Direct search for orthopositronium decay into two photons

S. Asai, S. Orito, T. Sanuki, M. Yasuda, T. Yokoi

Research output: Contribution to journalArticlepeer-review

25 Citations (Scopus)


We have performed the first direct search for orthopositronium decay into two photons in order to determine if this decay, forbidden by space-rotational invariance, is the cause of the reported discrepancy in the orthopositronium decay rate. A high-resolution measurement of the photon-energy spectrum by a germanium detector was used to discriminate the expected sharp peak of the two-photon decay from the broader peak of the pickoff annihilations, the magnitude of which was minimized by selecting a target of low-density silica powder and by placing it in vacuum. No evidence of such decay is observed, resulting in an upper limit on the branching ratio of 3.5×10-4, a factor of 6 below the level that could explain the decay-rate discrepancy.

Original languageEnglish
Pages (from-to)1298-1301
Number of pages4
JournalPhysical review letters
Issue number10
Publication statusPublished - 1991
Externally publishedYes

ASJC Scopus subject areas

  • Physics and Astronomy(all)


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