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\char21{}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 $(6.31\ifmmode\pm\else\textpm\fi{}0.28)\ifmmode\times\else\texttimes\fi{}{10}^{6}$ 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 $\mathrm{o}\mathrm{\text{\ensuremath{-}}}\mathrm{Ps}\ensuremath{\rightarrow}\mathrm{\text{invisible}}$ could be set: $\mathrm{Br}(\mathrm{o}\mathrm{\text{\ensuremath{-}}}\mathrm{Ps}\ensuremath{\rightarrow}\mathrm{\text{invisible}})l4.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}(90%\mathrm{C}.\mathrm{L}.)$ Our results provide a limit on the photon mirror-photon mixing strength $ϵ\ensuremath{\le}1.55\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}(90%\mathrm{C}.\mathrm{L}.)$ and rule out particles lighter than the electron mass with a fraction ${Q}_{x}\ensuremath{\le}3.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ of the electron charge. Furthermore, upper limits on the branching ratios for the decay of parapositronium $\mathrm{Br}(\mathrm{p}\mathrm{\text{\ensuremath{-}}}\mathrm{Ps}\ensuremath{\rightarrow}\mathrm{\text{invisible}})\ensuremath{\le}4.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}(90%\mathrm{C}.\mathrm{L}.)$ and the direct annihilation $\mathrm{Br}({e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\mathrm{\text{invisible}})\ensuremath{\le}2.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}(90%\mathrm{C}.\mathrm{L}.)$ could be set.