Measurement of the Rare Decayπ0→e+e−

Abstract

Evidence for the rare decay ${\pi }^0\to e^{+}{e}^{-}$ has been obtained in an experiment which measured the invariant mass spectrum of $e^{+}{e}^{-}$ pairs produced by 300-MeV/c ${\pi }^{-}$ mesons interacting in a liquid-hydrogen target. The branching ratio is (1.8±0.6)×${10}^{-7\mathrm{}}$, about four times larger than the unitarity lower limit.