Completing Natural Inflation
Jihn E. Kim, Hans Peter Nilles, Marco Peloso
TL;DR
The paper tackles the challenge that natural inflation typically requires a super-Planckian axion decay constant, which may clash with effective field theory and quantum gravity. It introduces a two-axion mechanism with two anomalous gauge groups that, when their couplings are nearly equal, creates a flat direction with an effectively large decay constant despite sub-Planckian fundamental scales. The authors derive that the orthogonal axion mode ξ gains a large effective decay constant $f_ξ = \frac{g_2 \sqrt{f^2+g_1^2}}{|g_1 - g_2|}$, while the heavier mode decouples, allowing ξ to drive inflation through the second potential term. This approach potentially resolves the quantum-gravity and EFT concerns for natural inflation and may be realizable in heterotic string theory.
Abstract
If the inflaton is a pseudo-scalar axion, the axion shift symmetry can protect the flatness of its potential from too large radiative corrections. This possibility, known as natural inflation, requires an axion scale which is greater than the (reduced) Planck scale. It is unclear whether such a high value is compatible with an effective field theoretical description, and if the global axionic symmetry survives quantum gravity effects. We propose a mechanism which provides an effective large axion scale, although the original one is sub-Planckian. The mechanism is based on the presence of two axions, with a potential provided by two anomalous gauge groups. The effective large axion scale is due to an almost exact symmetry between the couplings of the axions to the anomalous groups. We also comment on a possible implementation in heterotic string theory.