Abstract
The experimental setup and results of the first search for invisible decays of ortho-positronium (o-Ps) confined in a vacuum cavity are reported. No evidence 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 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 $\epsilon < 3.1 \times 10^{-7}$ (90% C. L.). This limit was obtained for the first time in vacuum free of systematic effects due to collisions with matter.
Highlights
The origin of dark matter is a question of great importance for both cosmology and particle physics
No evidence of invisible decays at a level Brðo-Ps → invisibleÞ < 5.9 × 10−4 (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
Note that o-Ps confined in a vacuum cavity will undergo collisions with the walls, whose rate can be modulated by tuning the o-Ps kinetic energy
Summary
The origin of dark matter is a question of great importance for both cosmology and particle physics. The latest Planck mission results [4] provide an accurate estimate of the abundance of baryons Ωb 1⁄4 0.048 and cold matter Ωc 1⁄4 0.258, leading to an abundance of cold matter ∼5 times larger than ordinary matter. The explanation of such observations is one of the strongest hints of the existence of new physics. The calorimeter was designed to ensure high hermeticity and minimize photon energy losses, verified by a detailed GEANT4 [51] simulation of the complete setup. With a kinetic energy Eo-Ps 1⁄4 440 meV, corresponding to the shallowest implantation energy Eeþ 1⁄4 2 keV, the total background due to energy losses and hermeticity is at a level of 10−7, below the experimental sensitivity
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