  We study the observability of new interactions which modify Higgs-pair
production via vector-boson fusion processes at the LHC and at future
proton-proton colliders. In an effective-Lagrangian approach, we explore in
particular the effect of the operator $h^2 W_{\mu\nu}^a W^{a,\mu\nu}$, which
describes the interaction of the Higgs boson with transverse vector-boson
polarization modes. By tagging highly boosted Higgs bosons in the final state,
we determine projected bounds for the coefficient of this operator at the LHC
and at a future 27 TeV or 100 TeV collider. Taking into account unitarity
constraints, we estimate the new-physics discovery potential of Higgs pair
production in this channel.
