Abstract
We searched for an exotic decay of orthopositronium into a single photon and a short-lived neutral boson ø 0 in hitherto unexplored mass region below 300 keV/ c 2. A high-resolution measurement of the associated photon energy spectrum was carried out with a germanium detector to discriminate the expected sharp peak of this decay from the broader peak of the pick-off annihilations. No evidence of such decay was observed, resulting in the upper-limits on the branching ratio of 3.0×10 −4 ( m Φ 0 <500 keV/ c 2). This clearly rules out the possibility of this exotic decay mode explaining the reported discrepancy in the orthopositronium decay rate.
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