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 $W$ bosonsproduced in top quarkdecays, 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 $\theta^*$ between the momenta of the down-type fermion and the top quarkin the $W$ boson rest framefor each top quarkdecay. A fit of the resulting \costheta distribution finds that the fraction of longitudinal $W$ bosons$f_0 = 0.425 \pm 0.166 \hbox{( stat.)} \pm 0.102 \hbox{(syst.)}$ and the fraction of right-handed $W$ bosons$f_+ = 0.119 \pm 0.090 \hbox{( stat.)} \pm 0.053 \hbox{(syst.)}$, which is consistent at the 30% C.L. with the standard model.