The search for the decay of the long-lived neutral kaon to a pion, an electron, and a positron

The KTeV/E799 experiment at Fermilab has finished analyzing the 1999–2000 data for the rare kaon decay K_L → π 0e+e− . This mode is of interest because it is expected to have a large CP-violating component. The measurement of its branching ratio could support Standard Model predictions through the CKM mechanism for CP violation or it could indicate the existence of new physics. One event is observed in the 1999–2000 data, with the expected background at 0.99 ± 0.22 events. We set a branching ratio upper limit of 3.5 × 10−10 at the 90% confidence level.