CP Violation
CP violation is the small breaking of combined charge-parity symmetry that distinguishes matter from antimatter and is a necessary ingredient for explaining the cosmic dominance of matter.
Definition
CP violation is the phenomenon in which the combined operation of charge conjugation and parity is not a symmetry of certain weak-interaction processes, so that particles and their antiparticle mirror images behave differently, as quantified in the Standard Model by a complex phase in the quark-mixing matrix.
Scope
This topic covers the discovery of CP violation in neutral kaon decays, its description within the Standard Model through the complex phase of the Cabibbo-Kobayashi-Maskawa quark-mixing matrix, and its observation in kaon, B-meson, and D-meson systems. It treats the connection between CP violation and the matter-antimatter asymmetry of the universe, and the requirement that this Standard Model source appears insufficient to account for the observed cosmic asymmetry.
Core questions
- How was CP violation first discovered, and in what system?
- How does the Standard Model accommodate CP violation through quark mixing?
- Why does CP violation require at least three generations of quarks?
- How is CP violation related to the matter-antimatter asymmetry of the universe?
Key concepts
- Combined CP symmetry
- Neutral kaon and B-meson systems
- Cabibbo-Kobayashi-Maskawa matrix
- Complex CP-violating phase
- Three-generation requirement
- Matter-antimatter asymmetry
Key theories
- Kobayashi-Maskawa mechanism
- Kobayashi and Maskawa showed that a complex phase in the quark-mixing matrix can generate CP violation, but only if there are at least three generations of quarks, predicting a third generation before it was found.
- CP violation in neutral mesons
- CP violation was discovered in the decays of neutral kaons and later established in B and D mesons, with measured asymmetries matching the pattern predicted by the quark-mixing phase.
Clinical relevance
CP violation is essential to the Sakharov conditions for generating a matter-antimatter imbalance in the early universe, and the apparent insufficiency of the known Standard Model source motivates extensive searches for additional CP violation in quark and lepton systems.
History
CP violation was discovered unexpectedly in 1964 by Cronin, Fitch, and collaborators, who observed the forbidden two-pion decay of the long-lived neutral kaon, a result honored with the 1980 Nobel Prize. In 1973 Kobayashi and Maskawa explained CP violation through a complex phase requiring three quark generations, a prediction confirmed by later quark discoveries and recognized with the 2008 Nobel Prize.
Debates
- Origin of the cosmic matter-antimatter asymmetry
- The CP violation observed in the Standard Model is too small to account for the observed excess of matter over antimatter, leaving open whether additional sources of CP violation, perhaps in the lepton sector, are responsible.
Key figures
- James Cronin
- Val Fitch
- Makoto Kobayashi
- Toshihide Maskawa
Related topics
Seminal works
- christenson1964
- kobayashimaskawa1973
Frequently asked questions
- Why does CP violation matter for cosmology?
- CP violation is one of the conditions required to generate an excess of matter over antimatter in the early universe. Without it, matter and antimatter would have been produced in equal amounts and largely annihilated, leaving no ordinary matter.
- Why does the Standard Model need three quark generations for CP violation?
- Kobayashi and Maskawa showed that a physically meaningful complex phase, the source of CP violation in the quark sector, can only appear in the mixing matrix if there are at least three generations of quarks.