Observation of a cusp-like structure in the pizero-pizero invariant mass   distribution from K+- ==> pi+- pizero pizero decay and determination of the   pi-pi scattering lengths
  We report the results from a study of ~23 Million K+- ==> pi+- pizero pizero decays recorded by the NA48/2 experiment at the CERN SPS, showing an anomaly in the pizero pizero invariant mass distribution in the region around 2m+, where m+ is the charged pion mass. This anomaly, never observed in previous experiments, can be interpreted as an effect due mainly to the final state charge exchange scattering process pi+ pi- ==> pizero pizero in K+- ==> pi+- pi+ pi- decay. It provides a precise determination of a0 - a2, the difference between the pi-pi scattering lengths in the isospin I=0 and I=2 states. 
The NA48/2 Collaboration J.R. Batley, C. Lazzeroni, D.J. Munday, M.W.   Slater, S.A. Wotton, et al