A search for massive neutral bosons in orthopositronium decay

We have searched for an exotic decay of orthopositronium into a single photon and a short-lived neutral boson in the hitherto unexplored mass region above 900 keV/c2, by noting that this decay is one of few remaining candidates which could explain the discrepancy of the orthopositronium decay rate. A high-resolution measurement of the associated photon energy spectrum was carried out with a germanium detector to search for a sharp peak from this two-body decay. Our negative result provides an upper limit of 2.0 × 10−4 on the branching ratio of such a decay in the mass region from 847 to 1013 keV/c2, and excludes the possibility of this decay mode explaining the discrepancy in the orthopositronium decay rate.