Strong CP problem: Difference between revisions

Revision as of 02:33, 29 February 2024

Revision as of 11:33, 29 February 2024

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==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 interaction|weak interactions]], but it is also expected{{Citation needed|reason=Needs a source that not only says that it is expected, but also describes why it is expected|date=February 2024}} 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: