Sterile neutrino dark matter from the cosmic QCD epoch

Dark matter may be comprised of keV-mass sterile neutrinos, which were mainly produced during the Quantum Chromodynamics (QCD) epoch of the early Universe. The production could be strongly affected by opacities (or damping rates) of active neutrinos, which receive non-perturbative QCD contributions. We investigate the importance of these contributions and find that they significantly affect the sterile neutrino yield only in the case of non-resonant production. The latter can however only provide a small fraction of the relic dark matter abundance, since it is subject to strong constraints from X-ray observations of sterile neutrino decay, as well as Lyman-$\alpha$ observations connected to structure formation. In the presence of large lepton asymmetries, the production can be resonantly enhanced and a significant fraction of the dark matter can be produced, while observational constraints are much weaker. In that case however, we find that the yield turns out to be insensitive to changes of the opacities. Thus, non-perturbative QCD contributions to the opacities will not affect this dark matter scenario. Furthermore, we obtain larger sterile neutrino yields than previous studies and thus weaker lower limits on the active-sterile mixing angle from big bang nucleosynthesis. In addition, we calculate lower limits on the mixing angle from the cosmic microwave background, which has not been done before in the context of sterile neutrino dark matter. These limits are weaker than the limits from big bang nucleosynthesis but also more robust due to smaller systematic uncertainties. Finally, we investigate a simple extension of the Standard Model in which a new scalar field, mediating self-interactions among active neutrinos, poses an alternative to large lepton asymmetries in helping to efficiently produce sterile neutrino dark matter. The new scalar field can also lead to resonant production, however its contribution to the active neutrino opacity is much higher than the Standard Model contribution and reduces the significance of resonances. Nevertheless, the available parameter space in which the dark matter abundance can be produced is quite large due to weak observational constraints in this scenario.