Abstract
We study the thermodynamic history of composite Dark Matter models. We start with classifying the models by means of the symmetries partially protecting the composite Dark Matter decays and constrain their lifetimes. For each model, we determine the impact of number-changing and number-conserving operators on its thermal history. We also develop the analytic formalism to calculate the asymptotic abundance of stable relics. We show how the relative strength between number-changing and number-conserving interactions together with the dark plasma lifetime affect the thermal fate of the various composite models. Additionally, we discover that the final dark relic density of composite particles can be diluted due to an entropy increase stemming from dark plasma decay. Finally, we confront the models with experimental bounds. We find that indirect detection experiments are most promising in testing this large class of models.
Highlights
The nature of dark matter (DM) is one of the most fascinating questions that remains to be addressed in particle physics
The beststudied scenario is based on the process DM þ DM → Standard Model (SM) þ SM: weakly interacting massive particle (WIMP) annihilation [19,20]
III we identify the composite dark matter candidates arising in gauge theories
Summary
The nature of dark matter (DM) is one of the most fascinating questions that remains to be addressed in particle physics. In a model in which the dark matter particle is in thermal equilibrium in the early Universe, the asymptotic relic abundance is uniquely determined by its decoupling process from the thermal plasma. In the confined phase the lightest composite particles can kinetically decouple from the Standard Model (SM) plasma and evolve as an independent dark thermal bath. This can significantly affect the relic abundance and late-time target cross section and we will elucidate this issue in several scenarios arising from a dark composite model. We observe that the relative strength between number-changing and -conserving interactions determines the thermal fate of the various composite models and further depends on the dark plasma lifetime. In Appendices A and B we offer a glossary of multifluid thermodynamics and in Appendix C we summarize the Boltzmann equation for number-changing processes
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