Strong interaction

Before 1971, physicists were uncertain as to how the atomic nucleus was bound together. It was known that the nucleus was composed of protons and neutrons and that protons possessed positive electric charge, while neutrons were electrically neutral. Based on established principles of electromagnetism, positive charges would be expected to repel one another and the positively charged protons would be expected to cause the nucleus to fly apart. However, this was never observed. New discoveries in physics were needed to explain this phenomenon. A stronger attractive force was postulated to explain how the atomic nucleus was bound despite the protons' mutual electromagnetic repulsion. This hypothesized force was called the strong force, which was believed to be a fundamental force that acted on the protons and neutrons that make up the nucleus. In 1964, Murray Gell-Mann, and separately George Zweig, proposed that baryons, which include protons and neutrons, and mesons were composed of elementary particles. Zweig called the elementary particles "aces" while Gell-Mann called them "quarks"; the theory came to be called the quark model. The strong attraction between nucleons was the side-effect of a more fundamental force that bound the quarks together into protons and neutrons. The theory of quantum chromodynamics explains that quarks carry what is called a color charge, although it has no relation to visible color. Quarks with unlike color charge attract one another as a result of the strong interaction, and the particle that mediates this was called the gluon. == Behavior of the strong interaction ==

The strong interaction is observable at two ranges, and mediated by different force carriers in each one. On a scale less than about 0.8 fm (roughly the radius of a nucleon), the force is carried by gluons and holds quarks together to form protons, neutrons, and other hadrons. On a larger scale, up to about 3 fm, the force is carried by mesons and binds nucleons (protons and neutrons) together to form the nucleus of an atom. In the former context, it is often known as the color force, and is so strong that if hadrons are struck by high-energy particles, they produce jets of massive particles instead of emitting their constituents (quarks and gluons) as freely moving particles. This property of the strong force is called color confinement. Within hadrons of the strong interaction, from left to right: (a) gluon radiation, (b) gluon splitting and (c,d) gluon self-coupling. The word strong is used since the strong interaction is the "strongest" of the four fundamental forces. At a distance of 10−15 m, its strength is around 100 times that of the electromagnetic force, some 106 times as great as that of the weak force, and about 1038 times that of gravitation. The strong force is described by quantum chromodynamics (QCD), a part of the Standard Model of particle physics. Mathematically, QCD is a non-abelian gauge theory based on a local (gauge) symmetry group called SU(3). The force carrier particle of the strong interaction is the gluon, a massless gauge boson. Gluons are thought to interact with quarks and other gluons by way of a type of charge called color charge. Color charge is analogous to electromagnetic charge, but it comes in three types (±red, ±green, and ±blue) rather than one, which results in different rules of behavior. These rules are described by quantum chromodynamics (QCD), the theory of quark–gluon interactions. Unlike the photon in electromagnetism, which is neutral, the gluon carries a color charge. Quarks and gluons are the only fundamental particles that carry non-vanishing color charge, and hence they participate in strong interactions only with each other. The strong force is the expression of the gluon interaction with other quark and gluon particles. All quarks and gluons in QCD interact with each other through the strong force. The strength of interaction is parameterized by the strong coupling constant. This strength is modified by the gauge color charge of the particle, a group-theoretical property. The strong force acts between quarks. Unlike all other forces (electromagnetic, weak, and gravitational), the strong force does not diminish in strength with increasing distance between pairs of quarks. After a limiting distance (about the size of a hadron) has been reached, it remains at a strength of about , no matter how much farther the distance between the quarks. Between hadrons (shown by the animation in the lead) with the individual quark constituents shown, to illustrate how the fundamental strong interaction gives rise to the nuclear force. Straight lines are quarks, while multi-colored loops are gluons (the carriers of the fundamental force). While color confinement implies that the strong force acts without distance-diminishment between pairs of quarks in compact collections of bound quarks (hadrons), at distances approaching or greater than the radius of a proton, a residual force (described below) remains. It manifests as a force between the "colorless" hadrons, and is known as the nuclear force or residual strong force (and historically as the strong nuclear force). The nuclear force acts between hadrons, known as mesons and baryons. This "residual strong force", acting indirectly, transmits gluons that form part of the virtual π and ρ mesons, which, in turn, transmit the force between nucleons that holds the nucleus (beyond hydrogen-1 nucleus) together. The residual strong force is thus a minor residuum of the strong force that binds quarks together into protons and neutrons. This same force is much weaker between neutrons and protons, because it is mostly neutralized within them, in the same way that electromagnetic forces between neutral atoms (van der Waals forces) are much weaker than the electromagnetic forces that hold electrons in association with the nucleus, forming the atoms. Unlike the strong force, the residual strong force diminishes with distance, and does so rapidly. The decrease is approximately as a negative exponential power of distance, though there is no simple expression known for this; see Yukawa potential. The rapid decrease with distance of the attractive residual force and the less rapid decrease of the repulsive electromagnetic force acting between protons within a nucleus, causes the instability of larger atomic nuclei, such as all those with atomic numbers larger than 82 (the element lead). Although the nuclear force is weaker than the strong interaction itself, it is still highly energetic: transitions produce gamma rays. The mass of a nucleus is significantly different from the summed masses of the individual nucleons. This mass defect is due to the potential energy associated with the nuclear force. Differences between mass defects power nuclear fusion and nuclear fission. == Unification ==

The so-called Grand Unified Theories (GUT) aim to describe the strong interaction and the electroweak interaction as aspects of a single force, similarly to how the electromagnetic and weak interactions were unified by the Glashow–Weinberg–Salam model into electroweak interaction. The strong interaction has a property called asymptotic freedom, wherein the strength of the strong force diminishes at higher energies (or temperatures). The theorized energy where its strength becomes equal to the electroweak interaction is the grand unification energy. However, no Grand Unified Theory has yet been successfully formulated to describe this process, and Grand Unification remains an unsolved problem in physics. If GUT is correct, after the Big Bang and during the electroweak epoch of the universe, the electroweak force separated from the strong force. Accordingly, a grand unification epoch is hypothesized to have existed prior to this. == See also ==