Abstract
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.
Highlights
Dynamical Dark Matter (DDM) [1,2] is a framework for dark-matter physics in which the dark sector is composed of a large ensemble of dark states exhibiting a variety of masses, lifetimes, and cosmological abundances
We review how the canonical relationship between abundance and mass arises within the weakly interacting massive particle (WIMP) paradigm and illustrate how this relationship can be altered through modifications of the particle physics alone, without any modification of the background cosmology
Within the DDM framework, the phenomenological viability of the dark-matter ensemble is the result of the interplay between three fundamental relations which govern how the masses, cosmological abundances, and decay widths of the individual ensemble constituents scale relative to one another across the ensemble as a whole
Summary
Dynamical Dark Matter (DDM) [1,2] is a framework for dark-matter physics in which the dark sector is composed of a large ensemble of dark states exhibiting a variety of masses, lifetimes, and cosmological abundances. In these and other realistic DDM scenarios, the masses, lifetimes, and abundances of these individual particles are not arbitrary Rather, these quantities follow directly from the underlying physics model and generally take the form of scaling relations which dictate how these quantities scale relative to one another across the ensemble as a whole. III, we undertake a somewhat more general study along the lines of this approach and derive a general expression for the freeze-out cosmological abundance of an individual ensemble constituent as a function of the mass, spin, and couplings of the particles involved In this way, we find that we can generate a broad range of negative scaling exponents γ and potentially even dial between them. V, we conclude with a summary of the implications of our results for DDM model-building in a thermal context and possible directions for future work
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