Search for long-lived neutral bosons in orthopositronium decay

We have searched for orthoposition decay into {γ} plus a feebly interacting boson X⁰, 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^-6, which is a factor of 60 more stringent than previous limits in the mₓ region below 30 keV. This experiment rules out the existence of light (mX800 GeV) Goldstone-like bosons having electron-coupling constants αXee larger than 1.1×{}10^-11.