Күшті CP мәселесі: Кванттық хромодинамикадағы CP симметриясының бұзылмауының себептері

Question of why quantum chromodynamics does seem to not break CP symmetry
The strong CP problem is a question in particle physics, which brings up the following quandary: why does quantum chromodynamics (QCD) seem to preserve CP symmetry? In particle physics, CP stands for the combination of charge conjugation symmetry (C) and parity symmetry (P). According to the current mathematical formulation of quantum chromodynamics, a violation of CP symmetry in strong interactions could occur. However, no violation of the CP symmetry has ever been seen in any experiment involving only the strong interaction. As there is no known reason in QCD for it to necessarily be conserved, this is a "fine tuning" problem known as the strong CP problem. The strong CP problem is sometimes regarded as an unsolved problem in physics, and has been referred to as "the most underrated puzzle in all of physics." There are several proposed solutions to solve the strong CP problem. The most well known is Peccei–Quinn theory, involving new pseudoscalar particles called axions.

The strong CP problem is solved automatically if one of the quarks is massless. In that case one can perform a set of chiral transformations on all the massive quark fields to get rid of their complex mass phases and then perform another chiral transformation on the massless quark field to eliminate the residual θ term without also introducing a complex mass term for that field. This then gets rid of all CP violating terms in the theory. The problem with this solution is that all quarks are known to be massive from experimental matching with lattice calculations. Even if one of the quarks was essentially massless to solve the problem, this would in itself just be another fine tuning problem since there is nothing requiring a quark mass to take on such a small value. The most popular solution to the problem is through the Peccei–Quinn mechanism. This introduces a new global anomalous symmetry which is then spontaneously broken at low energies, giving rise to a pseudo Goldstone boson called an axion. The axion ground state dynamically forces the theory to be CP symmetric by setting Axions are also considered viable candidates for dark matter and axion like particles are also predicted by string theory. Other less popular proposed solutions exist such as Nelson–Barr models. These set at some high energy scale where CP symmetry is exact but the symmetry is then spontaneously broken at low energies. The tricky part of these models is to account for why remains small at low energies while the CP breaking phase in the CKM matrix becomes large.