Measurement of the W Helicity in Fully Reconstructed Top Anti-Top Events using 1 fb<sup>-1</sup>  

Measurement of the W Helicity in Fully Reconstructed Top Anti-Top Events using 1 fb-1  

Public Page 



Thorsten Chwalek, 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 955 pb-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 = 0.59 +- 0.12 (stat.) + 0.07 - 0.06 (syst.) and F+ = - 0.03 +- 0.06 (stat.) + 0.04 -0.03 (syst.), which is well consistent with the standard model.



Final Results for 955 pb-1:


 

F0 = 0.59 +- 0.12 (stat.) + 0.07 - 0.06 (syst.)
F+ = - 0.03 +- 0.06 (stat.) + 0.04 - 0.03 (syst.)
F+ < 0.1 @ 95C.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 together with background estimation (EPS)
longitudinally polarized W
Measured cos(&theta*) distribution (error on the MC - expectation (red)). (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
cos(&theta*) distribution for data together with the curves for the F0- fit (red) and the SM expectation (blue dashed). (EPS) 		F<sub>0</sub>: Result
cos(&theta*) distribution for data together with the curves for the F+- fit (red) and the SM expectation (blue dashed). (EPS)
F<sub>0</sub>: Result
unfolded cos(&theta*) distribution for data together with the SM (F0=0.7) distribution. (EPS)
F<sub>+</sub>: Result
unfolded cos(&theta*) distribution for data together with the SM (F+ = 0) distribution. (EPS)
F<sub>0</sub>: Result
unfolded cos(&theta*) distribution for data together with two theoretical curves for different values of F0 (F0=0.5 /F0=0.8). (EPS)
F<sub>+</sub>: Result
unfolded cos(&theta*) distribution for data together with two theoretical curves for different values of F+ (F+ = 0.1 /F+ = 0.2). (EPS)
F<sub>+</sub>: Result
limit on F+. (EPS)

Sensitivity

Measured cos(&theta<sup>*</sup>)
expected sensitivity (statistical uncertainty only) for the F0 - measurement performing 500 PE - the sensitivity of our measurement is indicated by the arrow (EPS)
longitudinally polarized W
expected sensitivity (statistical uncertainty only) for the F+ - measurement performing 500 PE - the sensitivity of our measurement is indicated by the arrow . (EPS)
Measured cos(&theta<sup>*</sup>)
expected sensitivity (statistical uncertainty only) as a function of the integrated luminosity (EPS)
longitudinally polarized W
expected sensitivity (total uncertainty) as a function of the integrated luminosity. (EPS)


Systematic uncertainties


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

Extracting the helicity fractions - plots for our analysis

Measured cos(&theta<sup>*</sup>)
relative efficency (Pythia MC) - for explanation see public note. (EPS)
longitudinally polarized W
migration matrix (Pythia MC) - for explanation see public note. (EPS)
Measured cos(&theta<sup>*</sup>)
signal templates for F0 - for explanation see public note. (EPS)
longitudinally polarized W
signal templates for F+ - for explanation see public note. (EPS)

Last modified: Tue Jul 18 17:51:23 CEST 2006 by T. Chwalek