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 plus a long-lived neutral particle X, in the hitherto unexplored mass region of mx below 100 keV. Our negative result provides new upper limits of 6.4×10-5 to 7.6×10-6 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.