The Higgs Portal from LHC to ILC

Interpretations of searches for the Higgs boson are governed by model-dependent combinations of Higgs production cross sections and Higgs branching ratios. Mixing of the Higgs doublet with a hidden sector captures modifications from the Standard Model Higgs phenomenology in the standard search channels in a representative way, in particular because invisible Higgs decay modes open up. As a consequence, LHC exclusion bounds, which disfavor a heavy Standard Model Higgs can be consistently understood in terms of a standard-hidden mixed Higgs system. Shedding light on the possible existence of such an admixture with a hidden sector and quantifying the resemblance of an eventually discovered scalar resonance with the Standard Model Higgs crucially depends on measurement of invisible decays. This task will already be tackled at LHC, but eventually requires the clean environment of a future linear collider to be ultimately completed.