MEASUREMENT OF THE BRANCHING RATIO OF PI(0)-]E(+)E(-) FROM PI(0)S PRODUCED BY K(L)-]PI(0)PI(0)PI(0) DECAYS IN FLIGHT

The Fermilab E799 Collaboration has carried out a search for the rare decay pi0 --> e+e- using pi0's produced from K(L) --> pi0pi0pi0 decays in flight. We observe a signal of nine events with an e+e-mass close to the pi0 mass where only one event is predicted from other known processes. From this, we measure B(pi0 --> e+e-, (m(ee)/m(pi0))2 > 0.95) = [7.6(-2.8)+3.9(stat) +/- 0.5 (syst)] x 10(-8), over and above the contribution from pi0 Dalitz decays. Correcting for radiative effects, we find this result consistent with theoretical expectations from standard-model processes.