure in semileptonic b-hadron decays at the Z peak

The neutrino energy spectrum in semileptonic b-hadron decays with identified energetic electrons and muons has been measured. The observed relative energy sharing between the neutrino and the cahrged lepton is found to be well described with a W± polarization obtained from a free b-quark decay model with a (Vf-A)×(Vf-A) decay structure. The alternative of a (V+A)×(Vf-A) decay structure is excluded with a significance of more than 6 standard deviations. The possibility that hadronic corrections to the b-hadron decay destroy any W± polarization is disfavored by more than 3 standard deviations.