Stochastic Axion Mixing: A General Mechanism Beyond Decay Constant Constraints

TL;DR

This paper introduces stochastic axion mixing, a broad mechanism in multi-axion models that operates independently of decay constant hierarchies, impacting axion cosmology.

Contribution

It generalizes axion mixing beyond decay constant constraints, showing maximal mixing as a special case within this broader framework.

Findings

Stochastic mixing occurs naturally with distinct, ultra-light ALPs lighter than the QCD axion.

Maximal mixing is a specific subset of stochastic mixing under certain conditions.

The mechanism has significant implications for axion cosmology.

Abstract

We propose a novel and generalized mechanism, dubbed stochastic axion mixing. In a multi-axion framework, this mixing occurs naturally provided that the masses of all ultra-light axion-like particles (ALPs) are distinct and lighter than the zero-temperature mass of the QCD axion. Crucially, this mechanism is independent of the relative magnitudes of the axion decay constants. In contrast to the conventional maximal mixing scenario -- which strictly relies on specific decay constant hierarchies -- stochastic mixing represents a significantly broader formalism. Notably, maximal mixing emerges as a specific subset of stochastic mixing under restrictive conditions. This new mechanism offers profound implications for axion cosmology.