$Z_2$ SIMP Dark Matter

TL;DR

This paper explores a $Z_2$ symmetric SIMP dark matter model where 4-to-2 annihilation processes dominate relic density production, with a cold dark sector achieved through a freeze-in portal, exemplified by the Singlet Scalar model.

Contribution

It introduces a novel scenario for $Z_2$ symmetric SIMP dark matter where 4-to-2 processes dominate, and the dark sector remains colder via a freeze-in portal, exemplified by the Singlet Scalar model.

Findings

4-to-2 annihilation processes can produce the correct relic density.

A small portal can dynamically generate a colder dark sector.

The Singlet Scalar model exemplifies this scenario.

Abstract

Dark matter with strong self-interactions provides a compelling solution to several small-scale structure puzzles. Under the assumption that the coupling between dark matter and the Standard Model particles is suppressed, such strongly interacting massive particles (SIMPs) allow for a successful thermal freeze-out through N-to-N' processes, where N dark matter particles annihilate to N' of them. In the most common scenarios, where dark matter stability is guaranteed by a $\mathbb{Z}_2$ symmetry, the seemingly leading annihilating channel, i.e. 3-to-2 process, is forbidden, so the 4-to-2 one dominate the production of the dark matter relic density. Moreover, cosmological observations require that the dark matter sector is colder than the thermal bath of Standard Model particles, a condition that can be dynamically generated via a small portal between dark matter and Standard Model particles, \`a la freeze-in. This scenario is exemplified in the context of the Singlet Scalar dark matter model.