What is an axion?

An axion is a hypothetical elementary particle that arises in theoretical physics, particularly in the context of quantum field theory and cosmology. It was originally proposed in the 1970s to solve the strong CP (Charge Parity) problem, which is an outstanding issue in the Standard Model of particle physics related to the apparent absence of CP violation in strong interactions. Axions are predicted to be very light, electrically neutral particles that interact very weakly with ordinary matter, which makes them difficult to detect. They are also associated with a class of theories known as "pseudo-scalar" fields, where their properties are described by a scalar field with specific symmetries. In addition to their role in particle physics, axions have implications for cosmology; they are candidates for dark matter, which makes up a significant portion of the universe's mass yet does not emit light or interact with electromagnetic forces in a way that we can easily observe. Experimental efforts to detect axions or confirm their existence are ongoing, and they typically involve highly sensitive detectors and experiments designed to observe their potential interactions with photons or other particles.