Axion

Axion

Axions are a hypothetical type of particle first theorized by physicists in the 1970s as a solution to the strong CP problem in quantum chromodynamics (QCD). The strong CP problem is an unsolved issue in particle physics that questions why quantum chromodynamics does not violate the CP (charge parity) symmetry, as one would expect it to. Axions are proposed as a component of the dark matter that makes up much of the universe's mass, yet remains undetected by direct observation. The concept of axions emerged from the work of Roberto Peccei and Helen Quinn in 1977, who proposed a mechanism (now known as the Peccei-Quinn mechanism) that could solve the strong CP problem. This mechanism predicts the existence of a new, very light, and weakly interacting particle: the axion. The axion's properties, including its mass and interaction strengths, are determined by the energy scale at which the Peccei-Quinn symmetry is broken. Axions are of significant interest in both theoretical physics and astrophysics. In theoretical physics, they are a candidate for the cold dark matter that is believed to make up about 27% of the universe. In astrophysics, axions could be detected indirectly through their effects on astrophysical objects and processes. For example, axions could convert into photons (and vice versa) in the presence of a magnetic field, a process that could potentially be observed in astronomical observations. Despite extensive searches, axions have not yet been directly detected, and their existence remains hypothetical. Experiments such as the Axion Dark Matter Experiment (ADMX) and projects like the CERN Axion Solar Telescope (CAST) aim to detect axions by exploiting their predicted interactions with electromagnetic fields. These experiments are challenging due to the extremely weak interaction of axions with ordinary matter and their expected low mass. The search for axions is not only a quest to solve the strong CP problem but also a broader effort to understand the fundamental nature of our universe, including the composition of dark matter. If detected, axions would not only solve a longstanding problem in particle physics but also provide critical insights into the structure and evolution of the universe.