Isospin

Isospin (also known as isotopic spin and isobaric spin) is a symmetry of the strong interaction as it applies to the interactions of the neutron and proton. Isospin symmetry is a subset of the flavour symmetry seen more broadly in the interactions of baryons and mesons. Isospin symmetry remains an important concept in particle physics, and a close examination of this symmetry historically lead directly to the discovery and understanding of quarks.