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.
ASJC Scopus subject areas
- Physics and Astronomy(all)