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

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 multibody 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. arXiv:1810.09455v1 [astro-ph.CO] 22 Oct 2018 m 2 Pl ρ r ∼ 1 m 2 Pl g * T 4 , whereρ r = (π 2 /30)g * T 4 is the radiation energy density (g * counts the number of relativistic degrees of freedom as usual), i.e. H ∼ g 1/2 * T 2 /m Pl . Thus the criterion for freezeout becomes (g * )

Unitarity limits on thermal dark matter in (non-)standard cosmologies

“…However, the maximum values of the inelastic k → 2 reaction rates obtained in refs. [29,30] do not agree with our results in the next section, as the authors in refs. [29,30] did not maximize the inelastic rate while taking into account the fact that for a non-zero inelastic reaction rate, the elastic scattering cross-section is always non-zero as well [9,17,31].…”

“…[29,30] do not agree with our results in the next section, as the authors in refs. [29,30] did not maximize the inelastic rate while taking into account the fact that for a non-zero inelastic reaction rate, the elastic scattering cross-section is always non-zero as well [9,17,31]. As seen in this section, our derivation of the maximum rate of inelastic reaction cross-sections uses only the optical theorem and the matrix element and cross-section for the relevant 2 → 2 elastic scattering process.…”

“…[9], but agrees with the appearance of this extra symmetry factor (of 2, for 2 identical particles in the initial state) with ref. [29]. However, the maximum values of the inelastic k → 2 reaction rates obtained in refs.…”

“…(3.11), (3.12) and (3.13) do not agree with the results in refs. [29,30]. For identical particles in the initial state, we found in eq.…”

Boosting assisted annihilation for a cosmologically safe MeV scale dark matter

“…An Nð≥3Þ → 2 process driving thermal freeze-out can naturally lead to LDM [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. This type of annihilation process can be relevant if (i) the two-body cross section is forbidden or strongly suppressed, (ii) densities of the annihilating species are appreciable, and (iii) the velocity of the interacting particles are low [24]. One novel example of this class of models is the assisted annihilation, where, along with DM particles, there are standard model (SM)like assisters in the initial state facilitating the annihilation of LDM [8].…”