Resonant conversions of QCD axions into hidden axions and suppressed isocurvature perturbations

TL;DR

This paper investigates how resonant conversions between QCD axions and hidden axions can suppress axion abundance and isocurvature perturbations, potentially relaxing constraints on inflation models.

Contribution

It provides a detailed analysis of MSW-like resonant conversions, including marginal cases and anharmonic effects, revealing their impact on axion abundance and isocurvature bounds.

Findings

Resonant conversion can significantly suppress QCD axion abundance.

Incomplete conversion can reduce isocurvature perturbations without affecting non-Gaussianity.

Partial conversion relaxes inflation scale constraints from isocurvature bounds.

Abstract

We study in detail MSW-like resonant conversions of QCD axions into hidden axions, including cases where the adiabaticity condition is only marginally satisfied, and where anharmonic effects are non-negligible. When the resonant conversion is efficient, the QCD axion abundance is suppressed by the hidden and QCD axion mass ratio. We find that, when the resonant conversion is incomplete due to a weak violation of the adiabaticity, the CDM isocurvature perturbations can be significantly suppressed, while non-Gaussianity of the isocurvature perturbations generically remain unsuppressed. The isocurvature bounds on the inflation scale can therefore be relaxed by the partial resonant conversion of the QCD axions into hidden axions.