Dispersive representation of K -> 3 pi amplitudes and cusps
  The NA48/2 collaboration has shown clear experimental evidence for a cusp in the data for K -> pi pi^0 pi^0. This effect can be used to extract information on the pi-pi scattering lengths. We address this issue using a two-loop dispersive construction of pi pi -> pi pi and K -> pi pi pi amplitudes in the presence of isospin breaking. 
Karol Kampf, Marc Knecht, Jiri Novotny, Martin Zdrahal