We present a measurement of $W$ boson polarization in top-quark decays in $t\bar{ t}$ events with decays to dilepton final states using $5.1 { \rm fb^{ -1}}$ of integrated luminosity in $p\bar{ p}$ collisions collected by the CDF II detector at the Tevatron. A simultaneous measurement of the fractions of longitudinal ($f_0$) and right-handed ($f_+$) $W$ bosons yields the results $f_0 = 0.71 ^{ +0.18}_{ -0.17} { \rm (stat)} \pm 0.06 { \rm (syst)}$ and $f_+ = -0.07 \pm 0.09 { \rm (stat)} \pm 0.03 { \rm (syst)}$. Combining this measurement with our previous result based on single lepton final states, we obtain $f_0 = 0.84 \pm 0.09 { \rm (stat)} \pm 0.05 { \rm (syst)}$ and $f_{ +} = -0.16 \pm 0.05 { \rm (stat)} \pm 0.04 { \rm (syst)}$. The results are consistent with standard model expectation.
Categories: hep-ex