# Hidden strongly interacting massive particles

## TL;DR

This paper explores hidden sector SIMPs as a viable form of self-interacting dark matter, analyzing their thermalization and decoupling processes, and demonstrating their consistency within chiral perturbation theory.

## Contribution

It introduces a detailed study of hidden sector SIMPs, showing they can thermalize and decouple independently of the Standard Model, unlike simpler models.

## Key findings

- Hidden SIMPs can thermalize at a different temperature from the visible sector.
- Decoupling occurs as the Universe expands, consistent with cosmological evolution.
- Chiral perturbation theory effectively describes the thermalization of composite SIMPs.

## Abstract

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 thermalization 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. Thermalization and decoupling of a composite SIMP can be treated within the domain of validity of chiral perturbation theory unlike the simplest realizations of the SIMP, where the SIMP is in thermal equilibrium with the Standard Model.