Enhanced photon coupling of ALP dark matter adiabatically converted from the QCD axion

TL;DR

This paper explores how adiabatic conversion between the QCD axion and ALP in the early universe can enhance ALP-photon coupling, making ALP detection more feasible in future experiments.

Contribution

It refines the conditions for adiabatic conversion and shows that the ALP produced can have a smaller decay constant with enhanced photon coupling.

Findings

ALP-photon coupling can be enhanced by a few orders of magnitude.

The ALP decay constant can be significantly smaller than in the standard realignment scenario.

Enhanced coupling improves prospects for future ALP detection experiments.

Abstract

We revisit the adiabatic conversion between the QCD axion and axion-like particle (ALP) at level crossing, which can occur in the early universe as a result of the existence of a hypothetical mass mixing. This is similar to the Mikheyev-Smirnov-Wolfenstein effect in neutrino oscillations. After refining the conditions for the adiabatic conversion to occur, we focus on a scenario where the ALP produced by the adiabatic conversion of the QCD axion explains the observed dark matter abundance. Interestingly, we find that the ALP decay constant can be much smaller than the ordinary case in which the ALP is produced by the realignment mechanism. As a consequence, the ALP-photon coupling is enhanced by a few orders of magnitude, which is advantageous for the future ALP and axion-search experiments using the ALP-photon coupling.