Measurement of the branching ratio of ${\mathrm{\pi }}^0$→${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ from ${\mathrm{\pi }}^0$’s produced by ${\mathit{K}}_{\mathit{L}}$→${\mathrm{\pi }}^0$${\mathrm{\pi }}^0$${\mathrm{\pi }}^0$ decays in flight

The Fermilab E799 Collaboration has carried out a search for the rare decay ${\mathrm{\pi }}^0$→${\mathit{e}}^{+}$ ${\mathit{e}}^{\mathrm{-}}$ using ${\mathrm{\pi }}^0$’s produced from ${\mathit{K}}_{\mathit{L}}$→${\mathrm{\pi }}^0$ ${\mathrm{\pi }}^0$ ${\mathrm{\pi }}^0$ decays in flight. We observe a signal of nine events with an ${\mathit{e}}^{+}$ ${\mathit{e}}^{\mathrm{-}}$ mass close to the ${\mathrm{\pi }}^0$ mass where only one event is predicted from other known processes. From this, we measure B(${\mathrm{\pi }}^0$→${\mathit{e}}^{+}$ ${\mathit{e}}^{\mathrm{-}}$,(${\mathit{m}}_{\mathit{e}\mathit{e}}$/${\mathit{m}}_{\mathrm{\pi }}^0$ ${)}^2$≳0.95) =[7.$6_{\mathrm{-}2.8}^{+3.9\mathrm{}}$(stat)±0.5(syst)]×${10}^{\mathrm{-}8}$, over and above the contribution from ${\mathrm{\pi }}^0$ Dalitz decays. Correcting for radiative effects, we find this result consistent with theoretical expectations from standard-model processes.