A minimal model of fermion FIMP dark matter

TL;DR

This paper proposes a minimal extension of the standard model featuring a feebly interacting fermion as dark matter, produced via freeze-in, with a simple scalar and five free parameters, demonstrating its viability across various conditions.

Contribution

It introduces a minimal fermion FIMP dark matter model with a new $Z_4$ symmetry, detailing its production mechanisms and viable parameter space.

Findings

The model can produce the correct relic density over a wide parameter range.

Numerical analysis confirms the model's viability for different masses and couplings.

The scenario offers a minimal and novel fermion freeze-in dark matter solution.

Abstract

We investigate a simple extension of the standard model (SM) in which the dark matter consists of a feebly interacting fermion (FIMP), charged under a new $Z_4$ symmetry, that is produced in the early Universe by the freeze-in mechanism. The only other new particle included in the model is a singlet scalar, also charged under the $Z_4$, which couples to the fermion via Yukawa interactions and to the SM Higgs. The model is truly minimal, as it admits just five free parameters: two masses and three dimensionless couplings. Depending on their values, the freeze-in mechanism can be realized in different ways, each characterized by its own production processes. For all of them, we numerically study the relic density as a function of the free parameters of the model and determine the regions consistent with the dark matter constraint. Our results show that this scenario is viable over a wide range of couplings and dark matter masses. This model, therefore, not only offers a novel solution to the dark matter problem, but it also provides a minimal realization of freeze-in for fermion dark matter.