Strong CP problem: Difference between revisions

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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 (physics)|charge]] [[C-symmetry|conjugation symmetry]] (C) and [[Parity (physics)|parity]] symmetry (P). According to the current mathematical formulation of quantum chromodynamics, a [[CP violation|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

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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 interaction|weak interactions]], but it is also expected to be broken through [[strong interaction|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]].<ref>{{cite conference|url=https://www.osti.gov/servlets/purl/6260191|title=A Brief Introduction to the Strong CP Problem|last1=Wu|first1=D.|date=1991|publisher=|___location=Austin, Texas, United States|id=SSCL-548}}</ref> The most general mass term possible for the quark is a complex mass written as <math>m e^{i\theta' \gamma_5}</math> for some arbitrary phase <math>\theta'</math>. In that case the [[Lagrangian (field theory)|Lagrangian]] describing the theory consists of four terms

:<math>