Precision measurement of the ratio BR(K-S -> pi(+)pi(-)e(+)e(-))/BR(K-L -> pi(+)pi(-)pi(0)(D))

Abstract 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 D 0 normalization decays, the K S → π + π − e + e − branching ratio relative to the K L → π + π − π D 0 one was determined to be BR ( K S → π + π − e + e − ) / BR ( K L → π + π − π D 0 ) = ( 3.28 ± 0.06 stat ± 0.04 syst ) × 10 − 2 . This result was used to set the upper limit | g E1 / g BR | 3.0 at 90% CL on the presence, in the decay amplitude, of an E1 direct emission ( g E1 ) term relative to the dominant inner bremsstrahlung ( g BR ) term. The CP-violating asymmetry A ϕ in the sin ϕ cos ϕ distribution of K S → π + π − e + e − events, where ϕ is the angle between the π + π − and the e + e − decay planes in the kaon centre of mass, was found to be A ϕ = ( − 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.