Enhancing Dark Matter Annihilation Rates with Dark Bremsstrahlung
  Many dark matter interaction types lead to annihilation processes which
suffer from p-wave suppression or helicity suppression, rendering them
subdominant to unsuppressed s-wave processes. We demonstrate that the natural
inclusion of dark initial state radiation can open an unsuppressed s-wave
annihilation channel, and thus provide the dominant dark matter annihilation
process for particular interaction types. We illustrate this effect with the
bremsstrahlung of a dark spin-0 or dark spin-1 particle from fermionic dark
matter, $\overline{\chi}\chi\rightarrow \overline{f}f\phi$ or
$\overline{f}fZ'$. The dark initial state radiation process, despite having a
3-body final state, proceeds at the same order in the new physics scale
$\Lambda$ as the annihilation to the 2-body final state
$\overline{\chi}\chi\rightarrow \overline{f}f$. This is lower order in
$\Lambda$ than the well-studied lifting of helicity suppression via Standard
Model final state radiation, or virtual internal bremsstrahlung. This dark
bremsstrahlung process should influence LHC and indirect detection searches for
dark matter.
