Quarks had been postulated much earlier, in 1964, by American physicist Murray Gell-Mann (1929–) and, independently, by American physicist George Zweig (1937–). The theory describing quarks was called the quark model. In 1964 it was thought that there should be three different quarks. These different quarks each have a unique property called flavor. These first three quarks had flavors that were whimsically named up, down, and strange. Up-flavored quarks have an electric charge of (2/3)e, where e is the fundamental quantum of charge such as that of the negatively-charged electron. Down- and strange-flavored quarks have an electric charge of (-1/3)e. The quark model also says that quarks must remain bound inside their particles—in nature, quarks cannot exist by themselves. This idea is called quark confinement, and is based on the experimental observation that a free quark has never been seen. Since we cannot isolate quarks, it is very difficult to determine their masses.
In 1964 physicist Oscar W. Greenberg (1932–) suggested each quark has a quality he termed color. The label "color" for this quark property does not refer to the usual definition of color, but is just a way to keep track of quarks. Using this idea of color, the improved quark model says only overall-colorless particles can exist in nature. There are only three different kinds of color in the quark model, usually designated red, blue, and green. Color had to be introduced when a particle called the Δ++ (pronounced delta-plus-plus) baryon was discovered to avoid violating the Pauli exclusion principle. The Pauli exclusion principle says that each particle in a system of matter particles must have unique properties like electric charge, mass, and spin. The Δ++ baryon is made of three up quarks. Without color, each of its three up quarks cannot have its own properties. Color has been proven experimentally, and a theory called the standard model of elementary particles has updated the quark model.
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