First search for invisible decays of orthopositronium confined in a vacuum cavity

Abstract

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 Br(o−Ps→invisible)<5.9×10−1 (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 ϵ<3.1×10−7 (90% C.L.). This limit was obtained for the first time in vacuum free of systematic effects due to collisions with matter.