Ke4 decays and Wigner cusp

The NA48/2 experiment at the CERN SPS collected in 2003 and 2004 large samples of the decays K± -> pi+ pi- e± nu (Ke4±), K± -> pi0 pi0 e± nu (Ke400) and K± -> pi0 pi0 pi±. From the Ke4± form factors and from the cusp in the M00^2 distribution of the K± -> pi0 pi0 pi± events, the pi-pi scattering lengths a00 and a20 could be extracted. This measurement is a fundamental test of Chiral Perturbation Theory (ChPT). The branching fraction and form factors of the Ke400 decay were precisely measured, using a much larger data sample than in previous experiments. An improved measurement of the slope parameters for the decay K± -> pi0 pi0 pi± showed evidence for a non-zero value of k.