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

Abstract

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 $tbar{t}$ events in the dilepton and lepton plus jets channels collected by the D0 detector at the Fermilab Tevatron $pbar{p}$ 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 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 $f__s16 = 0.119 pm 0.090 hbox{(stat.)} pm 0.053 hbox{(syst.)}$, which is consistent at the 30% C.L. with the standard model.