# Simplified models of dark matter with a long-lived co-annihilation partner

## TL;DR

This paper introduces simplified models for dark matter with a long-lived co-annihilation partner, focusing on 3-point interactions with the tau lepton, and explores their collider detection prospects and theoretical frameworks.

## Contribution

It develops new simplified models incorporating 3-point interactions and long-lived co-annihilation partners, providing a novel approach to dark matter searches at the LHC.

## Key findings

- Long-lived charged particles can be detected via anomalous charged tracks at the LHC.
- Co-annihilation channels significantly influence relic abundance calculations.
- Models are compatible with supersymmetry, extra dimensions, or composite frameworks.

## Abstract

We introduce a new set of simplified models to address the effects of 3-point interactions between the dark matter particle, its dark co-annihilation partner, and the Standard Model degree of freedom, which we take to be the tau lepton. The contributions from dark matter co-annihilation channels are highly relevant for a determination of the correct relic abundance. We investigate these effects as well as the discovery potential for dark matter co-annihilation partners at the LHC. A small mass splitting between the dark matter and its partner is preferred by the co-annihilation mechanism and suggests that the co-annihilation partners may be long-lived (stable or meta-stable) at collider scales. It is argued that such long-lived electrically charged particles can be looked for at the LHC in searches of anomalous charged tracks. This approach and the underlying models provide an alternative/complementarity to the mono-jet and multi-jet based dark matter searches widely used in the context of simplified models with the s-channel mediators. We consider four types of simplified models with different particle spins and coupling structures. Some of these models are manifestly gauge invariant and renormalizable, others would ultimately require a UV completion. These can be realised in terms of supersymmetric models in the neutralino-stau co-annihilation regime, as well as models with extra dimensions or composite models.