Abstract
We present a thorough analysis of the sequential freeze-in mechanism for dark matter production in the early universe. In this mechanism the dark matter relic density results from pair annihilation of mediator particles which are themselves produced by thermal collisions of standard model particles. Below some critical value of the mediator coupling to standard model fields, this sequential channel dominates over the usual freeze-in where dark matter is directly produced from thermal collisions, even when the mediator is not in thermal equilibrium. The latter case requires computing the full non-thermal distribution of the mediators, for which finite temperature corrections are particularly important.
Highlights
The nature of the dark matter (DM) is perhaps the most acute open question in particle physics
We concentrated on the specific case of a 10 MeV scalar mediator that couples to DM and u-quarks
Our results for the DM relic density would apply in cases where φ couples to any of the light quark flavors
Summary
The nature of the dark matter (DM) is perhaps the most acute open question in particle physics. In this scenario, the DM is never in equilibrium with the SM in the early Universe and is instead produced by freezein through pair annihilation or decay of particles in the thermal bath [11,12]. We point out that even when the mediator coupling to the SM is too small for the mediator to ever reach equilibrium, a finite density of mediators can be produced through SM induced processes The contribution of such nonthermal mediators to DM production parametrically dominates over that of pair annihilation of SM particles. The Appendixes contain details on the reaction rates for mediator production as well as an approximate analytical solution for the mediator distribution
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