Semidark Higgs boson decays: Sweeping the Higgs neutrino floor

We study exotic Higgs decays h -> ZX, with X an invisible beyond the Standard Model (SM) particle, resulting in a semidark final state. Such exotic Higgs decays may occur in theories of axionlike particles (ALPs), dark photons or pseudoscalar mediators between the SM and dark matter. The SM process h -> Zvv over bar represents an irreducible "neutrino floor" background to these new physics searches, providing also a target experimental sensitivity for them. We analyze h -> Z + invisible searches at the LHC and a future ILC, showing that these exotic Higgs decays can yield sensitivity to unexplored regions of parameter space for ALPs and dark matter models.