Pion scattering lengths from the NA48/2 experiment at CERN

The NA48 experiment at CERN collected more than 3.1 × 10^9 K± → π±π+π− decays and more than 110 × 10^6 K± → π±πoπo decays for searching CP violating asymmetries in charged kaon decays. An unexpected effect involving strong interactions was found while studing the kinematics of those decays. A study of a partial sample of 23 × 10^7 K± → π±πoπo decays was reported at this workshop, whose results show an anomaly (cusp) in the distribution of the πoπo invariant mass (moo) in the region around moo = 2m+, where m+ is the charged pion mass. The cusp, never observed in the past, is interpreted as an effect due mainly to the final state charge exchange scattering process π+π− → πoπo in the K± → π±π+π− decay. The effect provides a new method for a precise determination of the difference a0−a2 between the scattering lengths in the isospin I = 0 and I = 2 states. The cusp effect was confirmed by the NA48 collaboration, by finding a similar anomaly in the study of the πoπo invariant mass in the KL → 3πo decays, collected in the year 2000 with a statistics exceeding 100 × 10^6.