Recent Progress in the Physics of Axions and Axion-Like Particles

TL;DR

This review summarizes recent advances in axion and axion-like particle physics, highlighting new theoretical ideas, experimental search strategies, and the significance of hierarchical couplings in understanding fundamental physics.

Contribution

The paper provides a comprehensive overview of recent progress in axion research, emphasizing the role of hierarchical couplings and their experimental accessibility.

Findings

Hierarchical axion couplings are most accessible experimentally.

New search strategies probe previously inaccessible parameter spaces.

Theoretical developments address tensions with quantum gravity.

Abstract

The axion is a light pseudoscalar particle postulated to solve issues with the Standard Model, including the strong CP problem and the origin of dark matter. In recent years, there has been remarkable progress in the physics of axions in several directions. An unusual type of axion-like particle termed the relaxion was proposed as a new solution to the weak scale hierarchy problem. There are also new ideas for laboratory, astrophysical, or cosmological searches for axions; such searches can probe a wide range of model parameters that were previously inaccessible. On the formal theory side, the weak gravity conjecture indicates a tension between quantum gravity and a trans-Planckian axion field excursion. Many of these developments involve axions with hierarchical couplings. In this article, we review recent progress in axion physics, with particular attention paid to hierarchies between axion couplings. We emphasize that the parameter regions of hierarchical axion couplings are the most accessible experimentally. Moreover, such regions are often where important theoretical questions in the field are addressed, and they can result from simple model-building mechanisms.