The dawn of FIMP Dark Matter: A review of models and constraints

Abstract

Abstract. We present an overview of scenarios where the observed Dark Matter (DM) abundance consists of Feebly Interacting Massive Particles (FIMPs), produced non-thermally by the so-called freeze-in mechanism. In contrast to the usual freeze-out scenario, frozen-in FIMP DM interacts very weakly with the particles in the visible sector and never attained thermal equilibrium with the baryon-photon fluid in the early Universe. Instead of being determined by its annihilation strength, the DM abundance depends on the decay and annihilation strengths of particles in equilibrium with the baryon-photon fluid, as well as couplings in the DM sector. This makes frozen-in DM very difficult but not impossible to test. In this review, we present the freeze-in mechanism and its variations considered in the literature (dark freeze-out and reannihilation), compare them to the standard DM freeze-out scenario, discuss several aspects of model building, and pay particular attention to observational properties and general testability of such feebly interacting DM.

Self-interacting spin-2 dark matter

“…3 suggests, unless we choose unphysically large values for the β parameters, such DM masses lead to σ SI /m FP 0.1 cm 2 /g. This means that DM selfinteractions are irrelevant, as already implicitly assumed in the freeze-in regime [53].…”

Multi-component scalar dark matter from a ZN symmetry: a systematic analysis

“…All in all, it is expected that these additional processes may significantly modify [23,34] the typical outcome for the relic density of each DM particle from the standard freeze-out process [35]. Another appealing alternative to explain the relic density and satisfy current experimental limits is via freeze-in [39,40]. If the Higgs portal couplings are all tiny, λ Hφ i 1, the new scalars would never reach thermal equilibrium in the early Universe, preventing a freeze-out process.…”

Dark matter, dark photon and superfluid He-4 from effective field theory

“…for a year of exposure and a kg of material, assuming zero background. We also report the current BBN bounds [66], as well as a combination of mass and cross section that would allow to explain the dark matter relic abundance via a freeze-in mechanism [9,67,68].…”