CP-Conservation in QCD and why only "invisible" Axions work

TL;DR

This paper discusses why only 'invisible' axions are viable solutions to the strong CP problem, considering theoretical, observational, and cosmological constraints, and addresses issues related to gravity's effect on global symmetries.

Contribution

It provides a comprehensive argument for the uniqueness of 'invisible' axions among natural solutions to the strong CP problem, including theoretical and cosmological considerations.

Findings

'Invisible' axions are the only natural solutions consistent with current constraints.

Gravity's effect on global symmetries challenges other axion models.

Cosmological constraints favor the narrow axion window.

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, cosmological constraints must be included.