Experimental Searches for the Axion and Axion-like Particles

TL;DR

This paper reviews recent experimental efforts to detect axions and axion-like particles across various methods, highlighting technological advances and the potential for discovery in the coming years.

Contribution

It provides a comprehensive overview of experimental approaches, progress, and future prospects in axion and axion-like particle searches.

Findings

Microwave cavity experiments are advancing in the microelectronvolt range.

NMR-based searches are being developed for nanoelectronvolt axions.

Laboratory and solar axion searches have achieved significant sensitivity improvements.

Abstract

Four decades after its prediction, the axion remains the most compelling solution to the Strong-CP problem and a well-motivated dark matter candidate, inspiring a host of elegant and ultrasensitive experiments based on axion-photon mixing. This report reviews the experimental situation on several fronts. The microwave cavity experiment is making excellent progress in the search for dark matter axions in the microelectronvolt range and may be plausibly extended up to 100 mu eV. Within the past several years however, it has been realized that axions are pervasive throughout string theories, but with masses that fall naturally in the nanoelectronvolt range, for which a NMR-based search is under development. Searches for axions emitted from the Sun's burning core, and purely laboratory experiments based on photon regeneration have both made great strides in recent years, with ambitious projects proposed for the coming decade. Each of these campaigns has pushed the state of the art in technology, enabling large gains in sensitivity and mass reach. Furthermore each modality has also been exploited to search for more generalized axion-like particles, that will also be discussed in this report. We are hopeful, even optimistic, that the next review of the subject will concern the discovery of the axion, its properties, and its exploitation as a probe of early universe cosmology and structure formation.