On the Possibility of Large Axion Decay Constants

TL;DR

This paper surveys string theory models to explore the possibility of achieving large axion decay constants, but finds that effective actions limit their inflationary potential, making them no better than Planck-scale axions.

Contribution

It provides a comprehensive survey of string theory axion-like fields and analyzes their decay constants, highlighting limitations for inflationary applications.

Findings

Decay constants can be larger than the Planck scale in some models

Effective actions include significant harmonics, limiting inflation potential

Large decay constants do not necessarily improve inflaton candidacy

Abstract

The decay constant of the QCD axion is required by observation to be small compared to the Planck scale. In theories of "natural inflation," and certain proposed anthropic solutions of the cosmological constant problem, it would be interesting to obtain a large decay constant for axion-like fields from microscopic physics. String theory is the only context in which one can sensibly address this question. Here we survey a number of periodic fields in string theory in a variety of string vacua. In some examples, the decay constant can be parameterically larger than the Planck scale but the effective action then contains appreciable harmonics of order $f_A/M_p$. As a result, these fields are no better inflaton candidates than Planck scale axions.