Color confinement

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 terakelvin (corresponding to energies of approximately 130–140 MeV per particle).

In addition, colorless glueballs formed only of gluons are also consistent with confinement, though difficult to identify experimentally.

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).

, the world average in the 3-flavour case is given by[6] When the renormalization group equation is solved exactly, the scale is not defined at all.

It is however incorrect since in QCD the Landau pole is unphysical,[7][8] which can be seen by the fact that its position at the confinement scale largely depends on the chosen renormalization scheme, i.e., on a convention.

Most evidence points to a moderately large coupling, typically of value 1-3 [7] depending on the choice of renormalization scheme.

[11] If the electroweak symmetry breaking scale were lowered, the unbroken SU(2) interaction would eventually become confining.

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.