So, what are axions? I’ve noticed more articles lately about axions. Why all the fuss, eh?

• Wiki

The axion is a hypothetical elementary particle postulated by the Peccei–Quinn theory in 1977 to resolve the strong CP problem [violation of the combined symmetries of charge conjugation and parity] 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 effective periodic strong CP-violating term, Θ-overline, appears as a Standard Model input – its value is not predicted by the theory, but must be measured. …

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-overline 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 independently by Frank Wilczek and Steven Weinberg, that fills the role of Θ-overline, naturally relaxing the CP-violation parameter to zero. Wilczek named this new hypothesized particle the “axion” after a brand of laundry detergent, while Weinberg called it “Higglet.” Weinberg later agreed to adopt Wilczek’s name for the particle. Because it has a non-zero mass [above 10^−11 times the electron mass; and with electric charge and spin = 0], the axion is a pseudo-Nambu–Goldstone boson.

So, how might axions be detected? Once again, an interplay of models and supercomputer simulations.

• Space.com > “Mystery particle may explain extreme X-rays shooting from the ‘Magnificent 7’ [neutron] stars” by Stephanie Pappas (January 22, 2021) – Never-before-seen particles called axions could be behind the mysterious X-rays.

… scientists have proposed a possible culprit: axions, theoretical particles that turn into light [electromagnetic radiation, in this case high-energy X-rays] particles when they are in the presence of a magnetic field.

[Lawrence Berkeley National Laboratory physicist] Safdi and his team found that axions might work a lot like neutrinos, another extremely light subatomic particle that has been shown to exist. Neutrinos are produced inside neutron stars when neutrons bump into one another; axions could be produced in the same way.

“We’re not claiming that we’ve made the discovery of the axion yet, but we’re saying that the extra X-ray photons can be explained by axions,” Raymond Co, a postdoctoral researcher at the University of Minnesota who collaborated on the study, said in the statement.

More to come … more re Wilczek (and his latest book) …