The Four Basic Ways of Creating Dark Matter Through a Portal
  We consider the possibility that along the thermal history of the Universe,
dark matter (DM) would have been created from Standard Model particles, either
through a kinetic mixing portal to an extra U(1) gauge field, or through the
Higgs portal. Depending solely on the DM particle mass, on the portal and on
the DM hidden sector interaction, we show how the observed DM relic density can
be obtained. There are four possible freeze-in/reannihilation/freeze-out
regimes, which together result in a simple characteristic relic density phase
diagram, with the shape of a "Mesa". In the case of the kinetic mixing portal,
we show that, unlike other freeze-in scenarios discussed in the literature, the
freeze-in regime can be probed by forthcoming DM direct detection experiments.
These results are well representative {of} any scenario where a DM hidden
sector would be created out of the Standard Model {sector}.
