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

TL;DR

This paper reports the first model-independent measurement of W boson helicity in top quark decays, using data from the Fermilab Tevatron, and finds results consistent with the Standard Model predictions.

Contribution

It introduces a novel, model-independent method to measure W boson helicity fractions in top decays using angular distributions from Tevatron data.

Findings

Measured longitudinal W fraction: 0.425 ± 0.166 (stat) ± 0.102 (syst)

Measured right-handed W fraction: 0.119 ± 0.090 (stat) ± 0.053 (syst)

Results are consistent with Standard Model expectations at 30% confidence level.

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 $t\bar{t}$ 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 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 $f_+ = 0.119 \pm 0.090 \hbox{(stat.)} \pm 0.053 \hbox{(syst.)}$, which is consistent at the 30% C.L. with the standard model.