The first steps in resolving the particle proliferation problem were taken by Murray Gell-Mann and Yuval Ne'eman in 1962. They realized that many of the known particle could be fit into a series of families based on an abstract mathematical construct (called the su(3) group). Gell-Mann called this the Eightfold Way, after the Buddha's eightfold path to truth.
Some of these families are illustrated below.
The lowest plane in this figure is of particular interest. At a meeting at CERN in 1962 the cascade (Greek letter xi) was announced. Gell-Mann and Ne'eman were both in the audience and both realized that the lower plane was being filled out according to their expectations -- only the last particle, the omega, was missing. Both raised their hands and Gell-Mann was called upon. He strode to the front and announced that a new particle must exist which he called the omega. Two years later it was discovered at Brookhaven, providing dramatic confirmation of the Eightfold Way.
But what explained the symmetry of the Eightfold Way? In 1964 Gell-Mann and George Zweig thought they knew -- a new class of fractionally charged particles which Gell-Mann called quarks (Zweig called them "aces"). A simple explanation of all of the particles known at the time could be made by assuming the existence of three types of quarks: the up, down, and strange (Perhaps these rather prosaic names are an indication of the decline in classical education in the latter half of the century. Or perhaps they were running out of Greek letters!). The lighter (now known as mesons rather than mesotrons) particles were constructed from a quark and an antiquark. While the hyperons (or baryons as they have become known) were built from three quarks.
Thus, for example, a proton contains two up quarks (of charge +2/3) and one down quark (of charge -1/3).
+1 = 2/3 + 2/3 - 1/3
A neutron is built from two down qaurks and a single up quark, resulting in a net charge of zero.
0 = 2/3 - 1/3 -1/3
Lastly, a positive kaon is constructed from an up quark and an antistrange quark (recall that antiparticles have opposite charge from their particle mates).
+1 = 2/3 + 1/3
As time went by, three more quarks were discovered. Thus there are six quarks, grouped in pairs as follows:
Up/Down, Charm/Strange, and Top/Bottom.
For each of these quarks there is a corresponding anti-quark.
Besides having fractional electric charge, quarks also carry another type of charge called color charge. Quarks interact via their electric and color charges. It is the latter which generates the strong forces which cause binding of nucleons (protons and neutrons) into the nucleus. The theory which describes the strong interactions of quarks is called Quantum Chromodynamics. It is studied by the Particle Theory group here at Pitt.