K -> 3 Pi Decays in Chiral Perturbation Theory
  A kaon decaying into three pions is an example of a weak process. However, since the quarks are confined into mesons, the strong interaction also plays an important part. The reaction is a low-energy one, meaning that it takes place in the non-perturbative region of QCD. In that region perturbative QCD doesn't give you any answers and other methods have to be used. The one we use is called Chiral Perturbation Theory. 
Fredrik Borg