Direct search for orthopositronium decay into two photons.

Abstract

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\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$, a factor of 6 below the level that could explain the decay-rate discrepancy.