Improved limit on invisible decays of positronium

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 (6.31 +/- 0.28 ) X 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 -> invisible could be set: Br(o-Ps -> invisible) < 4.2 X 10(-7) (90% C.L.) Our results provide a limit on the photon mirror- photon mixing strength epsilon <= 1.55 X 10(-7) (90% C.L.) and rule out particles lighter than the electron mass with a fraction Q(x) <= 3.4 X 10(-5) of the electron charge. Furthermore, upper limits on the branching ratios for the decay of parapositronium Br(p-Ps -> invisible) <= 4.3 X 10(-7) (90% C.L.) and the direct annihilation Br(e(+) e(-) -> invisible) <= 2.1 X 10(-8) (90% C.L.) could be set.