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 Na22 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.
ASJC Scopus subject areas
- Physics and Astronomy(all)