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 $(6.31\pm 0.28)\times {10}^6$ 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 $\mathrm{o}\mathrm{\text{−}}\mathrm{Ps}\to \mathrm{\text{invisible}}$ could be set: $\mathrm{Br}(\mathrm{o}\mathrm{\text{−}}\mathrm{Ps}\to \mathrm{\text{invisible}})<4.2\times {10}^{-7}(90\%\mathrm{C}.\mathrm{L}.)$ Our results provide a limit on the photon mirror-photon mixing strength $ϵ\le 1.55\times {10}^{-7}(90\%\mathrm{C}.\mathrm{L}.)$ and rule out particles lighter than the electron mass with a fraction $Q_x\le 3.4\times {10}^{-5}$ of the electron charge. Furthermore, upper limits on the branching ratios for the decay of parapositronium $\mathrm{Br}(\mathrm{p}\mathrm{\text{−}}\mathrm{Ps}\to \mathrm{\text{invisible}})\le 4.3\times {10}^{-7}(90\%\mathrm{C}.\mathrm{L}.)$ and the direct annihilation $\mathrm{Br}(e^{+}{e}^{-}\to \mathrm{\text{invisible}})\le 2.1\times {10}^{-8}(90\%\mathrm{C}.\mathrm{L}.)$ could be set.