Search for long-lived neutral bosons in orthopositronium decay

We have searched for orthoposition decay into plus a feebly interacting boson X0, by looking for a narrow peak in the energy spectrum of single taken under the condition of no additional 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 (mX<800 GeV) Goldstone-like bosons having electron-coupling constants ±Xee larger than 1.1×10-11.