Axions
Introduction
Axions are hypothetical elementary particles postulated by the Peccei-Quinn theory in 1977 to resolve the strong CP problem in quantum chromodynamics (QCD). They are a type of pseudo-Nambu-Goldstone boson arising from the spontaneous breaking of the Peccei-Quinn symmetry. Axions are also considered potential candidates for cold dark matter, which constitutes a significant portion of the universe's mass-energy content.
Theoretical Background
Peccei-Quinn Theory
The Peccei-Quinn theory was introduced to address the strong CP problem, which is the question of why quantum chromodynamics (QCD) does not seem to violate the charge-parity (CP) symmetry, despite allowing for such violations. The theory proposes a new global U(1) symmetry, known as the Peccei-Quinn symmetry, which is spontaneously broken, giving rise to the axion as a pseudo-Nambu-Goldstone boson.
Axion Properties
Axions are characterized by their extremely low mass and weak interactions with ordinary matter. The mass of the axion is inversely proportional to the Peccei-Quinn symmetry breaking scale, which is typically very high, leading to very light axions. Axions also couple to photons, electrons, and nucleons, but these interactions are extremely weak, making them difficult to detect.
Axions in Cosmology
Dark Matter Candidate
Axions are considered a promising candidate for cold dark matter due to their weak interactions and stability. They could have been produced in the early universe through various mechanisms, such as the misalignment mechanism or the decay of topological defects like cosmic strings and domain walls. The axion's contribution to the dark matter density depends on its mass and production mechanism.
Axion Haloscopes
Axion haloscopes are experiments designed to detect axions by converting them into photons in the presence of a strong magnetic field. The most well-known axion haloscope is the Axion Dark Matter eXperiment (ADMX), which uses a resonant cavity to detect the conversion of axions into microwave photons. Other experiments, such as the HAYSTAC and CASPEr, also aim to detect axions using similar principles.
Axion Detection Methods
Axion-Photon Conversion
One of the primary methods for detecting axions is through their conversion into photons in the presence of a magnetic field. This process is known as the Primakoff effect. Experiments like ADMX and the CERN Axion Solar Telescope (CAST) use this effect to search for axions by looking for excess photons in specific energy ranges.
Axion-Electron Interactions
Axions can also interact with electrons through the axioelectric effect, which is analogous to the photoelectric effect. In this process, an axion is absorbed by an electron, which is then ejected from an atom. Experiments like the XENON1T and the upcoming DARWIN experiment aim to detect axions through this interaction.
Axion-Nucleon Interactions
Axions can couple to nucleons, leading to potential signals in nuclear magnetic resonance (NMR) experiments. The CASPEr experiment aims to detect axions through their interactions with nucleons in a polarized material, looking for changes in the material's magnetization.
Astrophysical and Cosmological Constraints
Stellar Cooling
Axions can be produced in the cores of stars through various processes, such as the Primakoff effect and bremsstrahlung. These axions can escape the star, carrying away energy and affecting the star's cooling rate. Observations of stellar cooling, such as the cooling of white dwarfs and red giants, provide constraints on the axion's properties.
Cosmic Microwave Background
The presence of axions in the early universe can affect the cosmic microwave background (CMB) through their interactions with photons and other particles. Measurements of the CMB, such as those from the Planck satellite, provide constraints on the axion's mass and abundance.
Experimental Searches
Laboratory Experiments
Laboratory experiments aim to detect axions through their interactions with photons, electrons, and nucleons. These experiments include axion haloscopes like ADMX, helioscopes like CAST, and direct detection experiments like XENON1T and CASPEr.
Astrophysical Observations
Astrophysical observations provide indirect evidence for axions through their effects on stellar cooling, supernovae, and the cosmic microwave background. These observations help constrain the axion's properties and guide the design of laboratory experiments.
Future Prospects
Future experiments aim to improve the sensitivity of axion searches and explore new detection methods. These include next-generation axion haloscopes, helioscopes, and direct detection experiments, as well as new techniques like axion interferometry and axion-photon conversion in dielectric materials.