  When a top quark is highly boosted, the $W$ boson from its decay has a
substantial linear polarization that results in a $\cos 2\phi$ azimuthal
angular correlation among the top decay products. We show that this correlation
can be measured for hadronically decayed boosted tops, and its magnitude
provides a way to measure the longitudinal polarization of top quark, which is
an important probe of new physics that couples to top sector.
