Sub-Leading Effects and the Field Range in Axion Inflation
An attractive inflaton candidate is an axion slowly rolling down a flat potential, protected by a perturbative shift symmetry. Realisations of axion inflation within natural and monomial inflation have been recently disfavoured by observations and are difficult to embed in string theory.
We show that sub-leading, but significant non-perturbative corrections to the axion potential can superimpose sharp cliffs and gentle plateaus into the potential, whose overall effect is to enhance the number of e-folds of inflation.
Sufficient inflation is therefore achieved for smaller field ranges compared to the potential without such corrections and the
tensor-to-scalar ratio results un-observably small, but there is a large negative running of the spectral index.
This brings both single-field chaotic and natural inflation in UV complete theories like string theory, back into the favoured region of current observations with distinctive signatures. In passing, we derive an upper bound on the tensor-to-scalar ratio $r$, for models of inflation in perturbative string theory