Strong CP Violation and Axions

Abstract

Once upon a time there was a serious problem in physics. Experiments did not agree with the theory: energy-momentum and angular momentum conservation laws seemed to he violated in beta decay. In 1930 Pauli suggested a clever cure: a “light penetrating particle” which he called the neutron but which later on got the name “neutrino”. He wrote to a friend, “I’ve done a terrible thing today, something which no theoretical physicist should ever do. I have suggested something that can never be verified experimentally”. A few years later, Pauli, in a jolly mood, had remarked ¹⁾ that perhaps he should have called it (the neutrino) the hypothon. Now some fifty years later, there is again a serious problem in physics, however this time not due to any new experimental discovery, but because the theory has changed. The problem is that, to the best of our knowledge, quantum chromodynamics respects neither parity nor time reversal invariance. The cure this time is again “a light penetrating particle”, as we shall discuss in the following. This time the “hypothon” has many names: the axion, Achion, higglet, etc. In some of its versions, the “hypothon” of our time is expected to “never be verified experimentally”. That’s why it is sometimes called the “invisible axion”, or “phantom axion”, etc.