# Resonant Assisted Annihilation

## TL;DR

This paper investigates how resonant effects can enhance assisted annihilation processes to produce viable sub-GeV dark matter, with implications for detection constraints and relic density.

## Contribution

It introduces a resonant mechanism for $3 o 2$ assisted annihilation in a scalar extension of the Standard Model, enabling viable dark matter relic density.

## Key findings

- Resonant assisted annihilation can dominate dark matter freeze-out.
- Certain parameter regions are constrained by indirect detection and CMB data.
- Substantial parameter space remains unconstrained.

## Abstract

Assisted annihilation is a novel mechanism to generate viable sub-GeV thermal dark matter, where a pair of stable dark matter annihilates with an assister to Standard Model states. Typically such $3 \to 2$ annihilation topologies are flux suppressed compared to $2 \to 2$ processes. In this paper, we explore the possibility of a resonant $3 \to 2$ assisted annihilation dominantly driving the freeze-out of dark matter. We demonstrate that in a simple multipartite scalar extension of the Standard Model this can be realized in certain regions of parameter space to provide viable dark matter relic density, in agreement with observation. We demonstrate that for photophilic assisters parts of the parameter space are already constrained by indirect detection experiments and the measurements of CMB anisotropies while substantial regions remain beyond the present limit.