Measurement of ππ Scattering Lengths from Ke4 and K3π Decays

In the last few years it has become possible to study low energy ππ scattering with high precision in kaon decays, thanks to the high statistics measurements of K±→π±π0π0 (K3π) and K±→π+π-e±ν (Ke4) decays performed by the NA48/2 experiment at the CERN SPS. In K±→π±π0π0, at the π+π- threshold, the π0π0 mass spectrum exhibits a Wigner-cusp, from which the scattering lengths a0 and a2 are extracted. From the study of Ke4 decays a similar precision, but quite independent measurement of a0 and a2 can be obtained. The combination of both measurements provides one of the most stringent tests of Chiral Perturbation Theory.