Physical Review Letters 98,7 (2007) 072001;
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 (p) over bar collisions at root s=1.96 TeV acquired by the upgraded Collider Detector at Fermilab. For the decay t -> Wb ->center dot nu b (where center dot=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 +/- 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, an improvement by a factor of 2 on the previous best direct limit.
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