= Weak interaction
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The <weak interaction> changes particle species through the <charged current> and also acts through the <neutral current>. Familiar examples include <beta-minus decay> and <neutrino> scattering. It is short-range and chiral: its <W boson> and <Z boson> couplings distinguish <chirality>. Above the electroweak scale it belongs to the unbroken $SU(2)_L\times U(1)_Y$ <electroweak interaction>. At much lower <energies>, <W boson> exchange is approximated by the <Fermi interaction>.
A particularly direct window into the <weak interaction> is the <Wu experiment>: a sample with oriented nuclear <spins> emits <Electrons> preferentially toward one end of its <spin> axis. The comparison tests <parity conservation>, because <spin> is an <axial vector> and <momentum> is a <polar vector>. The <polarized beta-decay angular distribution> translates that geometric distinction into a measurable counting asymmetry.
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