Dark Forces in the Sky: Signals from Z' and the Dark Higgs
  We consider the indirect detection signals for a self-consistent hidden
$U(1)$ model containing a Majorana dark matter candidate, $\chi$, a dark gauge
boson, $Z'$, and a dark Higgs, $s$. Compared with a model containing only a
dark matter candidate and $Z'$ mediator, the addition of the scalar provides a
mass generation mechanism for the dark sector particles and is required in
order to avoid unitarity violation at high energies. We find that the inclusion
of the two mediators opens up a new two-body s-wave annihilation channel,
$\chi\overline\chi\rightarrow sZ'$. This new process, which is missed in the
usual single-mediator simplified model approach, can be the dominant
annihilation channel. This provides rich phenomenology for indirect detection
searches, allows indirect searches to explore regions of parameter space not
accessible with other commonly considered s-wave annihilation processes, and
enables both the $Z'$ and scalar couplings to be probed. We examine the
phenomenology of the sector with a focus on this new process, and determine the
limits on the model parameter space from Fermi data on dwarf spheriodal
galaxies and other relevant experiments.
