= Solution
An observable that reverses sign under <parity> but has a nonzero expectation in a suitably prepared experiment tests <parity violation>. A polar <momentum> reverses, whereas an axial <spin angular momentum> does not. Thus $\mathbf J\cdot\mathbf p$ and <helicity> are parity-odd; an apparatus asymmetry must be separated from a physical asymmetry by reversing the preparation or analyser.
In the <Wu experiment>, polarized <cobalt-60> nuclei preferentially emit <beta decay> <electrons> in one direction relative to the nuclear <spin angular momentum>. The distribution contains $A\mathbf P\cdot\mathbf p_e/E_e$; its sign changes under spatial inversion. Reversing the nuclear polarization reverses the observed asymmetry.<Experimental Test of Parity Conservation in Beta Decay by Chien-Shiung Wu and others>[Nuclear polarization and the beta angular asymmetry]{ref}
In the <Goldhaber experiment>, the sign of the circular polarization of selected <gamma rays> determines the sign of the <neutrino> <helicity>. A preference for negative <helicity> is parity-odd because spatial inversion would produce the positive-<helicity> state. This provides a different experimental observable from the <electron> angular distribution.
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