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The experimental setup and results of the first search for invisible decaysof ortho-positronium (o-Ps) confined in a vacuum cavity are reported. Noevidence of invisible decays at a level$\text{Br}\left(\text{o-Ps}\to\text{invisible}\right) < 5.9\times 10^{-4}$ (90%C. L.) was found. This decay channel is predicted in Hidden Sector models suchas the Mirror Matter (MM), which could be a candidate for Dark Matter. Analyzedwithin the MM context, this result provides an upper limit on the kineticmixing strength between ordinary and mirror photons of $\epsilon < 3.1 \times10^{-7}$ (90% C. L.). This limit was obtained for the first time in vacuum freeof systematic effects due to collisions with matter.

First search for invisible decays of ortho-positronium confined in a vacuum cavity