Observation of the rare decay KS→π0e+e−

Abstract

A search for the decay K S → π 0 e + e − has been made by the NA48/1 experiment at the CERN SPS accelerator. Using data collected during 89 days in 2002 with a high-intensity K S beam, 7 events were found with a background of 0.15 events. The branching fraction BR (K S →π 0 e + e − , m ee >0.165 GeV /c 2 )=(3.0 +1.5 −1.2 ( stat )±0.2( syst ))×10 −9 has been measured. Using a vector matrix element and a form factor equal to one, the measurement gives BR( K S → π 0 e + e − )=(5.8 +2.9 −2.4 )×10 −9 .