SIMPs

SIMPs, or strongly interacting massive particles, are dark matter candidates whose relic abundance is set primarily by self-interactions within a dark sector, rather than by annihilation into Standard Model particles.

In the canonical SIMP framework, 3→2 processes among dark matter particles reduce the number density in the early universe. The dark sector often has its own temperature, T', possibly distinct from the photon bath, and a light mediator to maintain kinetic contact with the Standard Model as needed. Freeze-out occurs when the 3→2 rate falls below the Hubble rate, fixing the relic abundance.

A key motivation for SIMPs is to generate sizeable self-interactions that can address small-scale structure issues, requiring a self-scattering cross section per mass of order 0.1–10 cm^2/g at dwarf-galaxy scales. This can be achieved with light mediators, but must respect constraints from clusters, cosmic microwave background, and structure formation.

Model-building commonly features a stable dark matter particle coupled to a light mediator (for example a dark photon or a light scalar) that mediates 3→2 dynamics and, via a portal coupling, equilibrates with or decays to the Standard Model. Direct detection signals are typically suppressed, while collider and astrophysical probes can test the parameter space.

Compared with WIMPs, SIMPs emphasize the role of dark-sector dynamics rather than 2→2 annihilations to the visible sector; viability depends on satisfying relic density, self-interaction, and cosmological constraints.