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The results of a new search for positronium decays into invisible final states are reported. Convincing detection of this decay mode would be a strong evidence for new physics beyond the standard model (SM): for example, the existence of extra--dimensions, of milli-charged particles, of new light gauge bosons or of mirror particles. Mirror matter could be a relevant dark matter candidate. In this paper the setup and the results of a new experiment are presented. In a collected sample of about (6. 310. 28) 10^6 orthopositronium decays, no evidence for invisible decays in an energy 0, 80 keV was found and an upper limit on the branching ratio of orthopositronium o-Ps could be set: Br (o-Ps) <4. 210^-7 (90%C. L. ) Our results provide a limit on the photon mirror-photon mixing strength ϵ1. 5510^-7 (90%C. L. ) and rule out particles lighter than the electron mass with a fraction Qₗ3. 410^-5 of the electron charge. Furthermore, upper limits on the branching ratios for the decay of parapositronium Br (p-Ps) 4. 310^-7 (90%C. L. ) and the direct annihilation Br (e^+e^-) 2. 110^-8 (90%C. L. ) could be set.
Badertscher et al. (Tue,) studied this question.