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

## TL;DR

This paper investigates how a weakly coupled scalar particle, produced via the Higgs portal during the electroweak epoch, affects cosmological observations and sets constraints on its properties.

## Contribution

It provides an improved calculation of freeze-in production of scalars via the Higgs portal, extending constraints to very small couplings and a wide mass range.

## Key findings

- Constraints on scalar couplings from cosmological data extend to mixing angles as small as 10^{-16}.
- The scalar mass range constrained spans from keV to 100 GeV.
- Cosmological probes effectively limit the parameter space of weakly coupled scalars.

## Abstract

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, $N_{\rm eff}$, and the consistency of BBN with observations. These probes constrain technically natural couplings for such scalars from $m_S \sim$ keV all the way to $m_S \sim 100$ GeV. The ensuing constraints are similar in spirit to typical beam bump limits, but extend to much smaller couplings, down to mixing angles as small as $\theta_{Sh} \sim 10^{-16}$, and to masses all the way to the electroweak scale.