Measurement of the rare decayπ0→e+e−

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Abstract

The branching ratio of the rare decay ${\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}$ has been measured precisely, using the complete data set from the KTeV E799-II experiment at Fermilab. We observe 794 candidate ${\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}$ events using ${K}_{L}\ensuremath{\rightarrow}3{\ensuremath{\pi}}^{0}$ as a source of tagged ${\ensuremath{\pi}}^{0}$s. The expected background is $52.7\ifmmode\pm\else\textpm\fi{}11.2$ events, predominantly from high ${e}^{+}{e}^{\ensuremath{-}}$ mass ${\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}\ensuremath{\gamma}$ decays. We have measured $\mathrm{B}({\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}},({m}_{{e}^{+}{e}^{\ensuremath{-}}}/{m}_{{\ensuremath{\pi}}^{0}}{)}^{2}>0.95)=(6.44\ifmmode\pm\else\textpm\fi{}{0.25}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{0.22}_{\mathrm{syst}})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$, which is above the unitary bound from ${\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$ and within the range of theoretical expectations from the standard model.

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