First observation and study of the K± → π±π0e+e− decay

Abstract The NA48/2 experiment at CERN reports the first observation of the K ± → π ± π 0 e + e − decay from an exposure of 1.7 × 10 11 charged kaon decays recorded in 2003–2004. A sample of 4919 candidates with 4.9% background contamination allows the determination of the branching ratio in the full kinematic region, B R ( K ± → π ± π 0 e + e − ) = ( 4.24 ± 0.14 ) × 10 − 6 . The study of the kinematic space shows evidence for a structure dependent contribution in agreement with predictions based on chiral perturbation theory. Several P- and CP-violating asymmetries are also evaluated.