W-boson polarization measurement in the tt¯ dilepton channel using the CDF II detector

We present a measurement of the W-boson polarization in top-quark decays in tt¯ events with decays to dilepton final states using data corresponding to 5.1 fb−1 of integrated luminosity in pp¯ collisions collected by the CDF II detector at the Tevatron. Assuming a top-quark mass of 172.5 GeV/c2, a simultaneous measurement of the fractions of longitudinal (f0) and right-handed (f+) W-bosons yields the results f0=0.70−0.17+0.18(stat)±0.06(syst) and f+=−0.09±0.09(stat)±0.03(syst). Combining this measurement with our previous measurement based on single-lepton final states, we obtain f0=0.84±0.09(stat)±0.05(syst) and f+=−0.16±0.05(stat)±0.04(syst). The results are consistent with the standard model expectation.