= Solution
In the <Wu experiment>, <Chien-Shiung Wu> and collaborators studied <beta-minus decay> of polarized <cobalt-60>,
$$
{}^{60}_{27}\mathrm{Co}\longrightarrow{}^{60}_{28}\mathrm{Ni}^{*}+e^-+\bar\nu_e.
$$
A thin <cobalt-60> layer on a <cerium magnesium nitrate> crystal was cooled by <adiabatic demagnetization> to a few millikelvin. A polarizing <magnetic field> produced <nuclear polarization>. An <anthracene> scintillation counter and <photomultiplier> detected the emitted <Electrons>; angular <gamma ray> observations from the daughter de-excitation monitored whether the nuclei remained polarized.<Experimental Test of Parity Conservation in Beta Decay by Chien-Shiung Wu and others>[Apparatus and polarization controls, pp. 1413–1414]{ref}
The <Electrons> were emitted preferentially opposite the parent <nuclear spin>. Reversing the <nuclear polarization> reversed the asymmetry, while warming the sample removed both polarization and the effect. Those controls distinguished a spin-correlated asymmetry from a fixed detector bias.
The relevant angular correlation is a <polarized beta-decay angular distribution> of the form
$$
W(\theta)\propto1+A_\beta P\frac vc\cos\theta,
\qquad \cos\theta=\widehat{\mathbf J}\cdot\widehat{\mathbf p}_e.
$$
Under <parity>, $\mathbf p_e\to-\mathbf p_e$ but $\mathbf J\to\mathbf J$, so the spin-momentum correlation changes sign. <Parity conservation> would require equal rates for these two directions relative to the same spin. The observed nonzero coefficient therefore demonstrated \b[parity violation in beta decay]. An unpolarized source would average this correlation to zero and could not provide the same test.
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