LAUSR

The experimental setup and results of the first search for invisible decays of orthopositronium (o-Ps) confined in a vacuum cavity are reported. No evidence of invisible decays at a level $\mathrm{Br}(\mathrm{o}\text{\ensuremath{-}}\mathrm{Ps}\ensuremath{\rightarrow}\text{invisible})l5.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ (90% C.L.) was found. This decay channel is predicted in hidden sector models such as the mirror matter (MM), which could be a candidate for dark matter. Analyzed within the MM context, this result provides an upper limit on the kinetic mixing strength between ordinary and mirror photons of $ϵl3.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ (90% C.L.). This limit was obtained for the first time in vacuum free of systematic effects due to collisions with matter.