New limits on exotic two-body decay of orthopositronium

Motivated by the recently reported discrepancy between measured and theoretical values for the orthopositronium lifetime, we have searched for exotic decay of orthopositronium into \ensuremath{\gamma} 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\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ to 7.6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$ 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.