Dispersive representation of K→3π amplitudes and cusps

The NA48/2 collaboration has shown clear experimental evidence for a cusp in the data for K→ππ0π0. This effect can be used to extract information on the ππ scattering lengths. We address this issue using a two-loop dispersive construction of ππ→ππ and K→πππ amplitudes in the presence of isospin breaking.