First observation and study of the $K^{\pm} \rightarrow \pi^{\pm} \pi^0 e^+ e^-$ decay with the NA48/2 experiment at CERN

The first observation of the rare decay $K^{\pm} \rightarrow \pi^{\pm} \pi^0 e^+ e^-$ is reported by the NA48/2 experiment at CERN, based on 4919 candidates with 4.9% background contamination. From the analysis of 1.7 × 10$^{11}$ kaon decays collected in 2003–2004, the branching ratio in the full kinematic region is measured to be BR ($K^{\pm} \rightarrow \pi^{\pm} \pi^0 e^+ e^-$)= (4.24 ± 0.14) × 10$^{-6}$. A detailed study of the kinematic space shows evidence for a structure dependent contribution. Both measured BR and structure dependent contribution are in perfect agreement with theoretical predictions based on Chiral Perturbation Theory. Several P- and CP-violating asymmetries are also evaluated.