Dynamical Dark Matter from Thermal Freeze-Out
  In the Dynamical Dark Matter (DDM) framework, the dark sector comprises a
large number of constituent dark particles whose individual masses, lifetimes,
and cosmological abundances obey specific scaling relations with respect to
each other. In particular, the most natural versions of this framework tend to
require a spectrum of cosmological abundances which scale inversely with mass,
so that dark-sector states with larger masses have smaller abundances. Thus
far, DDM model-building has primarily relied on non-thermal mechanisms for
abundance generation such as misalignment production, since these mechanisms
give rise to abundances that have this property. By contrast, the simplest
versions of thermal freeze-out tend to produce abundances that increase, rather
than decrease, with the mass of the dark-matter component. In this paper, we
demonstrate that there exist relatively simple modifications of the traditional
thermal freeze-out mechanism which "flip" the resulting abundance spectrum,
producing abundances that scale inversely with mass. Moreover, we demonstrate
that a far broader variety of scaling relations between lifetimes, abundances,
and masses can emerge through thermal freeze-out than through the non-thermal
mechanisms previously considered for DDM ensembles. The results of this paper
thus extend the DDM framework into the thermal domain and essentially allow us
to "design" our resulting DDM ensembles at will in order to realize a rich
array of resulting dark-matter phenomenologies.
