Weak isospin is a quantum number associated with the weak interaction, one of the four fundamental forces of nature responsible for processes like beta decay in atomic nuclei. It is a key concept in the electroweak theory, which unifies the electromagnetic force and the weak nuclear force. In the context of particle physics, weak isospin is analogous to the concept of isospin (or isotopic spin) used for strong interactions, but it is specifically related to the weak force.
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Weak isospin labels representations of the SU(2) factor in the electroweak gauge group. A left-handed lepton doublet has with third-component eigenvalues ; a right-handed charged-lepton singlet has . Along with electric charge, the third component determines the weak neutral current through .