Talk:Chiral perturbation theory

The most important thing about chiral perturbation theory is the pattern of spontaneously broken chiral symmetry of massles QCD. This symmetry breaking implies, due to the Golstone theorem, the existence of Goldstone bosons (pions for SU(2) and pions, kaons and eta for SU(3)). With this fields, whose existence is guaranted by the Goldstome theorem, the most general lagrangian compatible with the QCD symmetries is constructed. When the quark masses (which for the u,d and s quark are small enough to treat them as a perturbation) are introduced, the goldstone bosons acquire a little mass, but are still the lightest degrees of freedom of the theory. With this lagrangian it is possible to calculate observables as an expansion series on the momenta and masses of the goldstone bosons involved over the typically symmetry breaking scale (aprox. 1 GeV). So this lagrangian can be used only at low energies.