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