Measurement of the W helicity in fully reconstructed ttbar events using 1.9 fb-1
 
Thorsten Chwalek, Dominic Hirschbuehl, Thomas Muller, Jeannine Wagner-Kuhr, Wolfgang Wagner

  A very brief overview on our analysis can be found here as PowerPoint , OpenOffice , or PDF .


CDF public note




Abstract:

We present a measurement of the fraction of longitudinally polarized (F0) and right-handed (F+) W bosons in top quark decays in the lepton+jets channel. The analyzed dataset corresponds to an integrated luminosity of about 1.9 fb-1. As sensitive observable we use the cosine of the decay angle &theta* of the charged lepton in the W rest frame measured with respect to the direction of motion of the W boson in the top-quark rest-frame. In order to determine the cos(&theta*) distribution in the data, the kinematics of the ttbar events are fully reconstructed. We find F0=0.66 ± 0.10 (stat) ± 0.06 (syst) (with F+ fixed to zero) and F+=0.01 ± 0.05 (stat) ± 0.03 (syst) (with F0 fixed to 0.7). Fitting both fractions simultaneously we find F0=0.38 ± 0.21 (stat) ± 0.07 (syst) and F+=0.15 ± 0.10 (stat) ± 0.05 (syst).



Final results for the 1 parameter fit

result for F0 for 1 parameter fit
result for F+ for 1 parameter fit limit for F+ for 1 parameter fit

Final results for the 2 parameter fit

result for F0 for 2 parameter fit
result for F+ for 2 parameter fit

Theoretical cos(&theta*) distributions for the different polarizations of the W boson

theoretical cos(&theta<sup>*</sup>)
Theoretical cos(&theta*) distribution (EPS)

Measured cos(&theta*) distribution

Measured cos(&theta<sup>*</sup>) all events
cos(&theta*) distribution for entire sample (EPS)
Measured cos(&theta<sup>*</sup>) single-tag events
cos(&theta*) distribution for single-tag sample (EPS)
Measured cos(&theta<sup>*</sup>) double-tag events
cos(&theta*) distribution for double-tag sample (EPS)

1 parameter fit
log likelihood for F0
&Delta lnL(F0) for fixed F+ (EPS)
log likelihood for F+
&Delta lnL(F+) for fixed F0 (EPS)
sensitivity F0
expected statistical uncertainty for measurement of F0 (stat. uncertainty of the real measurement is indicated by the red arrow) (EPS)
sensitivity F+
expected statistical uncertainty for measurement of F+ (stat. uncertainty of the real measurement is indicated by the red arrow) (EPS)
expected fit result F0
expected fit result for measurement of F0 assuming SM (fit result of the real measurement is indicated by the red arrow) (EPS)
expected fit result F+
expected fit result for measurement of F+ assuming SM (fit result of the real measurement is indicated by the red arrow) (EPS)

Limit on F+ (for F0 = 0.7)
limit on F+
Limit on F+ for F0 = 0.7 (EPS)
expected limit
expected limit on F+ for fixed F0 (EPS)

2 parameter fit

log likelihood for 2 parameter fit
&Delta lnL(F0,(F+) for fitting both fractions simultaneously (EPS)
sensitivity F0
expected statistical uncertainty for measurement of F0 (stat. uncertainty of the real measurement is indicated by the red arrow) (EPS)
sensitivity F+
expected statistical uncertainty for measurement of F+ (stat. uncertainty of the real measurement is indicated by the red arrow) (EPS)
expected fit result F0
expected fit result for measurement of F0 assuming SM (fit result of the real measurement is indicated by the red arrow) (EPS)
expected fit result F+
expected fit result for measurement of F+ assuming SM (fit result of the real measurement is indicated by the red arrow) (EPS)

Corrected distributions
corrected cos(&theta<sup>*</sup>) distribution
Corrected cos(&theta*) distribution for the 1-parameter F0 - fit result (EPS)
corrected cos(&theta<sup>*</sup>) distribution
Corrected cos(&theta*) distribution for the 1-parameter F+ - fit result(EPS)
corrected cos(&theta<sup>*</sup>) distribution
Corrected cos(&theta*) distribution for the 2-parameter - fit result (EPS)

Corrected distributions (2)
corrected cos(&theta<sup>*</sup>) distribution
Corrected cos(&theta*) distribution for the 1-parameter F0 - fit result together with theory curves for different values of F0 (EPS)
corrected cos(&theta<sup>*</sup>) distribution
Corrected cos(&theta*) distribution for the 1-parameter F+ - fit result together with theory curves for different values of F+(EPS)


Systematic uncertainties


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







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Universität Karlsruhe