Measurement of the W Helicity in Fully Reconstructed Top Anti-Top Events  

Measurement of the W Helicity in Fully Reconstructed Top Anti-Top Events  

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Thorsten Chwalek, Martin Erdmann, Dominic Hirschbühl,

Thomas Müller, Jeannine Wagner, Wolfgang Wagner


CDF public note



Abstract:

We present a measurement of the fraction of longitudinally (F0) and right-handed (F+) polarized W bosons in top quark decays using an integrated luminosity of about 320pb-1 collected at the CDF II detector. We are selecting ttbar events with a lepton+jets signature in the detector. For our helicity measurement we use the cosine of the decay angle &theta* between the momentum of the charged lepton in the W boson rest frame and the W momentum in the top quark rest frame, which is highly sensitive to the W helicity. In order to determine the cos(&theta*) distribution in the data, the kinematics of the ttbar events are fully reconstructed. We find F0 = 0.85 +0.15 -0.22 (stat.) +- 0.06 (syst.) and F+ = 0.05 +0.11 -0.05 (stat.) +- 0.03 (syst.), which is well consistent with the standard model.



Final Results for 320 pb-1:


 

F0 = 0.85 +0.15 -0.22 (stat.) +- 0.06 (syst.)
F+ = 0.05 +0.11 -0.05 (stat.) +- 0.03 (syst.)
F+ < 0.26 at the 95% C.L. 






Introduction into W Helicity

Measured cos(&theta<sup>*</sup>)
Theoretical cos(&theta*) distribution (EPS)
longitudinally polarized W
cos(&theta*) for longitudinally polarized W. (EPS)
left handed polarized W
cos(&theta*) for left handed polarized W. (EPS)
right handed W
cos(&theta*) for right handed polarized W. (EPS)

Measured cos(&theta*) Distribution

Measured cos(&theta<sup>*</sup>)
Measured cos(&theta*) distribution is shown together with the background estimate. (EPS)
Measured cos(&theta<sup>*</sup>)
Measured cos(&theta*) distribution is shown together with the MC expectation. (EPS)

Extraction of F0 and F+

F0 Extraction (F+=0)

F+ Extraction (F0=0.7)
F<sub>0</sub>: Negative Likelihood function
Negative Likelihood function as a function of F0. The minimum represents the fit result. (EPS)
F<sub>+</sub>: Negative Likelihood function
Negative likelihood function as a function of F+. The minimum represents the fit result. (EPS)
F<sub>0</sub>: Result
corrected cos(&theta*) distribution for data together with the curve predicted for the fit result. (EPS)
F<sub>+</sub>: Result
corrected cos(&theta*) distribution for data together with the curve predicted for the fit result. (EPS)
F<sub>0</sub>: Result
cos(&theta*) distribution for data together with the SM curve and the curve predicted for the fit result. (EPS)
F<sub>+</sub>: Result
limit on F+. (EPS)


Statistical uncertainty



Distribution of the statistical uncertainty for pseudoexperiments. The statistical uncertainty of our analysis is indicated by the arrow.



Systematic uncertainties:


Systematic uncertainties
Summary of systematic uncertainties, the total error is calculated by adding all single uncertainties in quadrature.

Last modified: Wed May 24 04:03:31 CDT 2006 by T. Chwalek