Editing Quantum chromodynamics (section)

===Effective theories===
For specific problems, effective theories may be written down that give qualitatively correct results in certain limits. In the best of cases, these may then be obtained as systematic expansions in some parameters of the QCD Lagrangian. One such [[effective field theory]] is [[chiral perturbation theory]] or ChiPT, which is the QCD effective theory at low energies. More precisely, it is a low energy expansion based on the spontaneous chiral symmetry breaking of QCD, which is an exact symmetry when quark masses are equal to zero, but for the u, d and s quark, which have small mass, it is still a good approximate symmetry. Depending on the number of quarks that are treated as light, one uses either SU(2) ChiPT or SU(3) ChiPT. Other effective theories are [[heavy quark effective theory]] (which expands around heavy quark mass near infinity), and [[soft-collinear effective theory]] (which expands around large ratios of energy scales). In addition to effective theories, models like the [[Nambu–Jona-Lasinio model]] and the [[chiral model]] are often used when discussing general features.