Color confinement

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The color force favors confinement because at a certain range it is more energetically favorable to create a quark–antiquark pair than to continue to elongate the color flux tube. This is analogous to the behavior of an elongated rubber-band.
An animation of color confinement. Energy is supplied to the quarks, and the gluon tube elongates until it reaches a point where it "snaps" and forms a quark–antiquark pair.

In quantum chromodynamics (QCD), color confinement, often simply called confinement, is the phenomenon that color charged particles (such as quarks and gluons) cannot be isolated, and therefore cannot be directly observed in normal conditions below the Hagedorn temperature of approximately 2 trillion kelvin (corresponding to energies of approximately 130–140 MeV per particle).[1][2] Quarks and gluons must clump together to form hadrons. The two main types of hadrons are the mesons (one quark, one antiquark) and the baryons (three quarks). In addition, colorless glueballs formed only of gluons are also consistent with confinement, though difficult to identify experimentally. Quarks and gluons cannot be separated from their parent hadron without producing new hadrons.[3]

Origin[edit]

There is not yet an analytic proof of color confinement in any non-abelian gauge theory. The phenomenon can be understood qualitatively by noting that the force-carrying gluons of QCD have color charge, unlike the photons of quantum electrodynamics (QED). Whereas the electric field between electrically charged particles decreases rapidly as those particles are separated, the gluon field between a pair of color charges forms a narrow flux tube (or string) between them. Because of this behavior of the gluon field, the strong force between the particles is constant regardless of their separation.[4][5]

Therefore, as two color charges are separated, at some point it becomes energetically favorable for a new quark–antiquark pair to appear, rather than extending the tube 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.

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 is 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 with the opposite orientation has only a small area between the two loops.

Models exhibiting confinement[edit]

In addition to QCD in four spacetime dimensions, the two-dimensional Schwinger model also exhibits confinement.[6] Compact Abelian gauge theories also exhibit confinement in 2 and 3 spacetime dimensions.[7] Confinement has recently been found in elementary excitations of magnetic systems called spinons.[8]

Models of fully screened quarks[edit]

Besides the quark confinement idea, there is a potential possibility that the 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.[9] However, such classical solutions do not take into account non-trivial properties of QCD vacuum. Therefore, the significance of such full gluonic screening solutions for a separated quark is not clear.

See also[edit]

References[edit]

  1. ^ V. Barger, R. Phillips (1997). Collider Physics. Addison–Wesley. ISBN 0-201-14945-1. 
  2. ^ J. Greensite (2011). An introduction to the confinement problem. Springer. ISBN 978-3-642-14381-6. 
  3. ^ T.-Y. Wu, W.-Y. Pauchy Hwang (1991). Relativistic quantum mechanics and quantum fields. World Scientific. p. 321. ISBN 981-02-0608-9. 
  4. ^ T. Muta (2009). Foundations of quantum chromodynamics: an introduction to perturbative methods in gauge theories (3rd ed.). World Scientific. ISBN 978-981-279-353-9. 
  5. ^ A. Smilga (2001). Lectures on quantum chromodynamics. World Scientific. ISBN 978-981-02-4331-9. 
  6. ^ Wilson, Kenneth G. (1974-10-15). "Confinement of Quarks". Physical Review D. College Park, MD, USA: American Physical Society. 10: 2445–2459. Bibcode:1974PhRvD..10.2445W. ISSN 1550-2368. OCLC 55589778. doi:10.1103/PhysRevD.10.2445. Retrieved 2014-04-12. 
  7. ^ Schön, Verena; Michael, Thies (2000-08-22). "2d Model Field Theories at Finite Temperature and Density (Section 2.5)". At The Frontier of Particle Physics: 1945–2032. arXiv:hep-th/0008175v1Freely accessible [hep-th]. doi:10.1142/9789812810458_0041. 
  8. ^ closed access publication – behind paywall Lake, Bella; Tsvelik, Alexei M.; Notbohm, Susanne; Tennant, D. Alan; Perring, Toby G.; Reehuis, Manfred; Sekar, Chinnathambi; Krabbes, Gernot; Büchner, Bernd (2009-11-29). "Confinement of fractional quantum number particles in a condensed-matter system". Nature Physics. London, UK: Nature Publishing Group. 6 (1): 50–55. Bibcode:2010NatPh...6...50L. ISSN 1745-2481. OCLC 150143123. arXiv:0908.1038Freely accessible. doi:10.1038/nphys1462. Retrieved 2014-04-12. (Subscription required (help)). 
  9. ^ closed access publication – behind paywall Cahill, Kevin (1978-08-28). "Example of Color Screening". Physical Review Letters. American Physical Society. 41 (9): 599–601. Bibcode:1978PhRvL..41..599C. ISSN 1079-7114. OCLC 31492939. doi:10.1103/PhysRevLett.41.599. Retrieved 2014-04-12. (Subscription required (help)).