Gamma-ray signals from dark matter annihilation via charged mediators

Abstract

We consider a simplified model in which Majorana fermion dark matter annihilates to charged fermions through the exchange of charged mediators. We consider the gamma-ray signals arising from the processes $XX\ensuremath{\rightarrow}\overline{f}f\ensuremath{\gamma}$, $\ensuremath{\gamma}\ensuremath{\gamma}$, and $\ensuremath{\gamma}Z$ in the most general case, including nontrivial fermion mass and nontrivial left-right mixing and the $CP$-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 $\ensuremath{\sigma}(\ensuremath{\gamma}\ensuremath{\gamma})/\ensuremath{\sigma}(\ensuremath{\gamma}Z)$ and the helicity asymmetry in the $XX\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$ process, each as a function of the mixing angle and $CP$-violating phase. As an application, we apply these results to searches for a class of minimal supersymmetric Standard Model models.