CP violation

CP violation is the failure of physical processes to remain unchanged under the combined operations of charge conjugation, which replaces particles with antiparticles, and parity, which inverts spatial coordinates.

Charge conjugation: A symmetry operation that replaces each particle with its corresponding antiparticle. CP violation tests charge conjugation only in combination with spatial inversion.
Kaon: A meson containing one strange quark or antiquark and one up- or down-type antiquark or quark. Neutral kaons provided the first established case of CP violation.
CP symmetry: The proposed invariance of physical laws under charge conjugation followed by parity inversion. CP violation is the experimentally observed failure of this proposed symmetry.
Parity violation: The failure of physical processes to remain unchanged under spatial inversion alone. Parity violation can occur even when a process’s combined CP symmetry is preserved.
Baryogenesis: The set of processes that generate an excess of baryons over antibaryons in the universe. CP violation is necessary for baryogenesis, alongside baryon-number violation and departure from equilibrium.
Parity: A symmetry operation that reverses spatial coordinates, changing left-handed configurations into right-handed ones. Its combination with charge conjugation defines the symmetry that is violated.
Cronin–Fitch experiment: The 1964 experiment that observed long-lived neutral kaons decaying into two pions. Its result demonstrated that CP symmetry is not exact.
Tsung-Dao Lee: A Chinese-American physicist who helped show that parity conservation in weak interactions had not been experimentally established. Lee and Yang’s symmetry analysis set the stage for testing CP.
CPT symmetry: The symmetry combining charge conjugation, parity inversion, and time reversal, required by local Lorentz-invariant quantum field theory. CP violation does not by itself imply CPT violation.
Matter–antimatter asymmetry: The observed excess of matter over antimatter in the observable universe. Known Standard Model CP violation is too small to account for this excess.

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