Abstract
We investigate a simplified freeze-in dark-matter model in which the dark matter only interacts with the standard-model neutrinos via a light scalar. The extremely small coupling for the freeze-in mechanism is naturally realized in several neutrino-portal scenarios with the secret neutrino interactions. We study possible evolution history of the hidden sector: the dark sector would undergo pure freeze-in production if the interactions between the dark-sector particles are negligible, while thermal equilibrium within the dark sector could occur if the reannihilation of the dark matter and the scalar mediator is rapid enough. We investigate the relic abundance in the freeze-in and dark freeze-out regimes, calculate evolution of the dark temperature, and study its phenomenological aspects on BBN and CMB constraints, the indirect-detection signature, as well as the potential to solve the small scale structure problem.
Highlights
The SM particles are often assumed to be at the weak scale
We investigate a simplified freeze-in dark-matter model in which the dark matter only interacts with the standard-model neutrinos via a light scalar
We study possible evolution history of the hidden sector: the dark sector would undergo pure freeze-in production if the interactions between the dark-sector particles are negligible, while thermal equilibrium within the dark sector could occur if the reannihilation of the dark matter and the scalar mediator is rapid enough
Summary
In the s-channel neutrino model, we consider a dark sector which consists of a Dirac fermionic χ and a light scalar mediator φ. After electroweak spontaneous symmetry breaking, tree level interactions between the scalar φ and the SM neutrinos can be generated as follows:. For Scenario II, the φ − ν − ν interaction for a pseudoscalar φ could be generated in the so-called minimal Majoron model [41,42,43,44] In this type of model, we still introduce three right-handed neutrinos Ni but in addition a SM-singlet complex scalar field Φ above the electroweak scale. Lint ⊃ gΦN cN Φ − gH LH N + h.c. We assume that Φ develops a VEV and spontaneously breaks a global U(1) symmetry at a scale higher than MEW. Having demonstrated how Scenario I and Scenario II could be generated from UV theories, in what follows, we shall use the effective Lagrangian in eq (2.4) when discussing production of dark matter. We list all the Feynman diagrams relevant for our scenarios in figure 1
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