Measurement of the W Boson Helicity in Top Quark Decay at D0

We report on a model-independent measurement of the helicity of W bosons produced in top quark decays based on a 1 fb$^{-1}$ sample of ttbar events in the dilepton and lepton+jets channels. In the standard model, the fraction of longitudinal (right-handed) W bosons is predicted to be 0.7 (0). The deviation from these values would be a clear sign of new physics. The measurement is based upon the angle between the momenta of the down-type fermion and the top quark in the W boson rest frame.