| parity violation | |
|---|---|
| Name | Parity violation |
| Field | Quantum physics |
| Discovered | 1956–1957 |
| Discoverer | Chien-Shiung Wu (experimental), theoretical proposal by Tsung-Dao Lee and Chen Ning Yang |
| Related | Weak interaction, CP violation, Parity (physics) |
parity violation
Parity violation is the phenomenon in which physical processes fail to be invariant under spatial inversion (parity) transformations. It is a central feature of the weak interaction in modern Quantum field theory and has profound consequences for the formulation of fundamental symmetries in particle physics and cosmology.
Parity is the discrete symmetry that corresponds to inverting spatial coordinates (x → −x); the parity operator P acts on quantum states to produce their mirror images. Parity violation occurs when the dynamics or observed outcomes of a process change under the action of P, indicating that parity is not a conserved symmetry of the governing interaction. In Quantum mechanics and Quantum field theory, parity is represented by a unitary (or antiunitary in some contexts) operator whose commutation with the Hamiltonian determines conservation; failure of commutation signals symmetry breaking. Understanding parity violation elucidates the structure of the Standard Model and the nature of fundamental forces.
The theoretical suggestion that parity might not be conserved in weak interactions was made by Tsung-Dao Lee and Chen Ning Yang in 1956 in response to puzzles in beta decay and nuclear statistics. The decisive experimental confirmation was provided by the famous experiment of Chien-Shiung Wu and collaborators in 1957, which measured anisotropic emission of electrons in the beta decay of polarized Cobalt-60 nuclei and demonstrated maximal parity violation. Concurrent corroborating work included measurements by Leon Lederman and others in muon decay and hyperon decays, and subsequent accelerator experiments at institutions such as Brookhaven National Laboratory and CERN expanded the empirical basis. The 1957 Nobel Prize in Physics was awarded to Lee and Yang for the theoretical insight; experimentalists like Wu were widely recognized in the community.
Within relativistic quantum mechanics, parity acts on spinors and fields: for scalar fields φ(x) → φ(−x), for vector fields Aμ(x) → (A0(−x), −A⃗(−x)), and for Dirac spinors ψ(x) → γ^0 ψ(−x) up to phase. In quantum field theory, Lagrangians invariant under P have terms that transform appropriately; parity violation requires inclusion of chiral couplings that distinguish left- and right-handed components ψ_L = (1−γ^5)/2 ψ and ψ_R = (1+γ^5)/2 ψ. The V−A theory (vector minus axial vector), originally formulated by Feynman and Gell-Mann and related to the work of Marian Danysz and others, provides an effective description of charged-current weak interactions featuring maximal parity violation via left-handed currents. Gauge theories that incorporate parity violation include the Glashow–Weinberg–Salam model of electroweak unification based on the gauge group SU(2)_L × U(1)_Y, where only left-handed fermions transform under SU(2)_L.
The weak force shows intrinsic handedness: charged-current processes mediated by the W boson couple only to left-handed fermions and right-handed antifermions, while neutral-current interactions via the Z boson contain both vector and axial-vector components leading to partial parity violation. Observables such as asymmetries in polarized scattering, helicity-dependent decay rates, and polarization of emitted leptons reflect this structure. Parity violation distinguishes weak interactions from electromagnetism and strong interaction, which conserve parity in their standard formulations (neglecting nonperturbative effects like the strong CP problem). The pattern of parity violation across quark and lepton sectors is embedded in the Cabibbo–Kobayashi–Maskawa matrix and Pontecorvo–Maki–Nakagawa–Sakata matrix when mixing phenomena are considered.
Key experimental probes include nuclear beta decay asymmetries (Wu experiment), polarized muon decay studies by groups at Fermilab and SLAC National Accelerator Laboratory, polarized electron scattering experiments such as SAMPLE (experiment), Qweak, and measurements at LEP that constrained electroweak couplings. Parity-violating electron scattering provides precision tests of electroweak radiative corrections and searches for physics beyond the Standard Model. Atomic parity violation measurements in heavy atoms like Cesium use precision atomic theory to extract weak charges, while deep inelastic scattering and parity-violating asymmetries probe quark electroweak structure. Modern neutrino experiments including Super-Kamiokande and SNO test chiral properties of neutrinos and weak interactions; searches at the Large Hadron Collider seek parity-violating signals of new particles or interactions.
Parity violation forced revision of symmetry assumptions and motivated the development of the electroweak theory culminating in Sheldon Glashow, Steven Weinberg, and Abdus Salam's work on gauge unification (Nobel Prize 1979). Asymmetric weak interactions have implications for baryogenesis mechanisms, such as electroweak baryogenesis, which require violation of C, CP, and out-of-equilibrium dynamics to explain the baryon asymmetry of the universe. The chiral nature of weak interactions constrains models of Grand Unified Theory and left-right symmetric extensions like Pati–Salam model and Left–right symmetric model, which seek to restore parity at high energies. Parity-violating effects also inform searches for new sources of CP violation that could address cosmological puzzles.
Parity violation is tightly linked to concepts of chirality and helicity in particle descriptions; chiral gauge theories require careful treatment of anomalies such as the axial anomaly and Adler–Bell–Jackiw anomaly, relevant for consistency of the Standard Model. Related discrete-symmetry violations include charge conjugation (C) and combined CP violation observed in kaon and B meson systems (Nobel Prizes to Cronin and Fitch and later to Kobayashi and Maskawa), which together with parity violation shape our understanding of fundamental interactions. Practical applications of parity-violating phenomena exist in precision tests constraining dark sector models and in atomic, nuclear and condensed-matter experiments that exploit chirality, such as studies in spintronics and molecular optical activity where parity-odd interactions can induce subtle effects. Category:Symmetry in physics