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 Charge+Parity or Charge-conjugation Parity symmetry: the combination of [[charge (physics)|charge]] [[C-symmetry|conjugation symmetry]] (C) and [[Parity (physics)|parity]] symmetry (P). According to the current mathematical formulation of quantum chromodynamics, a violation of CP-symmetry in [[strong interaction]]s 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'''.
According to the current mathematical formulation of quantum chromodynamics, a violation of [[CP-symmetry]] in [[strong interaction]]s 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 [[List of unsolved problems in physics|unsolved problem in physics]], and has been referred to as "the most underrated puzzle in all of physics."<ref>{{cite conference |first=T. |last=Mannel |title=Theory and Phenomenology of CP Violation |book-title=Nuclear Physics B
