Freeze-in production of decaying dark matter in five steps

Abstract: We study the cosmological evolution and phenomenological properties of scalar bosons in the keV to MeV range that have a tiny mixing with the Standard Model Higgs boson. The mixing determines both the abundance of light scalars produced via the freeze-in mechanism and their lifetime. Intriguingly, the parameters required for such scalars to account for all of the dark matter in the present Universe generically predict lifetimes comparable to the sensitivity of present and future indirect detection experiments. In order to accurately determine the relic abundance of light scalars, we calculate freeze-in yields including effects from finite temperatures and quantum statistics and develop a new approach for solving the Boltzmann equation for number-changing processes in the dark sector. We find that light scalars can potentially explain the anomalous x-ray emission at 3.5 keV, while evading constraints from structure formation and predicting potentially observable self-interaction cross sections.