Measurement of the helicity of $W$ bosons in top-quark decays

We measure the branching fraction of the top quark to longitudinally and right-handed polarized $W$ bosons, $F_0$ and $F_{+}$, using approximately $200{\mathrm{pb}}^{-1}$ of $\overline{p}p$ collisions collected by the CDF II detector. We analyze two quantities sensitive to the $W$ helicity: the invariant mass of the charged lepton and the bottom-quark jet in the decay $t\to Wb\to \ell \nu b$ (where $\ell =e$ or $\mu$), and the transverse momentum of the charged lepton. Constrained fits yield $F_0={0.74}_{-0.34}^{+0.22}$, and $F_{+}<0.27$ at the 95% confidence level. These measurements are consistent with the standard model predictions.

Figure 1

Distributions of reconstructed $\cos {\theta }^{*}$ (upper plot) and lepton $p_T$ (lower) for top-quark decays to left-handed, right-handed, and longitudinally polarized $W$ bosons.

Figure 2

The reconstructed $\cos {\theta }^{*}$ distribution for the lepton plus jets sample, overlaid with signal and background templates according to their best-fit values. The left-handed template has been scaled up by a factor of 25.

Figure 3

Distribution of lepton $p_T$ for the lepton plus jets and dilepton samples, overlaid with the total signal and background templates according to their best-fit values.