CPviolation

CP violation is the phenomenon in particle physics where the combined symmetry of charge conjugation (C) and parity (P) is not preserved by certain weak interactions. Charge conjugation swaps particles with antiparticles, while parity inverts spatial coordinates. If CP were exact, the laws governing particles would be identical for matter and antimatter under these transformations. CP violation was first observed in 1964 in the decays of neutral K mesons by Cronin and Fitch, signaling that CP is not exact in nature. Subsequent measurements detected CP violation in B meson systems and other processes, establishing it as a fundamental part of the Standard Model.

In the Standard Model, CP violation arises primarily from a complex phase in the quark mixing matrix, the Cabibbo–Kobayashi–Maskawa (CKM) matrix. A nonzero phase allows differences between a process and its CP-conjugate. CP violation also potentially occurs in the lepton sector through the Pontecorvo–Maki–Nakagawa–Sakata (PMNS) matrix governing neutrino oscillations, though the size of this effect remains an experimental question. CP violation can be direct, in the decay amplitudes of particles, or indirect, through mixing of neutral mesons. Observables include measures like epsilon and sin(2beta) in B decays and epsilon'/epsilon in kaon decays.

The strong interaction in quantum chromodynamics could also violate CP via a theta term, but experiments bound CP violation in the strong sector to be extremely small, yielding the strong CP problem and prompting proposed solutions such as the Peccei–Quinn mechanism and axions.

CP violation is one of the Sakharov conditions required to generate the matter–antimatter asymmetry of the universe, though the CP violation present in the Standard Model appears insufficient to account for the observed baryon asymmetry, motivating searches for new sources of CP violation through ongoing experiments at colliders and flavor facilities.