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Measurement of W-Boson Helicity Fractions in Top-Quark Decays using 1.9fb-1.

Shulamit Moed (Harvard)
Andy Hocker, Ricardo Eusebi, Doug Glenzinski (FNAL)

E-mail W-Boson Helicity Group

Abstract:

We peresent a measurement of the fractions of longitudinal (f_0) and right-handed (f_+) W bosons produced in top-quark decays. The W boson helicity fractions are extracted through an analysis of the angular distribution of cos(theta*) - the angle between the lepton as measured in the W rest frame and the W boson as measured in the top rest frame.
This analysis is based on an integrated luminosity of 1.9 fb-1, out of which we select and fully reconstruct 430 events in the lepton+jets channel of the top decay. Fitting the longitudinal and right-handed fractions simultaneously we find f_0 = 0.65 ± 0.19 ± 0.03 and f_+ = -0.03 ± 0.07 ± 0.03 .
We also perform 1D measurements - while fitting to extract the fraction of the longitudinal W helicity (f_0) with f_+ fixed to 0, we find f_0 = 0.59 ± 0.11 ± 0.04, and fitting the right-handed W fraction while fixing f_0 to its SM value expectation with a top mass of 175 GeV, we find f_+ = -0.04 ± 0.04 ± 0.03 Using a bayesian method we set an upper limit on the fraction on the right-handed W bosons f_+ < 0.07 at 95% CL.

public note (CDF 9215) Measurement of W Boson Helicity Fractions in Top Quark Decays Using cosThetaStar

  • cos(theta*) distribution in data with longitudinal fraction and right handed fraction fit results.
     The observed cos(theta*) distribution in the data (points) overlaid with the background (dotted), background + right-handed (dash-dot), background + right\-handed + left-handed (dashed) and background + right-handed + left-handed + longitudinal (solid) distributions as determined by the fit for F+ (the right h\anded fraction).

  • 2D likelihood
     Contours of constant ln likelihood in the (f0,f+) plane from the fit to the data. The X shows the minimum of the ln likelihood, i.e our result.

  • Limit on right-handed fraction - exclusion plot
     The area of the (f+,f0) plane excluded by the data is shown as the hatched region. The unhatched region on the upper right is unphysical.