Search for V+A current in top quark decay in ppbar collisions at   sqrt{s}=1.96 TeV
  We report an upper limit on the fraction of V+A current, $f_{V+A}$, in top quark decays, using approximately 700 pb$^{-1}$ of $p\bar{p}$ collisions at $\sqrt{s}$=1.96 TeV acquired by the upgraded Collider Detector at Fermilab. For the decay $t \to Wb \to \ell \nu b$ (where $\ell=e$ or $\mu$), the invariant mass of the charged lepton and the bottom quark jet is sensitive to the polarization of the $W$ boson. We determine $f_{V+A} = -0.06 \pm 0.25$ given a top quark mass of 175 GeV/$c^{2}$. We set an upper limit on $f_{V+A}$ of 0.29 at the 95% confidence level, which represents an improvement by a factor of two on the previous best direct limit. 
A. Abulencia et al. (CDF Collaboration)