Low energy dynamics and the ππ scattering lengths from the NA48/2 experiment at CERN

The observation of a cusp-like structure in the M00 invariant mass distribution in the decay K ± → π±π0π0 by the NA48/2 experiment at CERN has given a renewed interest in the ππ scattering lengths. We report the result of a study of a partial sample of 2.3 × 10 decays, the observation of an anomaly in the ππ mass spectrum in the region around M00 = m+, where m+ is the mass of the charged pion, and the subsequent interpretation in terms of the ππ scattering lengths. This anomaly, never observed in previous experiments, has been interpreted as an effect due to the final state charge exchange scattering process π+π− → ππ in K± → π±π+π−. From the distribution we can determine precisely (a0 − a2), the difference between the ππ scattering lengths in the isospin I = 0 and I = 2 states.