Research has shown the proton to be made up of even smaller constituent particles. A proton is found to consist of two "up" quarks, each with a +2/3 electric charge, and one "down" quark, with a -1/3 electric charge. The individual quarks are held together by particles called gluons. The up and down quarks are currently believed to be two of the three fundamental particles of all matter. Recent research has revealed the possibility of an even deeper substructure, and further work could lead to new theories which may overturn the current model of the proton's structure.
See also Subatomic particles.
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Michael G. Roepel