Wigner-Cusp in Kaon Decays and Determination of pi pi Scattering Lengths

In the last few years it has become possible to study low energy pi pi scattering in kaon decays to three pions, thanks to the high statistics measurement of K± -> pi± pi0 pi0 decays performed by the NA48/2 experiment at the CERN SPS. At the pi+ pi- threshold, the pi0 pi0 mass spectrum exhibits a Wigner-cusp, from which the S-wave pi pi scattering lengths are extracted with high precision. This measurement is complementary to the extraction of the scattering lengths from K_e4 decays, which is also performed by the NA48/2 experiment.