Strong CP problem

The strong CP problem is a question in <a href="/facts/Particle_physics/YgIuyj2t">particle physics</a>, which brings up the following quandary: why does <a href="/facts/Quantum_chromodynamics/3cA2ctyb">quantum chromodynamics</a> (QCD) seem to preserve <a href="/facts/CP_violation/Nu5lCsPI">CP-symmetry</a>?
In particle physics, CP stands for the combination of <a href="/facts/C-symmetry/YJ1vjdfD">C-symmetry</a> (charge conjugation symmetry) and <a href="/facts/Parity_(physics)/emSbJ4hD">P-symmetry</a> (parity symmetry). According to the current mathematical formulation of quantum chromodynamics, a <a href="/facts/CP_violation/Nu5lCsPI">violation of CP-symmetry</a> in <a href="/facts/Strong_interaction/JjRsKcLg">strong interactions</a> 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 "<a href="/facts/Fine-tuning_(physics)/x7usTnqr">fine tuning</a>" problem known as the strong CP problem.
The strong CP problem is sometimes regarded as an <a href="/facts/List_of_unsolved_problems_in_physics/vJ3d1A83">unsolved problem in physics</a>, 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 <a href="/facts/Peccei%E2%80%93Quinn_theory/jUsESBWX">Peccei–Quinn theory</a>, involving new <a href="/facts/Pseudoscalar/3ahEmnIW">pseudoscalar</a> particles called <a href="/facts/Axion/ILG6maca">axions</a>.

Strong CP problem open-in-new

Strong CP problem