Model-Independent Measurement of theW-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 $1{\mathrm{fb}}^{-1}$ sample of candidate $t\overline{t}$ events in the dilepton and lepton plus jets channels collected by the D0 detector at the Fermilab Tevatron $p\overline{p}$ Collider. We reconstruct the angle ${\theta }^{*}$ 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 $\cos {\theta }^{*}$ distribution finds that the fraction of longitudinal $W$ bosons $f_0=0.425\pm 0.166(\mathrm{stat})\pm 0.102(\mathrm{syst})$ and the fraction of right-handed $W$ bosons $f_{+}=0.119\pm 0.090(\mathrm{stat})\pm 0.053(\mathrm{syst})$, which is consistent at the 30% C.L. with the standard model.