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Search for long-lived neutral bosons in orthopositronium decay.

We have searched for orthoposition decay into {gamma} plus a feebly interacting boson {ital X}{sup 0}, by looking for a narrow peak in the energy spectrum of single {gamma} taken under the condition of no additional {gamma} 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{times}10{sup {minus}6}, which is a factor of 60 more stringent than previous limits in the {ital m}{sub {ital x}} region below 30 keV. This experiment rules out the existence of light ({ital m}{sub {ital X}}{lt}800 GeV) Goldstone-like bosons having electron-coupling constants {alpha}{sub {ital X}{ital e}{ital e}} larger than 1.1{times}10{sup {minus}11}.