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

The NA48/2 experiment collected data in 2003 and 2004 with the main purpose to study direct CP violation in charged kaons decaying into three pions. Thanks to the huge statistics collected, other studies are also possible. This paper reports the first observation of the K$^{±}$ → π$^{±}$π$^0$e$^+$e$^−$ decay from an exposure of 1.7 x 10$^{11}$ charged kaon decays. The Branching Ratio has been determined to be BR(K$^{±}$ → π$^{±}$π$^0$e$^+$e$^−$) = (4.24 ± 0.14)x10$^{-6}$, using a sample of 4919 candidates with 4.9% background contamination. Studies of the kinematic space and of several P- and CP-violating asymmetries are also reported.