Axions In String Theory

TL;DR

This paper reviews axions in string theory, highlighting their potential to solve the strong CP problem and analyzing their coupling scales across various models, including GUT-like and Standard Model scenarios.

Contribution

It provides a comprehensive re-examination of axion coupling parameters in string models, identifying conditions under which F_a aligns with or falls below the GUT scale.

Findings

F_a is near or above the GUT scale in many models with GUT-like phenomenology.

In some models with Standard Model gauge fields on vanishing cycles, F_a can be below the GUT scale.

String theory axions remain plausible solutions to the strong CP problem across various model classes.

Abstract

In the context of string theory, axions appear to provide the most plausible solution of the strong CP problem. However, as has been known for a long time, in many string-based models, the axion coupling parameter F_a is several orders of magnitude higher than the standard cosmological bounds. We re-examine this problem in a variety of models, showing that F_a is close to the GUT scale or above in many models that have GUT-like phenomenology, as well as some that do not. On the other hand, in some models with Standard Model gauge fields supported on vanishing cycles, it is possible for F_a to be well below the GUT scale.