Dark Matter in the Higgs Decay Channel: A Review of Past Experiments and Future Prospects

The Higgs boson provides an important opportunity to investigate possible connections between the Standard Model and the dark sector. In particular, Higgs-portal models propose that dark matter may interact with ordinary matter through the Higgs field, motivating experimental searches for invisible and exotic Higgs decays. This review examines the theoretical foundations and experimental progress of these searches, with emphasis on results from the ATLAS and CMS collaborations at the Large Hadron Collider (LHC). Existing measurements have placed increasingly stringent constraints on the invisible branching fraction of the Higgs boson, restricting parts of the parameter space of several Higgs-portal dark matter models. The review also discusses effective field theory descriptions, theoretical constraints from unitarity and positivity, and exotic decay signatures involving hidden-sector particles and displaced vertices. In addition, it evaluates the prospects of future electron–positron colliders, including the Circular Electron Positron Collider (CEPC) and the Future Circular Collider in its electron–positron configuration (FCC-ee). Techniques such as recoil-mass reconstruction may enable more precise measurements of invisible Higgs decays, while improved detector capabilities could extend searches for long-lived particles. These developments will help test Higgs-portal scenarios and further constrain possible interactions between the visible and dark sectors.