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 β⁺ decay of ²²Na and stopped by an aerogel target. No invisible event is detected for a total of 10⁸ stopping positrons, thereby giving an upper limit of 2.8×{}10^-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^-8 on the photon--mirror-photon mixing, and rules out millicharged particles lighter than 500 keV.
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Mitsui et al. (1993) studied this question.
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