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So, confinement has obviously been shown by lattice gauge theory to be a predicted aspect of QCD. However, to what extent has it been observed in experimental physics?

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    $\begingroup$ Er ... it was a phenomenological observation before it was part of any theory. $\endgroup$ – dmckee Nov 27 '14 at 2:30
  • $\begingroup$ whoops. then why does confinement remain unexplained? $\endgroup$ – ciao Nov 27 '14 at 2:56
  • $\begingroup$ Like every other observed fact at the most fundamental level it is simply the way the universe appears to work. Some observations in science can be explained in terms of more basic observations (and learning to do that is one of the pillars of the discipline), but at some level everything is built on a foundation of observation. $\endgroup$ – dmckee Nov 27 '14 at 4:09
  • $\begingroup$ @dmckee is that true? surely the idea of confinement only existed after the first parton model in the 60's. it was first observed, i'd say, in particle collisions in which colored objects quickly hadronized.. $\endgroup$ – innisfree Nov 27 '14 at 12:24
  • $\begingroup$ @innisfree If you insist on the language of QCD, then you can't have it until you have that same language, but the observation that (a) nucleons have substructure and (b) you can't seem to knock the pieces loose is what prompted the need to deduce the sub-component properties from the hadron spectrum. Those facts are the observation of confinement. $\endgroup$ – dmckee Nov 27 '14 at 16:16
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Confinement, i.e. the fact that bound states in QCD are colour singlets (or in other words: there are no free quarks or gluons) is experimentally well-established. There is no doubt that it is not just a prediction of lattice simulations, but a feature of strongly interacting systems at low energies, as no free quarks (or gluons) have yet been found within the relevant energy regime. An example that free quarks are prohibited by interactions are jets: two quarks which are in the process of being separated quickly turn into hadrons, which are colour-neutral. But just because we observe it experimentally, it does not mean that it is fully understood theoretically. For more details on this issue, see this question (and my answer to it).

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  • $\begingroup$ The following is off topic, but there is some analogy with atoms. In atoms there are electron clouds of different configurations, namely, containing fractionally charged "sub-clouds". If the nucleus were lighter, we could speak of positive charge sub-clouds too. Elastic scattering section is expressed via integrals over these fractionally charged sub-clouds. Inelastic cross sections include transitions between different sub-cloud configurations. But when ionization happens, we observe some sort of "hadronization" - only integer charges in the final states. $\endgroup$ – Vladimir Kalitvianski Nov 27 '14 at 20:45
  • $\begingroup$ If we considered sub-clouds "elementary" entities, then they would not be observed in free states in full analogy with QCD. $\endgroup$ – Vladimir Kalitvianski Nov 27 '14 at 20:45

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