Isospin

Isospin is a quantum number associated with an approximate symmetry among particles that differ in electric charge. It groups protons and neutrons into a nucleon doublet despite their different charges and masses.

Proton: A positively charged baryon composed of two up quarks and one down quark. It is one member of the nucleon pair related by isospin.
Isospin quantum number: A quantum label specifying a state's total isospin and its component along a chosen axis. Its total value and third component distinguish members of an isospin multiplet.
Pion: A light meson occurring as positively charged, neutral, and negatively charged states. Its three charge states form an isospin-one multiplet.
Electric charge: A conserved property that determines how particles couple to the electromagnetic field. Isospin can relate states with different electric charges; it is not electric charge itself.
Neutron: An electrically neutral baryon composed of one up quark and two down quarks. It is the other member of the nucleon pair related by isospin.
Isospin multiplet: A set of particle states connected by transformations of an internal symmetry group. Particles with different charges can occupy one multiplet when strong interactions treat them alike.
Delta baryon: A family of short-lived spin-3/2 baryons with charges from −1 to +2. Its four charge states form an isospin-3/2 multiplet.
Spin: An intrinsic form of angular momentum carried by quantum particles. Isospin follows similar mathematics but acts on an internal particle label, not physical space.
Strong interaction: The fundamental interaction that binds quarks and acts between hadrons. Its near-independence from electric charge gives nucleon isospin approximate validity.
Clebsch–Gordan coefficients: Coefficients that relate coupled angular-momentum states to products of individual states. They combine particle isospins to determine total-isospin amplitudes.

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