Axions are hypothetical elementary particles that were first proposed in the 1970s as a solution to the CP (Charge Parity) problem in quantum chromodynamics (QCD). The CP problem concerns the question of why there is no observed violation of CP symmetry in the strong interactions, despite such violations being apparent in weak interactions. Axions are thought to restore this symmetry by dynamically relaxing the CP-violating terms to zero. This theoretical solution, known as the Peccei-Quinn mechanism, named after physicists Roberto Peccei and Helen Quinn, suggests that axions are linked to a new global symmetry which, when broken, gives rise to the axion particle.
In terms of properties, axions are neutral, have very low mass (possibly as light as 10^-5 eV), and interact only weakly with ordinary matter and radiation, making them extremely difficult to detect with current technology. They are considered bosons, similar to photons, and are predicted to have no electric or color charge. Axions would be produced thermally in the hot early universe, akin to other cosmic relics like neutrinos. However, due to their unique properties, axions could also be generated non-thermally through the misalignment mechanism in the early universe, leading to a cold dark matter component.
The potential of axions as a candidate for dark_matter has spurred numerous experiments aimed at detecting them. Instruments such as the Axion Dark Matter Experiment (ADMX) use a strong magnetic field and a tuned microwave cavity to convert axions into detectable photons, if they are present in the local dark matter halo. Another approach involves looking for axions produced in the sun, through helioscopes like the CERN Axion Solar Telescope (CAST). These experiments, along with others like the planned International Axion Observatory (IAXO), aim to probe a wide range of axion masses and coupling strengths, filling in the gaps of our understanding of the universe's dark matter composition.
Despite the extensive theoretical appeal and the ongoing experimental efforts, axions remain elusive, and no definitive detection has been confirmed. However, the discovery of axions would not only solve the CP problem in QCD but could also provide a substantial breakthrough in our understanding of cosmic inflation and the overall structure of the universe. Their existence would have profound implications across multiple areas of physics, including astrophysics, cosmology, and particle physics, potentially opening new avenues in each field. As such, the pursuit of axions represents a pivotal frontier in modern physics, embodying the intersection of theoretical ingenuity and experimental innovation.