  In the simplified dark matter models commonly studied, the mass generation
mechanism for the dark fields is not typically specified. We demonstrate that
the dark matter interaction types, and hence the annihilation processes
relevant for relic density and indirect detection, are strongly dictated by the
mass generation mechanism chosen for the dark sector particles, and the
requirement of gauge invariance. We focus on the class of models in which
fermionic dark matter couples to a spin-1 vector or axial-vector mediator.
However, in order to generate dark sector mass terms, it is necessary in most
cases to introduce a dark Higgs field and thus a spin-0 scalar mediator will
also be present. In the case that all the dark sector fields gain masses via
coupling to a single dark sector Higgs field, it is mandatory that the
axial-vector coupling of the spin-1 mediator to the dark matter is non-zero;
the vector coupling may also be present depending on the charge assignments.
For all other mass generation options, only pure vector couplings between the
spin-1 mediator and the dark matter are allowed. If these coupling restrictions
are not obeyed, unphysical results may be obtained such as a violation of
unitarity at high energies. These two-mediator scenarios lead to important
phenomenology that does not arise in single mediator models. We survey
two-mediator dark matter models which contain both vector and scalar mediators,
and explore their relic density and indirect detection phenomenology.
