Gamma-ray Signals from Dark Matter Annihilation Via Charged Mediators

Understand

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.