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)×10⁶ 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)<4.2×10^-7(90%C.L.) Our results provide a limit on the photon mirror-photon mixing strength γ≤1.55×10^-7(90%C.L.) and rule out particles lighter than the electron mass with a fraction Q_x≤3.4×10^-5 of the electron charge. Furthermore, upper limits on the branching ratios for the decay of parapositronium Br(p-Ps→invisible)≤4.3×10^-7(90%C.L.) and the direct annihilation Br(e⁺e^-→invisible)≤2.1×10^-8(90%C.L.) could be set.