Model-independent measurement of the W boson helicity in top quark decays at D0

We present the first model-independent measurement of the helicity of W bosons produced in top quark decays, based on a 1 fb-1 sample of candidate t(bar t) events in the dilepton and lepton plus jets channels collected by the D0 detector at the Fermilab Tevatron p(bar p) Collider. We reconstruct the angle θ* between the momenta of the down-type fermion and the top quark in the W boson rest frame for each top quark decay. A fit of the resulting cos θ* distribution finds that the fraction of longitudinal W bosons f0 = 0.390 ± 0.177 (stat.) ± 0.104 (syst.) and the fraction of right-handed W bosons f+ = 0.171 ± 0.102 (stat.) ± 0.058 (syst.), which is consistent at the 27% C.L. with the standard model