Measurement of the branching Ratio of π0 → e+ e- from π0'S produced by KL → π0 π0 π0decays in flight

The Fermilab E799 Collaboration has carried out a search for the rare decay π 0 →e + e - using π 0 's produced from K L →π 0 p 0 p 0 decays in flight. We observe a signal of nine events with an e + e - mass close to the π 0 mass where only one event is predicted from other known processes. From this, we measure B(π 0 →e + e - , (m ee /m π o) 2 >0.95)=[7.6 -2.8 +3.9 (stat)±0.5(syst)]×10 -8 , over and above the contribution from π 0 Dalitz decays. Correcting for radiative effects, we find this result consistent with theoretical expectations from standard-model processes