A search for massive neutral bosons in orthopositronium decay

TL;DR

This study conducted a high-resolution search for a hypothetical neutral boson in orthopositronium decay, setting upper limits on its occurrence and ruling out its role in explaining decay-rate discrepancies.

Contribution

It provides the first experimental upper limits on exotic decay modes of orthopositronium into a photon and a neutral boson above 900 keV/c^2.

Findings

No evidence of the exotic decay was observed.

Upper limits of 2.0 x 10^-4 on the branching ratio were established.

The decay mode is excluded as an explanation for the decay-rate discrepancy.

Abstract

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 ${\rm keV}/{\it c}^{2}$, 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 the upper-limits of\mbox{ }$2.0 \times 10^{-4}$ on the branching ratio of such a decay in the mass region from 847 to 1013 ${\rm keV}/{\it c}^{2}$, and excludes the possibility of this decay mode explaining the discrepancy in the orthopositronium decay-rate.