New experimental approaches in the search for axion-like particles (1801.08127v2)

Abstract: Axions and other very light axion-like particles appear in many extensions of the Standard Model, and are leading candidates to compose part or all of the missing matter of the Universe. They also appear in models of inflation, dark radiation, or even dark energy, and could solve some long-standing astrophysical anomalies. The physics case of these particles has been considerably developed in recent years, and there are now useful guidelines and powerful motivations to attempt experimental detection. Admittedly, the lack of positive signal of new physics at the high energy frontier, and in underground detectors searching for weakly interacting massive particles, is also contributing to the increase of the interest in axion searches. The experimental landscape is rapidly evolving, with many novel detection concepts and new experiments being proposed lately. An updated account of those initiatives is lacking in the literature. In this review we attempt to provide such a review. We will focus on the new experimental approaches and their complementarity, but will also review the most relevant recent results from the consolidated strategies and the prospects of new generation experiments under consideration in the field. We will also briefly review the latest developments of the theory, cosmology and astrophysics of axions and we will discuss the prospects to probe a large fraction of relevant parameter space in the coming decade.

• The paper provides a comprehensive review of experimental approaches for detecting ALPs, emphasizing both theoretical motivations and practical methodologies.
• It details advancements in haloscopes, helioscopes, and light-shining-through-wall experiments, highlighting enhanced sensitivity across broader axion mass ranges.
• It underscores the need for international collaboration and cross-disciplinary innovations to overcome experimental challenges in axion detection.

Insightful Overview of New Experimental Approaches in the Search for Axion-Like Particles

The paper under review, authored by Igor G. Irastorza and Javier Redondo, provides a comprehensive survey of the current state of research into axion-like particles (ALPs), including both theoretical motivations and experimental techniques aimed at their detection. The paper addresses the significant roles that ALPs, as theoretical extensions of the Standard Model, play in potentially solving key issues in cosmology, such as dark matter composition and astrophysical anomalies. This review focuses primarily on the recent advancements and proposals for experimental methodologies intended to detect these elusive particles, assessing their implications for both physics and cosmology.

Main Theoretical Motivations

Axions and ALPs emerge naturally in theories that extend the Standard Model, driven in part by the strong CP problem — a fundamental yet unresolved issue involving the symmetry of laws under charge conjugation and parity. The authors illustrate how these particles are candidates not only for dark matter but also as potential remedies for various cosmological phenomena, such as the observed discrepancies in cosmic microwave background radiation and the structure of the early universe.

Experimental Approaches

The paper meticulously details the evolution of experimental strategies pioneered for the detection of ALPs, building upon the historical context established by earlier efforts. Key experimental avenues discussed include:

1. Axion Haloscopes - Utilized for detecting ALPs by converting them into detectable microwave photons in strong magnetic fields. The review highlights the advances achieved by the Axion Dark Matter eXperiment (ADMX) and explores the new challenges and methodologies for seeking signals across broader mass ranges, notably through innovations such as dielectric haloscopes and improvements in magnet technology.
2. Axion Helioscopes - Focused on detecting solar axions as they convert into X-rays in a high-intensity magnetic field. The paper discusses the seminal work by CAST (CERN Axion Solar Telescope) and future efforts like IAXO (International Axion Observatory), aiming to expand sensitivity across larger axion mass ranges through dedicated magnet designs and enhanced X-ray focusing capabilities.
3. Light-Shining-Through-Wall Experiments - A method utilizing high-intensity laser beams and strong magnets to regenerate photons from ALPs behind an impenetrable barrier. Projects such as ALPS at DESY leverage resonant regeneration capabilities, indicating potential surpassing of astrophysical ALP search limits.
4. Fifth-Force Experiments - Evaluated for their potential to uncover non-standard interactions mediated by ALPs, which would manifest as observable deviations in gravitational force measurement experiments on short scales.

Implications and Future Perspectives

The authors project optimistic prospects for near-term breakthroughs, underlined by the diverse array of experimental technologies advancing in sensitivity toward theoretically motivated parameters spaces for ALPs. The coupling advancements in detector technologies with increased motivation for large-scale experimental collaborations indicate a promising engagement with the axion problem from both the theoretical and experimental fronts.

The authors cautiously acknowledge the challenges, particularly in new domains — such as the extended low- and high-mass ranges for axions — which require innovative methodologies to maintain and enhance sensitivity. They emphasize the necessity of international collaboration and cross-disciplinary synergies to address these challenges, ensuring competitiveness and effectiveness of future axion detection initiatives.

Conclusion

In summary, the paper presents an in-depth analysis of contemporary experimental approaches to the detection of ALPs, contributing significant insights into the operational frameworks, scientific objectives, and the standing challenges that researchers currently face. By balancing theoretical imperatives with experimental constraints, this work lays a critical foundation for future endeavors in axion research, nurturing the potential for pivotal discoveries with far-reaching impacts on physics and cosmology. Moreover, the anticipated engagements in larger scientific collaborations and the incorporation of high-precision technologies reflect a dynamic trajectory poised for substantial explorations into the elusive field of axion-like particles.