Measurement of the Branching Ratio of [pi][sup 0] [r arrow] e[sup +] e[sup [minus]] Using K[sub L] [r arrow] 3[pi][sup 0] Decays in Flight

The branching ratio of the rare decay [pi][sup 0][r arrow]e[sup +]e[sup [minus]] has been measured in E799-II, a rare kaon decay experiment using the KTeV detector at Fermilab. We observed 275 candidate [pi][sup 0][r arrow]e[sup +]e[sup [minus]] events, with an expected background of 21.4[plus minus]6.2 events which includes the contribution from Dalitz decays. We measured B[[pi][sup 0][r arrow]e[sup +]e[sup [minus]],thinsp (m[sub e[sup +]e[sup [minus]]]/m[sub [pi][sup 0]])[sup 2][gt]0.95]=(6.09[plus minus] 0.40[plus minus]0.24)[times]10[sup [minus]8] , where the first error is statistical and the second systematic. This result is the first significant observation of the excess rate for this decay above the unitarity lower bound. [copyright] [ital 1999] [ital The American Physical Society ]