Most particles are not elementary but composed of other particles -- quarks (Gell-Mann, Néeman and Zweig 1962-64). The heavy particles (baryons) such as the neutron, the proton, D etc are composed of three quarks. The medium heavy particles (mesons) such as p, K, r etc are composed of a quark and an anti quark.

In the 60's, all known baryons and mesons could be described as composed of three types of quarks - the up-quark u, the down-quark d and the strange-quark, s.

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