hadron multiplet

When you sort the known hadrons by their properties, they do not scatter randomly. They fall into neat clusters — groups of particles that are almost identical in mass and behave almost the same way, differing mainly in electric charge. The proton and neutron are one such pair; three pions are another such trio. Each of these tidy groups is called a multiplet.

A multiplet is a family of hadrons that the strong force treats as nearly the same thing. The reason is symmetry. Because the up and down quarks are so close in mass, the strong force barely cares which one is which, so swapping ups for downs hardly changes anything — that approximate symmetry is called isospin, and it groups the proton with the neutron, and the three pions together. Bringing in the strange quark extends this to a bigger symmetry that arranges hadrons into larger patterns of eight or ten members, often drawn as elegant hexagons and triangles when you plot charge against strangeness.

These patterns were the breakthrough that revealed the quark model. In 1961 Gell-Mann and Yuval Ne'eman noticed that the baryons fell into groups of eight — the eightfold way — and that one group of ten was missing a corner. The model predicted a new particle, the omega-minus, with definite mass and properties, to fill that gap. It was found in 1964 exactly as predicted, a triumph that turned the multiplet patterns from curiosities into proof that quarks underlie everything.