The two-field regime of natural inflation
  The simplest two-field completion of natural inflation has a regime in which
both fields are active and in which its predictions are within the Planck
1-$\sigma$ confidence contour. We show this for the original model of natural
inflation, in which inflation is achieved through the explicit breaking of a
U(1) symmetry. We consider the case in which the mass coming from explicit
breaking of this symmetry is comparable to that from spontaneous breaking,
which we show is consistent with a hierarchy between the corresponding energy
scales. While both masses are comparable when the observable modes left the
horizon, the mass hierarchy is restored in the last e-foldings of inflation,
rendering the predictions consistent with the isocurvature bounds. For
completeness, we also study the predictions for the case in which there is a
large hierarchy of masses and an initial period of inflation driven by the
(heavy) radial field.
