Experimental Searches for the Axion and Axion-like Particles (1602.00039v1)

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.

• The paper presents a comprehensive review of experiments targeting axions as dark matter candidates, with a focus on microwave cavity and NMR-based techniques.
• It outlines diverse methods including helioscope and light-shining-through-a-wall experiments that probe varying mass ranges and coupling sensitivities.
• The study emphasizes innovative detector technologies and improved signal processing methods, paving the way for next-generation axion discovery.

Overview of Experimental Axion and Axion-like Particle Searches

The paper of axions and axion-like particles (ALPs) continues to be a compelling pursuit in the field of particle physics, motivated by the need to resolve the Strong-CP problem and to elucidate the nature of dark matter. The paper "Experimental Searches for the Axion and Axion-like Particles" by Graham et al. presents a comprehensive review of experimental methodologies oriented towards detecting these elusive particles. This synthesis focuses on various experimental fronts, each tailored to different mass ranges and coupling sensitivities, and the corresponding theoretical underpinnings that guide these searches.

The axion, initially proposed to address the Strong-CP problem in quantum chromodynamics (QCD), represents a promising dark matter candidate with a mass predicted in the microelectronvolt to milli-electronvolt range for QCD axions. In the context of string theory, axions and ALPs naturally emerge across a much broader mass spectrum, often in the nanoelectronvolt range, necessitating a diverse array of experimental strategies.

Experimental Techniques

1. Microwave Cavity Experiments: These are currently the most sensitive experiments for detecting dark matter axions in the microelectronvolt mass range. The core principle involves the resonant conversion of the axion field into a microwave photon field in a high-Q cavity within a strong magnetic field. Experiments like the Axion Dark Matter Experiment (ADMX) have been instrumental, employing state-of-the-art superconducting technology and near-quantum-limited amplifiers to enhance sensitivity to the galactic dark matter axions.
2. Cosmic Axion Spin Precession Experiment (CASPEr): This innovative approach uses nuclear magnetic resonance (NMR) techniques to explore axions or ALPs with masses in the nanoelectronvolt range. By exploiting the axion-induced oscillatory electric dipole moments and direct coupling to nuclear spins, this method shows potential for uncovering axion properties by measuring subtle shifts in spin precession frequencies.
3. Helioscope Searches: These aim to detect axions produced in the Sun via the Primakoff effect. The CERN Axion Solar Telescope (CAST) represents a notable effort, with plans to extend to the International AXion Observatory (IAXO), which promises enhanced sensitivity by utilizing a large aperture magnet and x-ray focusing optics.
4. Light Shining Through a Wall (LSW) Experiments: These laboratory-based efforts search for ALPs by attempting to regenerate photons on the other side of a barrier after axion-photon conversion. The All-Optical Light Particle Search (ALPS) at DESY is leading this domain with sophisticated optical systems to amplify any potential axion-photon signal.

Implications and Future Directions

The broad spectrum of axion and ALP masses predicted by theory has necessitated a versatile and multi-pronged experimental approach. Continued advancements in detection technology, such as the development of higher-Q cavities, next-generation magnet technologies, and improved signal processing methods, hold the promise of probing deeper into axion parameter space.

The pursuit of axions and ALPs remains rigorous, with each experimental modality presenting a unique sensitivity frontier. The synergistic use of different detection strategies enhances the probabilistic landscape for potential axion discovery. If successfully detected, axions could provide a transformative window into both particle physics and cosmology, potentially revealing the structure of galaxies and offering new insights into quantum field theories beyond the Standard Model.

The upcoming experimental advancements promise to bolster our searching capabilities significantly, so much so that, should axions exist in nature, an overwhelming wave of confirmation across different experiments might soon offer irrefutable evidence of their presence and role in the universe. This multifaceted approach ensures that axion searches remain at the forefront of physics research, poised to unravel one of the most profound mysteries in contemporary science.