# Decay of Ultralight Axion Condensates

## TL;DR

This paper studies how axion self-interactions affect the stability, mass, and size of galactic-scale condensates, revealing a lower bound on stable fuzzy dark matter structures with lifetimes exceeding the universe's age.

## Contribution

It extends previous non-interacting models by including interactions, analyzing stability, and establishing bounds on the size and lifetime of stable axion condensates.

## Key findings

- Self-interactions reduce maximum condensate mass.
- Stable condensates have a minimum size bound.
- Lifetimes of stable condensates exceed the age of the universe.

## Abstract

Axion particles can form macroscopic condensates, whose size can be galactic in scale for models with very small axion masses $m\sim10^{-22}$ eV, and which are sometimes referred to under the name of Fuzzy Dark Matter. Many analyses of these condensates are done in the non-interacting limit, due to the weakness of the self-interaction coupling of axions. We investigate here how certain results change upon inclusion of these interactions, finding a decreased maximum mass and a modified mass-radius relationship. Further, these condensates are, in general, unstable to decay through number-changing interactions. We analyze the stability of galaxy-sized condensates of axion-like particles, and sketch the parameter space of stable configurations as a function of a binding energy parameter. We find a strong lower bound on the size of Fuzzy Dark Matter condensates which are stable to decay, with lifetimes longer than the age of the universe.