The Axion Quality Problem: Global Symmetry Breaking and Wormholes

TL;DR

This paper investigates how wormholes in gravity can break global symmetries, revealing that axions face a quality problem in both Einstein gravity and certain string theory models, impacting their role in solving the strong CP problem.

Contribution

It demonstrates that wormholes induce significant global symmetry breaking, challenging the viability of axions in gravity and string theory frameworks, and clarifies the ill-defined nature of global symmetries in these contexts.

Findings

Wormholes cause large global symmetry breaking affecting axion models.

Axions face a quality problem within non-perturbative Einstein gravity.

Axions also have a quality problem in certain open String Theory scenarios.

Abstract

Continuous global symmetries are expected to be broken by gravity, which can lead to important phenomenological consequences. A prime example is the threat that this poses to the viability of the Peccei-Quinn solution to the strong CP problem. In this paper, we explore the impact of wormholes as a source of global symmetry breaking by gravity. We review the current status of wormholes and global symmetries and note that, surprisingly, the axion has a quality problem within non-perturbative Einstein gravity. Although these wormholes lead to a large breaking of global symmetries, we show that their effect is nonetheless relevant for the model building of gauge protected axions. We also find wormhole solutions within two scenarios: (i) an extended global symmetry group within Einstein gravity, and (ii) U(1) wormholes within the low-energy limit of an open String Theory. The former allows us to show that the concept of a global symmetry in General Relativity is somewhat ill-defined. The latter illustrates that for motivated values of the string coupling constant, axions appear to have a quality problem within the open String Theory we consider.