Orthopositronium (o-Ps) decay or conversion into ‘‘invisible’’ final states is searched for by measuring the energy deposited in a hermetic photon detector for each positron produced from ${\mathrm{\beta }}^{+}$ decay of ${}_{}{}^{22}{}_{}{}^{}\mathrm{Na}$ and stopped by an aerogel target. No invisible event is detected for a total of ${10}^8$ stopping positrons, thereby giving an upper limit of 2.8×${10}^{\mathrm{-}6}$ on the o-Ps branching ratio, which is 200 times more stringent than previous limits. This experiment excludes an invisible decay as the origin of the reported discrepancy on o-Ps lifetime, provides a limit of ε<1.5×${10}^{\mathrm{-}8}$ on the photon–mirror-photon mixing, and rules out millicharged particles lighter than 500 keV.

• Received 21 January 1993

DOI:https://doi.org/10.1103/PhysRevLett.70.2265