  A method is proposed for distinguishing highly boosted hadronically decaying
W's (W-jets) from QCD-jets using jet substructure. Previous methods, such as
the filtering/mass-drop method, can give a factor of ~2 improvement in
S/sqrt(B) for jet pT > 200 GeV. In contrast, a multivariate approach including
new discriminants such as R-cores, which characterize the shape of the W-jet,
subjet planar flow, and grooming-sensitivities is shown to provide a much
larger factor of ~5 improvement in S/sqrt(B). For longitudinally polarized W's,
such as those coming from many new physics models, the discrimination is even
better. Comparing different Monte Carlo simulations, we observe a sensitivity
of some variables to the underlying event; however, even with a conservative
estimates, the multivariate approach is very powerful. Applications to
semileptonic WW resonance searches and all-hadronic W+jet searches at the LHC
are also discussed. Code implementing our W-jet tagging algorithm is publicly
available at http://jets.physics.harvard.edu/wtag
