Hadrons - multiplets and quark structure
The strongly interacting particles, the hadrons, vary in their physical properties. These "inner" qualities are determined by the mass (rest energy) and the following quantum numbers: spin angular momentum, and electric charge Q. Particles with nearly equal mass are grouped together and one assignes to them an additional property, the hypercharge Y, which is calculated from the double charge center (average of the charges within a group). Finally, one introduces the isospin I3 = Q - 2Y. Like particles, which differn only in their charge, belong to so-called isospin multiplets. For example, the nucleons - neutron and proton - form an isospin doublet (n,p). Among the mesons we find the triplet consisting of the pions ((π-,π0,π+). If the particle multiplets are plotted in a I3.
The origin of these symmetric patterns can be understood with the help of the SU(3) algebra. All multiplet shapes are built up from fundamental cells, the SU(3) triplets. These contain those three quarks which are the constituents of the hadrons in the multiplets: The up, down and strange quark can be combined to a quark triplet, while their antiparticles form an antitriplet.
According to the quark model, the baryons are bound states of three quarks, whereas mesons are bound states of a quark and an antiquark. One can verify that these combinations yield the required properties by adding the quark charges and hypercharges, as given in quark-antiquark-table, and comparing the results with the corresponding baryon and meson quantum numbers.