Enabling forbidden dark matter

Abstract

The thermal relic density of dark matter is conventionally set by two-body annihilations. We point out that in many simple models, 3 → 2 annihilations can play an important role in determining the relic density over a broad range of model parameters. This occurs when the two-body annihilation is kinematically forbidden, but the 3 → 2 process is allowed; we call this scenario not-forbidden dark matter. We illustrate this mechanism for a vector-portal dark matter model, showing that for a dark matter mass of m χ ∼ MeV-10 GeV, 3 → 2 processes not only lead to the observed relic density, but also imply a selfinteraction cross section that can solve the cusp/core problem. This can be accomplished while remaining consistent with stringent CMB constraints on light dark matter, and can potentially be discovered at future direct detection experiments.

Hunting for light dark matter with DUNE PRISM

“…In the right panel, we do not show the thermal relic abundance line below M A = 2M φ ; for M A < M φ , freeze-out occurs via the process φφ † → A A , independent of ε 2 . For M φ < M A < 2M φ , this process still contributes to freeze-out due to non-zero temperature effects [43,44], and a thermal relic target would require much smaller ε 2 than what we display here.…”

“…where m f stands for the mass of the SM fermion in the final state. The requirement that φ (χ) comprises ‡ In case the DM mass is below the A mass but not too much, the thermal DM distribution may still allow it to annihilate to A A , accounting for the correct relic abundance [43,44]. We do not investigate this scenario.…”

Fourth Exception in the Calculation of Relic Abundances

“…[5], we assume DM begins in thermal equilibrium, has its number diluted through 2-to-2 annihilations, and has a temperature that tracks the photon temperature (for studies that relax at least one of these assumptions see for example Refs. [14][15][16][17][18][19][20][21][22][23][24][25][26][27]). We consider the presence of two states charged under the symmetry that stabilizes DM: χ and ψ, where m χ < m ψ and χ is DM.…”

Cosmic abundances of SIMP dark matter

“…For instance, in the case of fermionic SIMP, the initial states in the 3 → 2 process can be all fermions as discussed in refs. [15,16] while the case of vector SIMP was discussed [6] or will be published elsewhere [7].…”

“…The results on thermal average can be generalized to the case with non-degenerate masses in the initial states of the 3 → 2 process [15,16], namely, the co-annihilation between multiple components of dark matter. In this case, we consider the momenta p i (i = 1, 2, 3) instead of velocities v i (i = 1, 2, 3) in the integration and the velocity expansion of the 3 → 2 annihilation cross section.…”