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In 1956, Tsung-Dao Lee and Chen Ning Yang examined the evidence for parity conservation in the weak interaction. Their paper, Question of Parity Conservation in Weak Interactions, distinguished an untested assumption from an established experimental result and proposed comparing electron emission along opposite directions relative to oriented nuclear spins, explicitly considering cobalt-60. It proposed a test; it did not report the later Wu experiment.^[ref]

The conceptual improvement is the choice of observable. An angle between two polar vectors is preserved by spatial reflection; its distribution can conceal parity violation. An angle between an axial vector and a polar vector changes to its supplement. Thus a spin-resolved directional measurement asks a different question from an unpolarized total decay rate.

Awarded jointly to Chen Ning Yang and Tsung-Dao Lee for investigating parity conservation laws, leading to discoveries about elementary particles.^[ref]

Tsung-Dao Lee co-developed experimentally testable challenges to parity conservation with Yang Chen-Ning. In a polarized nuclear decay, the parent spin supplies an axial direction while the emitted particle's momentum supplies a polar direction. Their scalar product changes sign under parity, so a preferred emission direction can expose a symmetry failure. The proposal made a foundational assumption into a laboratory question.^[ref]

A physicist.^[ref]

Yang Chen-Ning and Tsung-Dao Lee questioned whether parity conservation had actually been tested in the weak interaction. Their 1956 paper separates established evidence from an assumed symmetry and proposes concrete tests in nuclear and particle decays. An angular correlation that changes sign under spatial reflection can distinguish a process from its mirror image.^[ref]

A physicist.^[ref]