CP-violation

The redoubtable Bryan Appleyard writes a nice piece for the Sunday Times on supercomputer simulations of CP-violation in quantum chromodynamics.

In particle physics, a CP transformation is the combined operation which swaps positive charges and negative charges (Charge conjugation), and swaps right-handed particle states and left-handed particle states (Parity reversal). Given a left-handed, negatively charged particle, the corresponding antiparticle is a right-handed, positively charged particle. CP-violation occurs when particle processes are not invariant under a CP transformation. CP-violation is necessary to explain the asymmetry present in the early universe between the amount of matter and the amount of antimatter. However, whilst the standard model of particle physics predicts CP-violation in electroweak processes, and this CP-violation has been experimentally observed, the order of the violation is insufficient to account for the matter-antimatter asymmetry. The standard model is also capable of representing strong force CP-violation, but this has not been experimentally observed, and, once again, it transpires that this CP-violation, even if it does exist, cannot account for the exact magnitude of the matter-antimatter asymmetry in the early universe.

It seems then, that an explanation for the matter-antimatter asymmetry will reside in a Grand Unified Theory (GUT). Such a theory goes beyond the standard model by unifying the strong force with the electroweak force. (Note that a GUT does not incorporate gravity, and should not be confused with a Theory of Everything, which does just that.) CP-violation in a GUT will, it appears, be necessary to explain the matter-antimatter asymmetry. When energy densities in the early universe were sufficiently high, it is thought that the strong and electroweak forces were unified, and that particles mediating the unified force passed between quarks and leptons, permitting transmutations between these particles. If these reactions possessed CP-violation, then it could well explain the presence of a matter-antimatter asymmetry at the end of the GUT era.

Note, incidentally, that Appleyard's article equates the question 'Why was there more matter than antimatter in the early universe?' with the question 'Why is there something rather than nothing?' This is rather misleading. If there had been an exactly equal amount of matter and antimatter in the early universe, all the mass in the early universe would have been converted into a sea of photons (i.e., radiative energy). One would therefore have a space-time replete with energy, and although there would be no matter as such, this is hardly the same thing as nothing at all.

CP-Violation Standard Model Something rather than nothing Grand Unified Theories Bryan Appleyard