Minimal model of fermion FIMP dark matter

We investigate a simple extension of the standard model in which the dark matter consists of a feebly interacting fermion, charged under a new Z4 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 Z4, which couples to the fermion via Yukawa interactions and to the standard model 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.