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

The NA48/2 experiment at CERN reports the first observation of the $K^{\pm }\to {\pi }^{\pm }{\pi }^0{e}^{+}{e}^{-}$ decay from an exposure of $1.7\times {10}^{11}$ 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, $BR(K^{\pm }\to {\pi }^{\pm }{\pi }^0{e}^{+}{e}^{-})=(4.24\pm 0.14)\times {10}^{-6}$. 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.