One of the main rules of the standard quark model is that combinations of three quarks are called baryons. Protons and neutrons are the most important baryons. Protons are made of two up quarks and one down quark. Neutrons are made of two down quarks and one up quark. Since the quark model requires that naturally-occurring particles be colorless, a baryon must be made of a red, a blue, and a green quark. These combine to make a white, or colorless particle. Spin is also important in classifying baryons. Baryons are fermions and so have spins of one-half h-bar or three-halves h-bar. Table 3 summarizes several kinds of baryons, with masses in MeV/c 2 (millions of electron-volts divided by the speed of light squared) and spin in terms of h-bar.
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