Axion

The axion (/ˈæksiɒn/) is a hypothetical elementary particle postulated by the Peccei–Quinn theory in 1977 to resolve the strong CP problem in quantum chromodynamics (QCD). If axions exist and have low mass within a specific range, they are of interest as a possible component of cold dark matter. The axion (/ˈæksiɒn/) is a hypothetical elementary particle postulated by the Peccei–Quinn theory in 1977 to resolve the strong CP problem in quantum chromodynamics (QCD). If axions exist and have low mass within a specific range, they are of interest as a possible component of cold dark matter. As shown by Gerard 't Hooft, strong interactions of the standard model, QCD, possess a non-trivial vacuum structure that in principle permits violation of the combined symmetries of charge conjugation and parity, collectively known as CP. Together with effects generated by weak interactions, the effective periodic strong CP-violating term, Θ, appears as a Standard Model input – its value is not predicted by the theory, but must be measured. However, large CP-violating interactions originating from QCD would induce a large electric dipole moment (EDM) for the neutron. Experimental constraints on the currently unobserved EDM implies CP violation from QCD must be extremely tiny and thus Θ must itself be extremely small. Since a priori Θ could have any value between 0 and 2π, this presents a “naturalness” problem for the standard model. Why should this parameter find itself so close to zero? (Or, why should QCD find itself CP-preserving?) This question constitutes what is known as the strong CP problem. One simple solution exists: If at least one of the quarks of the standard model is massless, CP-violation becomes unobservable. However, empirical evidence strongly suggests that none of the quarks are massless. Consequently, particle theorists sought other resolutions to the problem of inexplicably conserved CP. In 1977, Roberto Peccei and Helen Quinn postulated a more elegant solution to the strong CP problem, the Peccei–Quinn mechanism. The idea is to effectively promote Θ to a field. This is accomplished by adding a new global symmetry (called a Peccei–Quinn symmetry) that becomes spontaneously broken. This results in a new particle, as shown by Frank Wilczek and Steven Weinberg, that fills the role of Θ, naturally relaxing the CP-violation parameter to zero. This hypothesized new particle is called the axion. The original Weinberg–Wilczek axion was ruled out. Axion models carefully chose coupling that could not have been detected in prior experiments. It had been thought that these “invisible axions” solved the strong CP problem while still being too small to have been observed before. Current literature discusses “invisible axion” mechanisms in two forms, called KSVZ (Kim–Shifman–Vainshtein–Zakharov) and DFSZ (Dine–Fischler–Srednicki–Zhitnitsky). The very weakly coupled axion is also very light because axion couplings and mass are proportional. Satisfaction with “invisible axions” changed when it was shown that any very light axion would have been overproduced in the early universe and therefore must be excluded. The critical mass is of order 10−11 times the electron mass. With a mass above 10−11 times the electron mass, axions could account for dark matter, thus be both a dark-matter candidate and a solution to the strong CP problem. A mass value between 0.05 and 1.50 meV for the axion was reported in a paper published in November 2016 (Borsanyi, S. et al.). The result was calculated by simulating the formation of axions during the post-inflation period on a supercomputer. Pierre Sikivie published a modification of Maxwell's equations that arise from a light, stable axion in 1983. He showed that these axions could be detected on Earth by converting them to photons, using a strong magnetic field, hence leading to several experiments: the ADMX; Solar axions may be converted to X-rays, as in CAST; Other experiments are searching laser light for signs of axions. If magnetic monopoles exist then there is a symmetry in Maxwell's equations where the electric and magnetic fields can be rotated into each other with the new fields still satisfying Maxwell's equations. Luca Visinelli showed that the duality symmetry can be carried over to the axion-electromagnetic theory as well. Assuming the existence of both magnetic charges and axions, Maxwell's equations read