The Strong CP problem

The Strong CP problem is a question in particle physics that asks why quantum chromodynamics (QCD) seems 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 the Peccei–Quinn theory, involving new pseudoscalar particles called axions. In essence, the strong CP problem is a discrepancy between the theoretical predictions of the Standard Model of particle physics and experimental observations.