Everything about Quark Confinement totally explained
Color confinement, often called just
confinement, is the
physics phenomenon that
color charged particles (such as
quarks) can't be isolated. The quarks are confined with other quarks by the
strong interaction to form
pairs or
triplets so that the net color is neutral, to obey the
Pauli exclusion principle. Quarks in mesons must be of a color and the corresponding anti-color to achieve color neutralism; in baryons a red-green-blue mixture (or its anti-color equivalent in an antiparticle ) must be achieved. Quark color bears no relation to
visual color but is purely a label for a property of quarks.
The reasons for quark confinement are somewhat complicated; there's no analytic proof that
quantum chromodynamics should be confining, but intuitively 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
nuclei. However, as two
quarks separate, the gluon fields form narrow
tubes (or strings) of color charge, which tend to bring the quarks together as though they were some kind of rubber bands. This is quite different in behavior from electrical charge. Because of this behavior, the color
force experienced by the quarks in the direction to hold them together, remains constant, regardless of their distance from each other.
The color force between quarks is large, even on a macroscopic scale, being on the order of 100 newtons. As discussed above, it's constant, and does
not decrease with increasing distance after a certain point has been passed.
When two quarks become separated, as happens in
particle accelerator collisions, at some point it's more energetically favorable for a new quark/anti-quark pair to "pop" out of the
vacuum, than to allow the quarks to separate 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.
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.
Further Information
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