Cosmological beam dump: Constraints on dark scalars mixed with the Higgs boson

Precision cosmology provides a sensitive probe of extremely weakly coupled states due to thermal freeze-in production, with subsequent decays impacting physics during well-tested cosmological epochs. We explore the cosmological implications of the freeze-in production of a new scalar S via the super-renormalizable Higgs portal. If the mass of S is at or below the electroweak scale, peak freeze-in production occurs during the electroweak epoch. We improve the calculation of the freeze-in abundance by including all relevant QCD and electroweak production channels. The resulting abundance and subsequent decay of S is constrained by a combination of x-ray data, cosmic microwave background anisotropies and spectral distortions, N-eff, and the consistency of big bang nucleosynthesis with observations. These probes constrain technically natural couplings for such scalars from m(S) similar to 10 keV all the way to m(S) similar to 100 GeV. The ensuing constraints are similar in spirit to typical beam dump limits, but extend to much smaller couplings, down to mixing angles as small as theta(Sh) similar to 10(-16), and to masses all the way to the electroweak scale.