A Measurement of the Helicity of W Bosons Produced in Top-Quark Decays

We have measured the fraction of longitudinally polarized W bosons produced top-quark decays. The result of the analysis in single-lepton t$\bar{t}$ events is fully consistent with the Standard Model expectation. The 2σ discrepancy in the dilepton analysis is suggestive of new phenomena. However, given the significance of the discrepancy, any claim of new physics based on this analysis is highly speculative. It is worth noting that another analysis of the kinematic properties of these data finds a similar discrepancy [63]. It will be interesting to see if this discrepancy persists as more data are collected during run II of the Tevatron. In order to make a stronger statement about the nature of the tWb coupling with this method, larger statistics are required. However alternative methods, particularly the matrix-element method developed at D0 [36, 35], should be especially powerful, even with limited statistics.