Enhanced n-body annihilation of dark matter and its indirect signatures

TL;DR

This paper explores multi-body dark matter annihilation processes, which can be significantly enhanced at low velocities, potentially producing observable signals in the cosmic microwave background and astrophysical sources, with implications for indirect detection.

Contribution

It introduces the concept of multi-body dark matter annihilation with non-standard density scaling, analyzing its effects on cosmological signals and detection prospects, including a specific model with dominant 3-body annihilation.

Findings

Multi-body annihilation can be strongly velocity-enhanced without violating unitarity.

Signals from such processes can originate from the end of the cosmic dark ages.

Detection may be possible from dense dark matter clumps in the Milky Way.

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.