coannihilation

Coannihilation is a concept in particle physics and cosmology describing how the present-day abundance of a dark matter candidate is influenced by annihilations not only of the dark matter particle with itself, but also by interactions with other, nearly degenerate species in the early universe.

When several particles χ_i with masses m_i close to the dark matter mass m_χ are in thermal equilibrium in the early universe, the species’ number densities n_i follow Boltzmann distributions. If the mass splittings Δ_i ≈ (m_i − m_χ)/m_χ are small, the partner particles remain sufficiently abundant around freeze-out to participate in annihilations of the form χ_i χ_j → Standard Model particles. The overall depletion rate is captured by an effective annihilation cross-section, σ_eff v, which sums over all relevant species and channels with weights determined by their equilibrium abundances. The evolution of the total number density is described by a single Boltzmann equation using σ_eff; freeze-out occurs when the expansion rate of the universe, the Hubble rate, exceeds the interaction rate, and the final relic density reflects the combined depletion from all coannihilating channels.

Coannihilation can significantly alter relic densities, opening or closing regions of model parameter space. It is especially important in supersymmetric theories where the lightest neutralino is near-degenerate with sleptons or charginos, and in models with extra dimensions where Kaluza-Klein partners participate. Observationally, precise measurements of the dark matter relic density constrain such scenarios. Computational tools routinely compute σ_eff and solve the coupled Boltzmann equations to predict the relic abundance Ω_DM h^2.