The first observation of the K± → π± π0 e+ e- rare decay by the NA48/2 experiment at CERN SPS is reported, based on ∼ 2x10^11 K± decays recorded in 2003-2004. From a sample of 4919 candidates with 4.9% background the branching ratio in the full kinematic region is measured to be BR(K± → π± π0 e+ e-) = (4.24±0.14)x10^-6 .
The study of the kinematic space shows evidence for a structure dependent contribution in agreement with predictions based on Chiral Perturbation Theory. The CP-violating asymmetry between K+ → π+ π0 e+ e- and K- → π- π0 e+ e- decay rates has been measured, providing an upper limit | A_CP | < 4.82 x 10^-2 at 90% CL.
The NA48/2 combined measurement of the charged kaon semileptonic form factors, based on 4.4 million K± → π0 e± ν_e and 2.3 million K± → π0 μ± ν_μ events collected in 2004, is presented. This result represents the most precise form factor measurement from a combined Kl3 analysis.