Observation of the rare decay KS→π0e+e−

Abstract A search for the decay K S → π 0 μ + μ − has been made by the NA48/1 Collaboration at the CERN SPS accelerator. The data were collected during 2002 with a high-intensity K S beam. Six events were found with a background expectation of 0.22 − 0.11 + 0.18 events. Using a vector matrix element and unit form factor, the measured branching ratio is B ( K S → π 0 μ + μ − ) = [ 2.9 − 1.2 + 1.5 ( stat ) ± 0.2 ( syst ) ] × 10 − 9 .