Thermal Dark Matter From A Highly Decoupled Sector
  It has recently been shown that if the dark matter is in thermal equilibrium
with a sector that is highly decoupled from the Standard Model, it can
freeze-out with an acceptable relic abundance, even if the dark matter is as
heavy as ~1-100 PeV. In such scenarios, both the dark and visible sectors are
populated after inflation, but with independent temperatures. The lightest
particle in the dark sector will be generically long-lived, and can come to
dominate the energy density of the universe. Upon decaying, these particles can
significantly reheat the visible sector, diluting the abundance of dark matter
and thus allowing for dark matter particles that are much heavier than
conventional WIMPs. In this paper, we present a systematic and pedagogical
treatment of the cosmological history in this class of models, emphasizing the
simplest scenarios in which a dark matter candidate annihilates into hidden
sector particles which then decay into visible matter through the vector,
Higgs, or lepton portals. In each case, we find ample parameter space in which
very heavy dark matter particles can provide an acceptable thermal relic
abundance. We also discuss possible extensions of models featuring these
dynamics.
