A measurement of the polarization of the $W$ boson from top quark decay is an 
excellent test of the $V-A$ form of the charged-current weak interaction in 
the standard model.  Since the longitudinal $W$ boson is intimately related to 
the electroweak symmetry breaking mechanism, and the standard model gives a 
specific prediction for the fraction of longitudinal $W$ bosons from top decays,
it is of particular interest for study.  This thesis presents a measurement of 
$W$ boson polarization in top quark decays through an analysis of the 
$\cos\theta^*$ distribution in the lepton-plus-jets channel of $t\overline{t}$ 
candidate events from $p\overline{p}$ collisions at $\sqrt{s}=1.96$ TeV.  This 
measurement uses an integrated luminosity of $\sim$162 pb$^{-1}$ of data 
collected with the CDF Run II detector, resulting in 31 $t\overline{t}$ 
candidate events with at least one identified $b$ jet.  Using a binned 
likelihood fit to the $\cos\theta^*$ distribution from the  $t\overline{t}$ 
candidate events found in this sample, the fraction of $W$ bosons with 
longitudinal polarization is determined to be $F_0 = 0.99^{+0.29}_{-0.35}\mbox{
(stat.) }\pm 0.19\mbox{ (syst.)}$, $F_0 > 0.33\mbox{ @ }95\%\mbox{ CL}$.  This 
result is consistent with the standard model prediction, given a top quark 
mass of 174.3 GeV$/$c$^2$, of $F_0 = 0.701\pm0.012$.


Posted to /cdf/pub/thesis/cdf7614_tvickey_thesis.pdf
    http://www-cdf.fnal.gov/thesis/cdf7614_tvickey_thesis.pdf

Posted by vickey