Resonant Landau-Zener Conversion In Multi-Axion Systems

TL;DR

This paper analytically studies the cosmological evolution of multi-axion systems, focusing on resonant conversions during the Universe's cooling, and explores implications for axion detection experiments.

Contribution

It introduces an analytic approach using Landau-Zener formalism to describe non-adiabatic axion conversions in multi-axion systems, applicable to arbitrary two-state mass matrices.

Findings

Landau-Zener accurately models non-adiabatic axion conversions.

Analytic relic abundance predictions for axion fields.

Mixing of QCD axion with axion-like particles affects detection prospects.

Abstract

Multiple axions may emerge in the low-energy effective theory of Nature. Generically, the potentials describing these axion fields are non-diagonal, leading to mass mixing between axion states which can be temperature-dependent due to QCD instanton effects. As the temperature of the Universe drops, level crossing can occur, causing resonant conversion between axion states. In this work, we present an analytic study of the cosmological evolution of multi-axion systems including adiabatic and non-adiabatic resonant conversion from one axion state into another during the misalignment process. We show how the Landau-Zener formalism accurately captures the non-adiabatic resonant conversion, permitting an analytic description of the relic abundances of each axion field for nearly any arbitrary two-state axion mass matrix. As an application, we study the mixing of a QCD axion with an axion-like-particle for specific potentials to identify the predictions for haloscope experiments. We conclude that the detection of an axion off the expected QCD mass-coupling line predicts other haloscope targets if it mixes with the QCD axion.