Editing Quantum chromodynamics (section)

===Area law and confinement===
Detailed computations with the above-mentioned Lagrangian<ref>See all standard textbooks on the QCD,  e.g., those noted above</ref> show that the effective potential between a quark and its anti-quark in a [[meson]] contains a term that increases in proportion to the distance between the quark and anti-quark (<math>\propto r</math>), which represents some kind of "stiffness" of the interaction between the particle and its anti-particle at large  distances, similar to the [[entropic force|entropic elasticity]] of a [[rubber]] band (see below). This leads to ''confinement''&nbsp;<ref>''Confinement'' gives way to a [[quark–gluon plasma]] only at extremely large pressures and/or temperatures, e.g. for <math>T \approx 5\cdot 10^{12}</math>&nbsp;&nbsp;K or larger.</ref> of the quarks to the interior of hadrons, i.e. [[meson]]s and [[nucleon]]s, with typical radii ''R''<sub>c</sub>, corresponding to  former "[[Bag model]]s" of the hadrons<ref>[[Kenneth Alan Johnson]]. (July 1979). The bag model of quark confinement. ''Scientific American''.</ref> The order of magnitude of the "bag radius" is 1&nbsp;fm (=&nbsp;10<sup>&minus;15</sup>&nbsp;m). Moreover, the above-mentioned stiffness is quantitatively related to the so-called "area law" behavior of the expectation value of the Wilson loop product ''P''<sub>W</sub> of the ordered coupling constants around a closed loop ''W''; i.e. <math>\,\langle P_W\rangle</math> is proportional to the ''area'' enclosed by the loop. For this behavior the non-abelian behavior of the gauge group is essential.