Measurement of the Branching Ratio ofπ0→e+e−UsingKL→3π0Decays in Flight

The branching ratio of the rare decay ${\pi }^0\to e^{+}{e}^{-}$ has been measured in E799-II, a rare kaon decay experiment using the KTeV detector at Fermilab. We observed 275 candidate ${\pi }^0\to e^{+}{e}^{-}$ events, with an expected background of $21.4\pm 6.2$ events which includes the contribution from Dalitz decays. We measured $B[{\pi }^0\to e^{+}{e}^{-},(m_{e^{+}{e}^{-}}{/m}_{{\pi }^0}{)}^2>0.95\mathrm{}]=(6.09\pm 0.40\pm 0.24)\times {10}^{-8\mathrm{}}$, 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.