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Many dark matter interaction types lead to annihilation processes whichsuffer from p-wave suppression or helicity suppression, rendering themsubdominant to unsuppressed s-wave processes. We demonstrate that the naturalinclusion of dark initial state radiation can open an unsuppressed s-waveannihilation channel, and thus provide the dominant dark matter annihilationprocess for particular interaction types. We illustrate this effect with thebremsstrahlung of a dark spin-0 or dark spin-1 particle from fermionic darkmatter, $\overline{\chi}\chi\rightarrow \overline{f}f\phi$ or$\overline{f}fZ'$. The dark initial state radiation process, despite having a3-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 StandardModel final state radiation, or virtual internal bremsstrahlung. This darkbremsstrahlung process should influence LHC and indirect detection searches fordark matter.

Enhancing Dark Matter Annihilation Rates with Dark Bremsstrahlung