Abstract
The existence of a light mediator is beneficial to some phenomena in astroparticle physics, such as the core-cusp problem and diversity problem. It can decouple from Standard Model to avoid direct detection constraints, generally realized by retard decay of the mediator. Their out-of-equilibrium decay process changes the dark matter (DM) freeze-out via temperature discrepancy. This type of hidden sector (HS) typically requires a precision calculation of the freeze-out process considering HS temperature evolution and the thermal average of the cross-section. If the mediator is light sufficiently, we can not ignore the s-wave radiative bound state formation process from the perspective of CMB ionization and Sommerfeld enhancement. We put large mass splitting between DM and mediator, different temperature evolution on the same theoretical footing, discussing the implication for DM relic density in this HS. We study this model and illustrate its property by considering the general Higgs-portal dark matter scenario, which includes all the relevant constraints and signals. It shows that the combination of BBN and CMB constraint favors the not-too-hot HS, rinf< 102, for the positive cubic interaction of mediator scenario. On the other hand, the negative cubic interaction is ruled out except for our proposed blind spot scenario.
Highlights
In this paper, for simplicity, the decoupled hidden sector (HS) only contains two particles: dark matter (DM) and mediator
To avoid large elastic scattering between DM and nucleon, we hypothesize the coupling between the mediator and visible sector (VS) particles is ultra-tiny so that the HS is completely decoupled from the VS thermal bath
When DM and mediator as a system effectively decoupled from SM thermal bath, the two sectors undergo different temperature evolution, which is convenient to define the ratio of the HS and VS temperatures, r = Th/T
Summary
When DM and mediator as a system effectively decoupled from SM thermal bath, the two sectors undergo different temperature evolution, which is convenient to define the ratio of the HS and VS temperatures, r = Th/T. Even though the temperatures of the two sectors are independent of each other, the conservation of comoving entropy densities, d(sa3)/dt = 0, determines the evolution of ratio as follows [24, 26]. Assume that the temperature at inflation is much higher than the mass of any particle in two sectors, d.o.f. at inflation equal Assuming the chemical potential are negligible, effective entropy d.o.f. g∗s as a function of T for a particle with a mass m and intrinsic d.o.f. g can be obtained by. Within the temperature ratio evolution r known, the Boltzmann equation describing.
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