Measurement of the Helicity of W Bosons in Top Quark Decays

TL;DR

This study measures the helicity fractions of W bosons from top quark decays using lepton momentum spectra in proton-antiproton collisions, providing results consistent with the Standard Model.

Contribution

It presents the first measurement of W boson helicity in top decays at the Tevatron, using a novel analysis of lepton spectra in a large dataset.

Findings

Measured W helicity fraction F_0 = 0.91 +/- 0.37 (stat) +/- 0.13 (syst)

Results are consistent with the Standard Model prediction of F_0=0.70

Analysis used 106 pb^{-1} of data from the CDF detector

Abstract

We use the transverse momentum spectrum of leptons in the decay chain t-->bW with W-->l nu to measure the helicity of the W bosons in the top quark rest frame. Our measurement uses a ttbar sample isolated in 106 +/- 4 inverse picobarns of data collected in ppbar collisions at sqrt(s)=1.8 TeV with the CDF detector at the Fermilab Tevatron. Assuming a standard V--A weak decay, we find that the fraction of W's with zero helicity in the top rest frame is F_0 = 0.91 +/- 0.37 (stat) +/- 0.13 (syst), consistent with the standard model prediction of F_0=0.70 for a top mass of 175 GeV/c**2.