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Chapter 1
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Until recently it was thought that protons and neutrons (the building blocks of atomic nuclei) were indivisible units. Physicists now model these subatomic particles as composed of still more elementary units. In the Standard Model of Particle Physics, the most widely accepted theory, units called quarks comprise the proton and neutron, and gluons, as their name suggests, are mediating particles responsible for binding the quarks together. For more information on quarks, gluons, and their interactions, visit:
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