In this work, we present a new technique to measure the longitudinal and transverse polarization fractions of hadronic decays of boosted $W$ bosons. We introduce a new jet substructure observable denoted as $p_theta$, which is a proxy for the parton level decay polar angle of the $W$ boson in its rest-frame. We show that the distribution of this observable is sensitive to the polarization of $W$ bosons and can therefore be used to reconstruct the $W$ polarization in a model-independent way. As a test case, we study the efficacy of our technique on vector boson scattering processes at the high luminosity Large Hadron Collider and we find that our technique can determine the longitudinal polarization fraction to within $pm 0.15$. We also show that our technique can be used to identify the parity of beyond Standard Model scalar or pseudo-scalar resonances decaying to $W$ bosons with just 20 events.
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