Isospin (source code)
= Isospin
{wiki}
Isospin is the approximate $SU(2)$ flavor symmetry rotating <up quarks> and <down quarks>. Their masses and mass difference are small relative to the hadronic strong-interaction scale, so a useful first approximation treats them as interchangeable in the strong dynamics.
The <proton> and <neutron> form an isospin doublet, while the three <pions> form a triplet. Isospin is an internal label, distinct from ordinary spatial <spin>. Combining isospins organizes reaction channels and relates amplitudes, explaining patterns beyond electric-charge counting alone. Different up/down masses and the electromagnetic interaction break the symmetry, since the two quarks have different electric charges. Threshold and phase-space differences must therefore be retained when testing a quantitative isospin relation.
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