The strong CP problem is a puzzling question in particle physics: 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. Theory CP-symmetry states that physics should be unchanged if particles were swapped with their antiparticles and then left-handed and right-handed particles were also interchanged. This corresponds to performing a charge conjugation transformation and then a parity transformation. The symmetry is known to be broken in the Standard Model through weak interactions, but it is also expected to be broken through strong interactions which govern quantum chromodynamics (QCD), something that has not yet been observed. To illustrate how the CP violation can come about in QCD, consider a Yang–Mills theory with a single massive quark. The most general mass term possible for the quark is a complex mass written as for some arbitrary phase . In that case the Lagrangian describing the theory consists of four terms The first and third terms are the CP-symmetric kinetic terms of the gauge and quark fields. The fourth term is the quark mass term which is CP violating for non-zero phases while the second term is the so-called θ-term, which also violates CP-symmetry. Quark fields can always be redefined by performing a chiral transformation by some angle as which changes the complex mass phase by while leaving the kinetic terms unchanged. The transformation also changes the θ-term as due to a change in the path integral measure, an effect closely connected to the chiral anomaly. The theory would be CP invariant if one could eliminate both sources of CP violation through such a field redefinition. But this cannot be done unless . This is because