Abstract
We examine the possible indirect signatures of dark matter annihilation processes with a non-standard scaling with the dark matter density, and in particular the case where more than two dark matter particles participate in the annihilation process. We point out that such processes can be strongly enhanced at low velocities without violating unitarity, similar to Sommerfeld enhancement in the standard case of two-body annihilation, potentially leading to visible signals in indirect searches. We study in detail the impact of such multi-body annihilations on the ionization history of the universe and consequently the cosmic microwave background, and find that unlike in the two-body case, the dominant signal can naturally arise from the end of the cosmic dark ages, after the onset of structure formation. We examine the complementary constraints from the Galactic Center, Galactic halo, and galaxy clusters, and outline the circumstances under which each search would give rise to the strongest constraints. We also show that if there is a population of ultra-compact dense dark matter clumps present in the Milky Way with sufficiently steep density profile, then it might be possible to detect point sources illuminated by multi-body annihilation, even if there is no large low-velocity enhancement. Finally, we provide a case study of a model where 3-body annihilation dominates the freezeout process, and in particular the resonant regime where a large low-velocity enhancement is naturally generated.
Highlights
Indirect detection signals could be present if the final state of the annihilation process involves Standard Model (SM) particles, or unstable particles which subsequently decay back to the SM; this feature is not present in all models with higher-body annihilation processes, but even when it is, the conventional wisdom is that these signals will always be unobservably tiny long after freezeout, since they are suppressed by an extra power of the dark matter (DM) density compared to the usual case of 2-body annihilation
We have explored the possible indirect-detection signatures of n-body DM annihilation processes that lead to the production of SM particles, either directly or through a decaying mediator
We have argued that such processes can be more strongly enhanced at low velocities than standard 2-body annihilation without violating partial-wave unitarity constraints; if the DM follows an approximately Maxwell-Boltzmann distribution, the unitarity-saturating temperature scaling is σvn−1 ∝ T (5−3n)/2
Summary
In recent years there has been great interest in dark matter (DM) models with modified thermal histories, where DM annihilation is still responsible for setting the late-time abundance of DM, but the annihilation involves three or more DM (or DM-like) particles (e.g. [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]). We will explore these three caveats, to address the question of whether it could ever be possible to detect SM particles produced by n-body annihilation in such models, with n > 2, in cases where (at least some of) the immediate products of annihilation decay back to the SM if they are not SM particles themselves
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
