= Wu experiment
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The <Wu experiment> was performed by <Chien-Shiung Wu>, <Ernest Ambler>, <Raymond Webster Hayward>, <Dale D. Hoppes>, and <Ralph Percy Hudson>; its 1957 report was Experimental Test of Parity Conservation in Beta Decay. Their experiment supplied the evidence that the <Lee-Yang proposal for a parity test> had called for.<The Fall of Parity Photo Gallery with Short Biographies by National Institute of Standards and Technology>[Experimentalists: the five named biographies; theorists: Lee and Yang]{ref}
A <cobalt-60> surface layer on a <cerium magnesium nitrate> <crystal> was cooled by <adiabatic demagnetization>; a solenoid's <magnetic field> polarized its <atomic nuclei>. An internal <anthracene> counter detected <Electrons> by <particle scintillation>; a light pipe carried flashes to a <photomultiplier>. Polar and equatorial <gamma ray> counters monitored <nuclear polarization> and effective <temperature>; bulk thermometry could miss surface heating.<Experimental Test of Parity Conservation in Beta Decay by Chien-Shiung Wu and others>[p. 1413, Fig. 1 and apparatus paragraphs; p. 1414, demagnetization followed by vertical solenoid]{ref}
Reversing <nuclear polarization> reversed the <Electron> excess; warming erased it with <gamma ray> anisotropy. Warm rates were field-independent. Cooling-field reversal checked remanence; poorly polarized sources checked magnetic deflection. <Electrons> favored the direction opposite <spin>.<Experimental Test of Parity Conservation in Beta Decay by Chien-Shiung Wu and others>[p. 1414, Fig. 2; warm rates, cooling-field reversal, sign, and two control-source preparations]{ref}
\Image[/wu-parity-apparatus-and-inversion.png]
{title=What the apparatus measures, and what spatial inversion changes}
{description=Functional layout, with paths separated for readability. The right-hand construction inverts every spatial coordinate: a polar momentum reverses while an axial spin remains fixed. It is a geometric argument, not a second historical apparatus.}
The <gamma ray> anisotropy diagnoses the preparation; the <Electron> asymmetry diagnoses <parity violation>. These are different jobs. A directionally uneven <gamma ray> signal alone need not violate <parity conservation>: a distribution even in the cosine of the angle can distinguish an axis from its perpendicular without distinguishing its two ends. The <spin>-<momentum> correlation instead distinguishes those ends.
The inference requires control of changes in <detection efficiency>, <Electron> transport and <nuclear polarization>. A fixed detector under <spin> reversal avoids physically exchanging detector positions, but still requires checks for <magnetic field> effects. The result establishes <parity violation> in this <weak interaction> process. A precise microscopic coupling requires more information than the existence or sign of a counting asymmetry; the following model makes that distinction explicit.
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