Abstract:
We investigate the freeze-in production of Dirac dark matter in an anomaly-free U(1)X extension of the Standard Model with an alterna-
tive charge assignment for right-handed neutrinos. The dark matter
candidate χ originates from the right-handed neutrino sector, and its
production involves the additional gauge boson Z′ and scalar ϕ. We
solve coupled Boltzmann equations for the abundances of χ, Z′, and ϕ.
We examine distinct mass hierarchies. We also investigate the cases of
vanishing and nonzero Higgssinglet scalar mixing. A nonzero scalar
mixing strongly enhances the production of ϕ, whereas in the sα = 0
limit the scalar abundance is highly suppressed and dark matter pro-
duction becomes predominantly vector-mediated. For each scenario,
we identify benchmark parameter sets that reproduce the observed
dark matter relic abundance, ΩDMh2 ≃0.12, and determine the domi-
nant production channels. The relative importance of scalar and vector
production channels varies by many orders of magnitude across the in-
vestigated regimes, demonstrating the strong dependence of freeze-in
dynamics on the mediator mass hierarchy and scalar mixing.