Cusps in K --> 3pi decays: a theoretical framework
  Based on the analysis of 6.031*10^7 K+- --> pi0 pi0 pi+- decays, the NA48/2 collaboration has recently determined the S-wave pi-pi scattering lengths a_0-a_2 with high precision. In addition, the scattering length a_2 has been independently measured, although less precisely so. The present article discusses in detail one of the theoretical frameworks used in the data analysis. 
Juerg Gasser, Bastian Kubis, Akaki Rusetsky
hep-ph hep-ex