Probing off-shell Higgs portals at colliders

We present the reach of current and future colliders on the vector boson fusion production of invisible scalars through an off-shell Higgs boson. The two leading Higgs portals, {\it renormalizable} and {\it derivative}, are discussed and their sizable differences highlighted. We consider a variety of future high-energy lepton and hadron colliders, emphasizing the unique potential of a multi-TeV muon collider to probe this “nightmare scenario” for new physics. The impact on strongly motivated theoretical models is assessed, including pseudo-Goldstone WIMP dark matter, electroweak baryogenesis, and neutral naturalness. Based on arXiV:1910.04170 [hep-ph] (published in SciPost Physics 8 (2020) 027), and work in progress.