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

A minimal model of fermion FIMP dark matter