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

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.

18 pages, 4 figures, v2: typos corrected, conclusions unchanged

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• An M Theory Solution to the Strong CP Problem and Constraints on the Axiverse
• The Need for Purely Laboratory-Based Axion-Like Particle Searches
• Update of axion CDM energy density
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• Induced Domain Walls of QCD Axion, and Gravitational Waves
• Cosmological Perturbations of Axion with a Dynamical Decay Constant
• Phenomenology of the companion-axion model: photon couplings
• Resolving the QCD Axion Domain Wall Problem with a Light Axion
• ALP Anarchy
• Cosmology of the companion-axion model: dark matter, gravitational waves, and primordial black holes
• Gravitational redshift from large-scale structure: nonlinearities, antisymmetries, and the dipole
• An Analytic Treatment of Underdamped Axionic Blue Isocurvature Perturbations