We study exotic Higgs decays $h\to ZX$, 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 $h\to Z\mathrm{<ruby><rb>\nu </rb><rt>にゅー</rt></ruby>}\overline{\mathrm{<ruby><rb>\nu </rb><rt>にゅー</rt></ruby>}}$ represents an irreducible “neutrino floor” background to these new physics searches, providing also a target experimental sensitivity for them. We analyze $h\to Z+\mathrm{invisible}$ 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.

• Received 20 June 2022
• Accepted 26 November 2022

DOI:https://doi.org/10.1103/PhysRevD.106.115023

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Published by the American Physical Society