# WIMPs with GUTs: Dark Matter Coannihilation with a Lighter Species

## TL;DR

This paper introduces a novel thermal freeze-out mechanism for ultra-heavy dark matter involving coannihilation with a lighter unstable species, leading to unique late decay phenomenology consistent with astrophysical constraints.

## Contribution

It proposes a new coannihilation-based freeze-out process for ultra-heavy dark matter linked to GUT-scale physics, expanding dark matter model possibilities.

## Key findings

- Exponential enhancement of annihilation rate due to coannihilation.
- Long-lived unstable species necessary for consistency with observations.
- Potential for observable late decays of ultra-heavy dark matter.

## Abstract

We propose a new thermal freeze-out mechanism for ultra-heavy dark matter. Dark matter coannihilates with a lighter unstable species, leading to an annihilation rate that is exponentially enhanced relative to standard WIMPs. This scenario destabilizes any potential dark matter candidate. In order to remain consistent with astrophysical observations, our proposal necessitates very long-lived states, motivating striking phenomenology associated with the late decays of ultra-heavy dark matter, potentially as massive as the scale of grand unified theories, $M_\text{GUT} \sim 10^{16}$ GeV.