Abstract
Dark sectors with purely gravitational couplings to the Standard Model are unavoidably populated from the SM plasma by graviton exchange, and naturally provide dark matter candidates. We examine the production in the relativistic regime where the dark sector is approximately scale invariant, providing general analytical formulas that depend solely on the central charge of the dark sector. We then assess the relevance of interactions that can lead to a variety of phenomena including thermalisation, non-perturbative mass gaps, out-of-equilibrium phase transitions and cannibalism in the dark sector. As an illustrative example we consider the dark glueball scenario in this light and show it to be a viable dark matter candidate due to the suppression of gravitational production. We go on to extend these results to strongly coupled CFTs and their holographic duals at large-N with the dark dilaton as the dark matter candidate.
Highlights
The dark sector under discussion can only be produced from the SM plasma via graviton exchange.1 Let us note that even though the process happens at tree level, gravitational production relies on the quantum nature of gravity
We assess the relevance of interactions that can lead to a variety of phenomena including thermalisation, non-perturbative mass gaps, out-of-equilibrium phase transitions and cannibalism in the dark sector
The dark sector is populated by the freeze-in mechanism [15] from the hot SM plasma, which itself is well approximated by a thermal conformal field theories (CFTs)
Summary
The universality of gravitational interactions provides an unavoidable source of DM. Two mechanisms have been proposed in the literature: production from the SM plasma through tree-level graviton exchange [1] and production through quantum fluctuations in an expanding background [8]. We will focus on the first production mechanism as the latter requires explicit breaking of conformal invariance that is very suppressed in our models. In the relativistic regime breaking of conformal invariance is non generic. Massless gauge theories and fermions are automatically conformally invariant. Tree-level gravitational production is very predictive being determined by the reheating temperature
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