Dynamic Freeze-In: Impact of Thermal Masses and Cosmological Phase
  Transitions on Dark Matter Production
  The cosmological abundance of dark matter can be significantly influenced by
the temperature dependence of particle masses and vacuum expectation values. We
illustrate this point in three simple freeze-in models. The first one, which we
call kinematically induced freeze-in, is based on the observation that the
effective mass of a scalar temporarily becomes very small as the scalar
potential undergoes a second order phase transition. This opens dark matter
production channels that are otherwise forbidden. The second model we consider,
dubbed vev-induced freeze-in, is a fermionic Higgs portal scenario. Its scalar
sector is augmented compared to the Standard Model by an additional scalar
singlet, $S$, which couples to dark matter and temporarily acquires a vacuum
expectation value (a two-step phase transition or `vev flip-flop'). While
$\langle S \rangle \neq 0$, the modified coupling structure in the scalar
sector implies that dark matter production is significantly enhanced compared
to the $\langle S \rangle = 0$ phases realised at very early times and again
today. The third model, which we call mixing-induced freeze-in, is similar in
spirit, but here it is the mixing of dark sector fermions, induced by non-zero
$\langle S \rangle$, that temporarily boosts the dark matter production rate.
For all three scenarios, we carefully dissect the evolution of the dark sector
in the early Universe. We compute the DM relic abundance as a function of the
model parameters, emphasising the importance of thermal corrections and the
proper treatment of phase transitions in the calculation.
