Dark matter bound states in the early universe The role of scalar mediators

Dark matter searches are currently at the onset of the exploration of the multi-TeV regime with a variety of existing and upcoming telescopes observing high-energy cosmic rays, such as H.E.S.S., IceCube, CTA and KM3Net. The experimental exploration of the multi-TeV scale
urges the comprehensive theoretical understanding of the dynamics and possibilities.
In this mass regime, within the thermal-relic scenario, the dark matter interactions are expected to manifest as long-range, in the sense that dark matter is significantly heavier than the force mediators to which it couples. The long-range nature of the interactions gives rise to non-perturbative effects, in particular the Sommerfeld effect and the existence of bound states. These effects may operate in the early universe during the dark matter thermal decoupling, as well as inside dark matter haloes today, and significantly alter the dark matter phenomenology.
This thesis focuses on light scalar mediators, and explores their role in the existence and formation of bound states. This is then employed to investigate the effect of metastable bound states on the dark matter relic abundance. A particularly compelling possibility are models where dark matter couples to the Higgs doublet. We find that in a class of these models, the bound state effects associated with the Higgs doublet can dramatically alter the dark matter thermal decoupling in the early universe, and consequently the predictions for the dark matter mass and couplings. This, in turn, is expected to affect all experimental signatures, and suggests that this type of dark matter may be much heavier than previously anticipated, thereby solidifying the physics case for the experimental exploration of the multi-TeV regime.