Higgs-portal interactions of fermionic dark matter -- in contrast to fermions
coupled via Yukawa interactions -- can have a stabilizing effect on the
standard-model Higgs potential. A non-perturbative renormalization-group
analysis reveals that, similar to higher-order operators in the Higgs potential
itself, the fermionic portal coupling can increase the metastability scale by
only about one order of magnitude. Furthermore, this regime of very weakly
coupled dark matter is in conflict with relic-density constraints. Conversely,
fermionic dark matter with the right relic abundance requires either a low
cutoff scale of the effective field theory or a strongly interacting scalar
sector. This results in a triviality problem in the scalar sector which
persists at the non-perturbative level. The corresponding breakdown of the
effective field theory suggests a larger dark sector to be present not too far
above the dark-fermion mass-scale.Comment: 12 pages; 3 figure