Inroduction to a mindblowing piece
It was Peccei-Quinn in 1977 who stipulated the axion, and as I believe, it was created to resolve a problem in the strong-CP for QCD. The following will be the reasoning why it was predicted… and I’ll explain anything else too.
History like you never seen it before
Until the 1950's, the view was that the laws of quantum physics remained absolutely unchanged when one changes the sign of spatial coordinates, such as x, y, z into -x, -y and -z; this particular mirror structure is called the parity, symbolized with P. However, C.S. Wu realized that the left-handed neutrino had no mirror reflection, (the left-handed neutrino), thus the symmetry of 'weak interaction' was violated through the parity; the symmetry could only be revived, if one considered that P could not be alone, and one must introduce the invariant of CP into the equation - - - hence, the CP Violation. 'C' stands for 'charge conjugate', which is the transformation of a particle into its antipartner, such as an electron into a positron. In Wu's case, a left-handed neutrino can be transformed into a right-handed antineutrino.
In 1964, physicists James Cronin and Val Vitch found a symmetry violation in the CP transformation, in observations on the K-meson, or Kaon particle. In short, they showed that the Antikaon is not the absolute mirror symmetry of the neutral Kaon. The Antikaon was shown to have a smaller life expectancy than its neutral partner. They received a Nobel Prize for their discovery.
The increasing studies into CP Violation, and fundamental asymmetry is having scientists ask why matter should be dominant in this universe, and not just a soup of gamma ray energy; the result when a particle comes into contact with an antiparticle. Even though the universe is vastly wide, and that ordinary matter covers only a mere 1% of spacetime, an even smaller percentage of this should be made up of antimatter - perhaps a mere 0.011%... though Cp Violation is telling us possibly why there isn't that much antimatter in the universe. Other theorists have concluded that a very heavy boson, dubbed the X-boson, might decay in such a way it is able to violate CP... However, the X-boson comes at a price; it would state that the proton is not infinitely stable! If the proton does decay, it's lifetime is expected to be something like 10^30 years. Though, so much controversy clouds the lifespan of the proton, if it even has a calculable lifespan. One might ask why we have never observed the X-boson... The X-Boson cannot be verified objectively, though it could be proven indirectly through proton decay.
I wish I was dead
Together with the effects of the weak violation, the strong violation given as, is found to be an intrinsic parameter of the theory, even though it isn’t predicted! This was what led to a paradox of physics, because the strong effective violation would cause the neutron to eventually have a dipole moment… and as we know, the neutron is a neutral particle with no charge, but it seems to work, because of certain predictions concerning the neutron. But it is also predicted to be very rare. The theory seems to ask… ‘’why is this variable [priori] close to vanishing numbers?’’ An interesting solution came about. If at least one of the quarks in this model, creating the neutron are massless, then the variable becomes unobservable. But this doesn’t seem right from a quantum physical view, because none of the quarks are predicted to be massless besides semen… (But I like the idea).
Then a second solution was devised by R. Peccei and H Quinn creating the ‘’global symmetry solution of Peccei-Quinn’’. They invited a new particle into the mix for the solution of the strong CP problem, which involves also spontaneous symmetry violations creating this new particle. This will now make the following… priori = 0, instead of some non-zero total close to zero. As I am sure you can guess, this was when the dark matter particle ‘’axion’’ was created.
It turns out that the axion has no electric charge, and has a vanishingly small mass of 10^-6 and super low interactions with the weak and strong forces. They can interact with other corporeal particles, but only very rarely. This is, it appears, the reason why they have the properties they have.