Gamma-ray Signals from Dark Matter Annihilation Via Charged Mediators
  We consider a simplified model in which Majorana fermion dark matter
annihilates to charged fermions through exchange of charged mediators. We
consider the gamma-ray signals arising from the processes $XX \rightarrow \bar
f f \gamma$, $\gamma \gamma$, and $\gamma Z$ in the most general case,
including non-trivial fermion mass and non-trivial left-right mixing and
$CP$-violating phase for the charged mediators. In particular, we find the most
general spectrum for internal bremsstrahlung, which interpolates between the
regimes dominated by virtual internal bremsstrahlung and by final state
radiation. We also examine the variation in the ratio $\sigma(\gamma \gamma) /
\sigma (\gamma Z)$ and the helicity asymmetry in the $XX \rightarrow \gamma
\gamma$ process, each as a function of mixing angle and $CP$-violating phase.
As an application, we apply these results to searches for a class of MSSM
models.
