Precision measurement of the ratio BR(K_S→π⁺π^-e⁺e^-)/BR(K_L→π⁺π^-π_D⁰)

The K_S→π⁺π^-e⁺e^- decay mode was investigated using the data collected in 2002 by the NA48/1 Collaboration. With about 23 k K_S→π⁺π^-e⁺e^- events and 59 k K_L→π⁺π^-π_D⁰ normalization decays, the K_S→π⁺π^-e⁺e^- branching ratio relative to the K_L→π⁺π^-π_D⁰ one was determined to be BR(K_S→π⁺π^-e⁺e^-)/BR(K_L→π⁺π^-π_D⁰)=(3.28±0.06stat±0.04syst)×10^-2. This result was used to set the upper limit |gE1/gBR|<3.0 at 90% CL on the presence, in the decay amplitude, of an E1 direct emission (gE1) term relative to the dominant inner bremsstrahlung (gBR) term. The CP-violating asymmetry Aø in the sinøcosø distribution of K_S→π⁺π^-e⁺e^- events, where ø is the angle between the π⁺π^- and the e⁺e^- decay planes in the kaon centre of mass, was found to be Aø=(-0.4±0.8)%, consistent with zero. These results are in good agreement with a description of the K_S→π⁺π^-e⁺e^- decay amplitude dominated by the CP-even inner bremsstrahlung process.