Isospin, or isobaric spin, is a concept in particle physics that is used to describe the symmetry properties of particles, particularly those involved in strong interactions, such as protons and neutrons. It was introduced by the physicist Eugene Wigner in the 1930s as a way to categorize the nucleons (protons and neutrons) in a manner analogous to how spin describes intrinsic angular momentum.
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Isospin is the approximate flavor symmetry rotating the up and down quarks. It is accurate because their masses are close and both are small compared with the QCD scale.