Determination of the relative decay rate KS→πeν/KL→πeνKS→πeν/KL→πeν

The decay rate of KS→πeνKS→πeν relative to the rate of KL→πeνKL→πeν has been measured by the NA48 Collaboration in a neutral kaon beam originating from 400 GeV proton-Be interactions at the CERN SPS. The result is 0.993±0.026stat±0.022syst0.993±0.026stat±0.022syst, compatible with 1 in agreement with the Standard Model prediction at tree level. It implies BR(KS→πeν)=(7.05±0.18stat±0.16syst)×10−4BR(KS→πeν)=(7.05±0.18stat±0.16syst)×10−4.