Enhancing Dark Matter Annihilation Rates with Dark Bremsstrahlung

Understand

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 $\overline{\chi}\chi\rightarrow \overline{f}f$.