Strong interaction

The strong interaction also binds neutrons and protons to create atomic nuclei, where it is called the nuclear force.

[2] Because the force is mediated by massive, short lived mesons on this scale, the residual strong interaction obeys a distance-dependent behavior between nucleons that is quite different from when it is acting to bind quarks within hadrons.

A stronger attractive force was postulated to explain how the atomic nucleus was bound despite the protons' mutual electromagnetic repulsion.

In 1964, Murray Gell-Mann, and separately George Zweig, proposed that baryons, which include protons and neutrons, and mesons were composed of elementary particles.

[5] The strong attraction between nucleons was the side-effect of a more fundamental force that bound the quarks together into protons and neutrons.

[6] 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.

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.

The strong force is described by quantum chromodynamics (QCD), a part of the Standard Model of particle physics.

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.

In QCD, this phenomenon is called color confinement; as a result, only hadrons, not individual free quarks, can be observed.

The elementary quark and gluon particles involved in a high energy collision are not directly observable.

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.

An animation of color confinement , a property of the strong interaction. If energy is supplied to the quarks as shown, the gluon tube connecting quarks elongates until it reaches a point where it "snaps" and the energy added to the system results in the formation of a quark– antiquark pair. Thus single quarks are never seen in isolation.
An animation of the strong interaction between a proton and a neutron, mediated by pions . The colored small double circles inside are gluons .
The fundamental couplings of the strong interaction, from left to right: (a) gluon radiation, (b) gluon splitting and (c,d) gluon self-coupling.
A Feynman diagram (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).