Nonminimal Couplings and the Forgotten Field of Axion Inflation
  We study the multifield dynamics of axion models nonminimally coupled to
gravity. As usual, we consider a canonical $U(1)$ symmetry-breaking model in
which the axion is the phase of a complex scalar field. If the complex scalar
field has a nonminimal coupling to gravity, then the (oft-forgotten) radial
component can drive a phase of inflation prior to an inflationary phase driven
by the axion field. In this setup, the mass of the axion field is dependent on
the radial field because of the nonminimal coupling, and the axion remains
extremely light during the phase of radial inflation. As the radial field
approaches the minimum of its potential, there is a transition to natural
inflation in the angular direction. In the language of multifield inflation,
this system exhibits ultra-light isocurvature perturbations, which are
converted to adiabatic perturbations at a fast turn, namely the onset of axion
inflation. For models wherein the CMB pivot scale exited the horizon during
radial inflation, this acts to suppresses the tensor-to-scalar ratio $r$,
without generating CMB non-Gaussianity or observable isocurvature
perturbations. Finally, we note that the interaction strength between axion and
gauge fields is suppressed during the radial phase relative to its value during
the axion inflation phase by several orders of magnitude. This decouples the
constraints on the inflationary production of gauge fields (e.g., from
primordial black holes) from the constraints on their production during
(p)reheating.
