Strong Interactions
The strong force is a fundamental force that acts across the short range of the atomic nucleus to bind it together. The strong force reflects an interact between quarks that is mediated by the carrier particle (boson) termed the gluon. In addition to binding nucleons (i.e., neutrons and protons), the strong force controls interactions between mesons and baryons.
At the end of the nineteen century, two fundamental forces were known, the electromagnetic force and the gravitational force. As physicists began to study atoms and sub atomic structure, however, they discovered more forces at work. The first new force to be found was the strong force, responsible for binding protons and neutrons into atomic nuclei. Initially, the nucleus presented a puzzle to physicists because, according to electromagnetic theory, nuclei with larger numbers of protons should not be stable. The positive charges would repel each other, and the attraction to neutrons could only be explained by gravity. The gravitational force between protons is dwarfed by the electric force, however, and is entirely negligible. The strong force was postulated because there was no other possible explanation for the stability of nuclei.
The strong force in nuclei acts between nucleons. The strong force does not differentiate between whether a nucleon is a proton or a neutron; it simply binds them together. Because of this "flavor-blindness" in the strong force, theorists sought a theory of the strong force based on a symmetry principle where the theory is invariant under exchange of protons and neutrons. In the language of quantum field theory, the strong force between nucleons is mediated (carried) by gluons. To describe all the possible interactions between gluons with differing colors (i.e., charges of the strong force) there are eight gluons, each of which generally carries a mixture of a color and a differing color anticolor. In connection with the strong force, the term color relates to three possible strong force charges not to the common use of the term color as related to the electromagnetic spectrum.
The true nature of the strong force was not discovered until the invention of the quark model. High-energy scattering experiments involving protons produced strange effects that could not be explained by assuming that protons were elementary particles. In the quark model, the proton was assumed to be constructed out of three smaller elementary particles, named quarks by American physicist Murray Gell-Mann (1929-). This model was successful in predicting the observed experimental results. Furthermore, it was found that the quarks possessed a new type of charge, named color. There are three distinct color charges. A nucleon consists of three quarks each of a different color, while a pion (the lightest form of meson) consists of a quark-antiquark pair of the same color. The force-carrier particle associated with the strong interaction is a massless boson, the gluon. There are eight different gluons necessary for interactions among all combinations of the three color charges. In terms of the quark model, the strong force between nucleons is a result of the residual force "leaking out" of the nucleons through the exchange of particles. This theory of color charges is known as quantum chromodynamics (QCD), and has been verified by further experiments. QCD is the strong force analog of quantum electrodynamics (QED).
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