= Solution
The <Goldhaber experiment> inferred <neutrino helicity> from the <circular polarization> of a nuclear <gamma ray>. A spin-zero metastable europium-152 nucleus captures an inner-shell <electron>, producing a spin-one excited samarium-152 daughter and an <electron neutrino>; the daughter promptly emits a roughly $960\,\mathrm{keV}$ <gamma ray> to its spin-zero ground state. For a nearly stationary initial atom, the daughter recoils opposite the <neutrino>. Selecting <photons> emitted along that recoil relates the daughter <spin> projection, the <photon helicity> and the <neutrino helicity>.
The selection used resonant absorption and scattering in samarium. Nuclear recoil normally shifts an emitted <gamma ray> below the absorption resonance; the daughter recoil from <electron capture> supplies a compensating forward Doppler shift. Resonant scattering therefore preferentially selects forward photons, fixing their direction relative to the otherwise invisible <neutrino>. A samarium scatterer and scintillation counting identify this resonant component. Transmission through magnetized iron, whose <Compton scattering> depends on the relative electron and photon <spins>, then analyzes the <circular polarization>. Reversing the magnetization reverses the analyzing asymmetry and helps control instrumental offsets.
The selected <photon> and <neutrino> normalized helicities have the same sign: their <momenta> are opposite, while the daughter angular-momentum accounting gives opposite spin projections along the recoil axis. The observed negative photon helicity therefore establishes predominantly left-helicity <neutrinos>, consistent with the <V-A interaction>. It measures a helicity correlation, not a tiny <neutrino mass> or a literal observation of the neutral particle's trajectory.<Helicity of Neutrinos by Maurice Goldhaber and others>[1958, pp. 1015–1017: capture cascade, resonant selection and polarization measurement]{ref}
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