Measurement of the Helicity Fractions of W Bosons from Top Quark Decays Using Fully Reconstructed Top-Antitop Events with CDF II

We present a measurement of the fractions F 0 and F + of longitudinally polarized and right-handed W bosons in top quark decays using data collected with the CDF II detector. The data set used in the analysis corresponds to an integrated luminosity of approximately 318 pb − 1 . We select t ¯ t candidate events with one lepton, at least four jets, and missing transverse energy. Our helicity measurement uses the decay angle θ ∗ , which is defined as the angle between the momentum of the charged lepton in the W boson rest frame and the W momentum in the top quark rest frame. The cos θ ∗ distribution in the data is determined by full kinematic reconstruction of the t ¯ t candidates. We find F 0 = 0 . 85 +0 . 15 − 0 . 22 (stat) ± 0 . 06 (syst) and F + = 0 . 05 +0 . 11 − 0 . 05 (stat) ± 0 . 03 (syst), which is consistent with the standard model prediction. We set an upper limit on the fraction of right-handed W bosons of F + < 0 . 26 at the 95% confidence level.