PeV-Scale Dark Matter as a Thermal Relic of a Decoupled Sector
  In this letter, we consider a class of scenarios in which the dark matter is
part of a heavy hidden sector that is thermally decoupled from the Standard
Model in the early universe. The dark matter freezes-out by annihilating to a
lighter, metastable state, whose subsequent abundance can naturally come to
dominate the energy density of the universe. When this state decays, it reheats
the visible sector and dilutes all relic abundances, thereby allowing the dark
matter to be orders of magnitude heavier than the weak scale. For concreteness,
we consider a simple realization with a Dirac fermion dark matter candidate
coupled to a massive gauge boson that decays to the Standard Model through its
kinetic mixing with hypercharge. We identify viable parameter space in which
the dark matter can be as heavy as ~1-100 PeV without being overproduced in the
early universe.
