Axion

The axion is a hypothetical elementary particle arising from a proposed solution to the strong CP problem in quantum chromodynamics. If sufficiently light and stable, axions could also make up dark matter.

Strong CP problem: The unexplained absence of observable CP violation from the strong interaction, despite terms in quantum chromodynamics that permit it. The problem the axion was proposed to solve.
Dark matter: Matter inferred from gravitational effects that emits, absorbs, or reflects little or no light. Axions could account for some or all of the universe’s dark matter.
Roberto Peccei: A theoretical physicist who, with Helen Quinn, proposed a mechanism to solve the strong CP problem. His work introduced the symmetry underlying the axion proposal.
QCD axion: An axion whose mass and interactions are tied to quantum chromodynamics by the Peccei–Quinn mechanism. This is the particle that specifically solves the strong CP problem.
Axion dark matter abundance: The amount of cosmic dark matter that could consist of axions produced in the early universe. Its predicted value depends on uncertain early-universe history and axion mass.
Peccei–Quinn mechanism: A proposed global symmetry mechanism that dynamically drives the strong interaction’s CP-violating angle toward zero. Its spontaneous breaking produces the axion and resolves the strong CP problem.
Axion haloscope: A detector that searches for dark-matter axions converting into microwave photons in a strong magnetic field. Haloscopes target axions in the Milky Way halo through their photon coupling.
Helen Quinn: A theoretical physicist who, with Roberto Peccei, proposed a mechanism to solve the strong CP problem. Her work established the mechanism from which the axion follows.
KSVZ axion: A QCD axion model in which the Peccei–Quinn symmetry is carried by new heavy quarks. It provides one benchmark pattern of axion couplings for experiments.
Axion isocurvature: Primordial fluctuations in axion dark-matter density that are not matched by fluctuations in radiation density. Cosmic microwave background limits constrain axion production scenarios involving inflation.

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