The Phenomenological Motivation of Axions: A Review

TL;DR

This review discusses the strong CP problem in quantum chromodynamics and how axions are proposed as a solution, highlighting their theoretical motivation and ongoing experimental searches.

Contribution

It provides a comprehensive overview of the phenomenological motivation for axions and summarizes current experimental efforts to detect them.

Findings

Axions can dynamically solve the strong CP problem.

Experimental searches for axions are actively ongoing.

Theoretical models support axions as a solution to CP violation in QCD.

Abstract

Setting aside anthropic arguments, there is no reason for CP symmetry to be obeyed within the theory of quantum chromodynamics. However, no such violation of CP symmetry has ever been observed in a strongly interacting experiment. This is known as the strong CP problem which, in its simplest manifestations, can be quantitatively formulated via a calculation of the pion masses and the neutron electric dipole moment. The former yields a larger mass for the neutral pion than its charged counterparts, the latter yields a far larger result than is experimentally measured, where in both cases the discrepancies are parameterised by the physical quantity $\bar{\theta}$. The strong CP problem can be solved via the inclusion of a new particle, the axion, which dynamically sets $\bar{\theta}$ to zero, eliminating these two manifestations. Thus, experimental searches for such a particle are an active field of research. This dissertation acts as a review of the aforementioned concepts.