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The strong interaction or '''strong force''' is today understood to represent the Interaction s between Quark s and Gluon s as detailed by the theory of Quantum Chromodynamics (QCD). The strong force is the Fundamental Force mediated by Gluons , acting upon Quark s, Antiquarks , and the Gluon s themselves. The strong force only acts ''directly'' upon elementary particles. However, a residual of the force is observed between Hadron s (the best known example being the force that acts between Nucleon s in atomic nuclei) as the Nuclear Force . Here the strong force acts indirectly, transmitted as gluons which form part of the virtual pi and rho Mesons which classically transmit the nuclear force (see this topic for more). As has been shown by many failed Free Quark searches, the elementary particles affected are unobservable directly. This phenomenon is called Confinement , a theory which allows only hadrons to be seen. HISTORY Before the 1970s , when Proton s and Neutron s were thought to be Fundamental Particle s, the phrase "strong force" referred to what is today known as the Nuclear Force or the residual strong force. What were being observed were the "residual" effects of the strong force, which act on hadrons, both Baryon s and Meson s. This force was postulated to overcome the Electric Repulsion between protons in the Nucleus , and for its strength (at short distances) it was dubbed the "strong force". After the discovery of quarks, scientists realized that the force was actually acting upon the quarks and gluons making up the protons, not the protons themselves. For some time after this realization, the older notion was referred to as the residual strong force, and the "new" strong interaction was called '''colour force'''. DETAILS Quantum Chromodynamics , a part of the Standard Model of particle physics, is a typical Non-Abelian Gauge Theory based on a local (gauge) Symmetry Group called SU(3) . All the particles in this theory interact with each other, through the strong force. The strength of the interaction is parametrized by the strong Coupling Constant . This strength is, as usual, modified by the gauge Colour Charge of the particle. This really refers to a Group Theoretical property whose meaning is explained in the article on Colour Charge and has nothing to do with Colour as such. Quarks and gluons are the only fundamental particles which carry non-vanishing colour charge, and hence participate in the strong interactions. SEE ALSO
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