't Hooft mechanism, anomalous gauge U(1), and "invisible" axion from string

TL;DR

This paper discusses the 'invisible' axion as a natural solution to the strong CP problem, exploring its theoretical origins, cosmological constraints, and the role of anomalous gauge U(1) symmetries in string theory.

Contribution

It proposes a connection between the 'invisible' axion, the 't Hooft mechanism, and anomalous gauge U(1) symmetries within string theory frameworks.

Findings

The 'invisible' axion remains a viable solution within the narrow window.

Global symmetry breaking and gravity effects are critical in axion viability.

Cosmological constraints, including domain wall issues, are addressed.

Abstract

Among solutions of the strong CP problem, the "invisible" axion in the narrow axion window is argued to be the remaining possibility among natural solutions on the smallness of $\bar{\theta}$. Related to the gravity spoil of global symmetries, some prospective "invisible" axions from theory point of view are discussed. In all these discussions including the observational possibility and cosmological constraints, including a safe domain wall problem, must be included.