Motivated by the recently reported discrepancy between measured and theoretical values for the orthopositronium lifetime, we have searched for exotic decay of orthopositronium into γ plus a long-lived neutral particle X, in the hitherto unexplored mass region of $m_x$ below 100 keV. Our negative result provides new upper limits of 6.4×${10}^{\mathrm{-}5}$ to 7.6×${10}^{\mathrm{-}6}$ on the branching ratio of such decay. Consequently we exclude the hypothesis of Samuel, i.e., that a new pseudoscalar particle with mass less than 5.7 keV is responsible for the discrepancy of the orthopositronium lifetime.

• Received 30 May 1989

DOI:https://doi.org/10.1103/PhysRevLett.63.597