= Lee-Yang proposal for a parity test
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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>.<Question of Parity Conservation in Weak Interactions by Tsung-Dao Lee and Chen Ning Yang>[pp. 254–255; evidence review and “Possible Experimental Tests of Parity Conservation in beta Decays,” first proposal]{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>.
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