Axion

An axion is a hypothetical elementary particle. Scientists believe it might exist to solve a major mystery in physics. This mystery is called the strong CP problem. It involves how certain symmetries work in quantum chromodynamics, or QCD. QCD is the study of the strong interactions in the Standard Model. If axions exist, they could also be a major part of cold dark matter. Dark matter is an invisible substance that makes up much of the universe.

The strong CP problem arose from a mismatch in nature. In the Standard Model, QCD has a complex vacuum structure. This structure can permit the violation of charge conjugation and parity symmetries. These are known together as CP symmetry. If CP violation from QCD were large, it would cause a large electric dipole moment (EDM) for the neutron. However, experiments show that this EDM is extremely tiny. This means CP violation must be very small. Scientists wonder why this parameter is so close to zero. This question is the core of the strong CP problem.

In 1977, Roberto Peccei and Helen Quinn proposed a solution. They suggested the Peccei–Quinn mechanism. This idea promotes the CP-violating term to a new field. This is done by adding a new global symmetry called Peccei–Quinn symmetry. When this symmetry is spontaneously broken, it creates a new particle. Frank Wilczek and Steven Weinberg independently predicted this particle in 1978. The particle acts to relax the CP-violation parameter to zero. This naturally solves the problem. Wilczek named the particle the "axion" after a laundry detergent. He felt the particle "cleaned up" the CP problem.

Axions are also strong candidates for dark matter. Because they have a non-zero mass, they are called pseudo-Nambu–Goldstone bosons. If axions have a low mass, they can act as cold dark matter. The way they form depends on the early universe. One scenario is the pre-inflationary scenario. This happens if the Peccei–Quinn symmetry breaks during cosmic inflation. In this case, inflation smooths out the field. This creates a homogeneous value across the universe. Another way is the post-inflationary scenario. This occurs if the symmetry is not restored after inflation. In this case, the field takes different values in different patches.

Scientists use various methods to search for these particles. Many experiments rely on the Primakoff effect. This effect allows axions to convert into photons in a magnetic field. One major experiment is the Axion Dark Matter Experiment, or ADMX. It is a haloscope located at the University of Washington. It uses a strong magnetic field to turn axions into microwave photons. ADMX searches for axions within the galactic dark matter halo. Other experiments include HAYSTAC and ORGAN. Another method uses helioscopes. These point magnets at the Sun to catch axions from its core. The CERN Axion Solar Telescope, or CAST, is a famous example.

Researchers have found interesting connections in other fields. In 2008, scientists studied topological insulators. These materials show an effective axion description of electrodynamics. This leads to a quantized magnetoelectric effect. In 2019, a team at the Max Planck Institute detected an axion insulator phase. They found this in a Weyl semimetal material. In this phase, electrons behave together like an axion-like quasiparticle. This discovery supports the math used to search for real axions. It shows that axion electrodynamics is a consistent way to describe particle interactions.

Even though axions have not been found, they remain a major focus of physics. For over 40 years, scientists have studied their potential effects. They have developed many ways to detect their very weak interactions. Some models suggest "invisible axions" with very small couplings. These include the Kim–Shifman–Vainshtein–Zakharov and Dine–Fischler–Srednicki-Zhitnitsky models. These particles would be very light. Finding them would explain both the strong CP problem and the nature of dark matter. The search continues through many different global experiments.