Search for long-lived neutral bosons in orthopositronium decay

We have searched for orthoposition decay into γ plus a feebly interacting boson ${\mathit{X}}^0$, by looking for a narrow peak in the energy spectrum of single γ taken under the condition of no additional γ detected in a hermetic photon detector. No evidence of such decay is observed, resulting in a new upper limit on the branching ratio of 1.1×${10}^{\mathrm{-}6}$, which is a factor of 60 more stringent than previous limits in the ${\mathit{m}}_{\mathit{x}}$ region below 30 keV. This experiment rules out the existence of light (${\mathit{m}}_{\mathit{X}}$<800 GeV) Goldstone-like bosons having electron-coupling constants ${\mathrm{\alpha }}_{\mathit{X}\mathit{e}\mathit{e}}$ larger than 1.1×${10}^{\mathrm{-}11}$.