Parity (physics)

Parity is the spatial inversion that sends every position vector x to x. In quantum physics, it is represented by an operator whose eigenvalues distinguish states that are even or odd under inversion.

Parity operator: The operator that implements spatial inversion on quantum states or wavefunctions. It expresses coordinate inversion as an operation on the states used in quantum mechanics.
Lee–Yang proposal: The 1956 proposal by Tsung-Dao Lee and Chen-Ning Yang that weak interactions might violate parity. It challenged the assumption that mirror symmetry held in every fundamental interaction.
Spatial inversion: The transformation that reverses all spatial coordinates through a chosen origin. Parity is the quantum-mechanical symmetry associated with this geometric operation.
Atomic parity violation: A small parity-violating effect in atomic transitions caused by weak interactions between electrons and nuclei. It provides precision tests of the weak interaction through parity-forbidden atomic amplitudes.
Charge conjugation: The transformation that replaces particles with their corresponding antiparticles. It changes internal charge identities rather than reversing spatial coordinates.
Parity eigenstate: A quantum state that changes only by a sign under spatial inversion. Its eigenvalue, +1 or −1, identifies even or odd parity.
Wu experiment: Chien-Shiung Wu's 1956–1957 experiment measuring beta decay of polarized cobalt-60 nuclei. The experiment found an asymmetry in electron emission that demonstrated parity violation.
Symmetry (physics): A transformation that leaves the laws or relevant structure of a physical system unchanged. Parity is one discrete symmetry whose conservation can be tested experimentally.
Parity doublet: A pair of nuclear or molecular states with similar energies and opposite parity. Near-degenerate opposite-parity states can amplify weak-interaction effects.
Time reversal: The transformation that reverses the direction of time in physical equations. It is a distinct discrete symmetry, though often discussed alongside parity.

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