Axion inflation with cross-correlated axion isocurvature perturbations
  We study the inflation scenarios, in the framework of superstring theory,
where the inflaton is an axion producing the adiabatic curvature perturbations
while there exists another light axion producing the isocurvature
perturbations. We discuss how the non-trivial couplings among string axions can
generically arise, and calculate the consequent cross-correlations between the
adiabatic and isocurvature modes through concrete examples. Based on the Planck
analysis on the generally correlated isocurvature perturbations, we show that
there is a preference for the existence of the correlated isocurvature modes
for the axion monodromy inflation while the natural inflation disfavors such
isocurvature modes.
