LAUSR

We consider dark matter as Strongly Interacting Massive Particles (SIMPs) in a hidden sector, thermally decoupled from the Standard Model heat bath. Due to its strong interactions, the number changing processes of the SIMP lead to its thermalisation at temperature $T_{\rm{D}}$ different from the visible sector temperature $T$, and subsequent decoupling as the Universe expands. We study the evolution of the dark SIMP abundance in detail and find that a hidden SIMP provides for a consistent framework for self-interacting dark matter. Thermalisation and decoupling of a composite SIMP can be treated within the domain of validity of chiral perturbation theory unlike the simplest realisations of the SIMP, where the SIMP is in thermal equlibrium with the Standard Model.