Color confinement, often simply called confinement, is the physics phenomenon that color charged particles (such as quarks) cannot be isolated singularly, and therefore cannot be directly observed. Quarks, by default, clump together to form groups, or hadrons. The two types of hadrons are the mesons (one quark, one antiquark) and the baryons (three quarks). The constituent quarks in a group cannot be separated from their parent hadron, and this is why quarks can never be studied or observed in any more direct way than at a hadron level.
The reasons for quark confinement are somewhat complicated; no analytic proof exists that quantum chromodynamics should be confining. The current theory is that confinement is due to the force-carrying gluons having color charge. As any two electrically charged particles separate, the electric fields between them diminish quickly, allowing (for example) electrons to become unbound from atomic nuclei. However, as a quark-antiquark pair separates, the gluon field forms a narrow tube (or string) of color field between them. This is quite different from the behavior of the electric field of a pair of positive and negative electric charges, which extends into the whole surrounding space and diminishes at large distances. Because of this behavior of the gluonic field, there acts a strong force between the quark pair which remains constant, regardless of their distance, with a strength of around 160,000 newtons, corresponding to the weight of 16 tons.
When two quarks become separated, as happens in particle accelerator collisions, at some point it is more energetically favorable for a new quark–antiquark pair to spontaneously appear, than to allow the tube to extend further. As a result of this, when quarks are produced in particle accelerators, instead of seeing the individual quarks in detectors, scientists see "jets" of many color-neutral particles (mesons and baryons), clustered together. This process is called hadronization, fragmentation, or string breaking, and is one of the least understood processes in particle physics.
The confining phase is usually defined by the behavior of the action of the Wilson loop, which is simply the path in spacetime traced out by a quark–antiquark pair created at one point and annihilated at another point. In a non-confining theory, the action of such a loop is proportional to its perimeter. However, in a confining theory, the action of the loop is instead proportional to its area. Since the area will be proportional to the separation of the quark–antiquark pair, free quarks are suppressed. Mesons are allowed in such a picture, since a loop containing another loop in the opposite direction will have only a small area between the two loops.
Models exhibiting confinement 
Besides QCD in 4D, another model which exhibits confinement is the Schwinger model. Compact Abelian gauge theories also exhibit confinement in 2 and 3 spacetime dimensions. Confinement has recently been found in elementary excitations of magnetic systems called spinons.
Models of fully screened quarks 
Besides the quark confinement idea, there is a potential possibility, that color charge of quarks gets fully screened by the gluonic color, surrounding the quark. Exact solutions of SU(3) classical Yang-Mills theory, which provide full screening (by gluon fields) of the color charge of a quark have been found. However, such classical solutions do not take into account non-trivial properties of QCD vacuum. Therefore, a significance of such full gluonic screening solutions for a separated quark is not clear.
See also 
- Gluon field strength tensor
- Asymptotic freedom
- Center vortices
- Deconfining phase
- Quantum mechanics
- Particle physics
- Fundamental force
- Dual superconducting model
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