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Sub-eV scalar dark matter through the super-renormalizable Higgs portal

The Higgs portal of the Standard Model provides the opportunity for coupling to a very light scalar field $ϕ$ via the super-renormalizable operator $ϕ(H^\dagger H)$. This allows for the existence of a very light scalar dark matter that has coherent interaction with the Standard Model particles and yet has its mass protected against radiative corrections. We analyze ensuing constraints from the fifth-force measurements, along with the cosmological requirements. We find that the detectable level of the fifth-force can be achieved in models with low inflationary scales, and certain amount of fine-tuning in the initial deviation of $ϕ$ from its minimum.