Subleading Effects and the Field Range in Axion Inflation

TL;DR

This paper demonstrates that non-perturbative corrections to axion inflation potentials can increase the number of e-folds, allowing models with smaller field ranges to align with observations and produce distinctive signatures.

Contribution

It shows that subleading non-perturbative effects can modify axion inflation potentials to achieve sufficient inflation with sub-Planckian decay constants.

Findings

Enhanced e-folds from potential corrections

Natural inflation with sub-Planckian decay constant possible

Large negative running of the spectral index

Abstract

An attractive candidate for the inflaton is an axion slowly rolling down a flat potential protected by a perturbative shift symmetry. Realisations of this idea within large field, natural and monomial inflation have been disfavoured by observations and are difficult to embed in string theory. We show that subleading, but significant non-perturbative corrections can superimpose sharp cliffs and gentle plateaus into the potential, whose overall effect is to enhance the number of e-folds of inflation. Sufficient e-folds are therefore achieved for smaller field ranges compared to the potential without such corrections. Thus, both single-field natural and monomial inflation in UV complete theories like string theory, can be restored into the favour of current observations, with distinctive signatures. Tensor modes result un-observably small, but there is a large negative running of the spectral index. Remarkably, natural inflation can be achieved with a single field whose axion decay constant is sub-Planckian.