An Apparatus To Search For Mirror Dark Matter

Among several interesting motivations for experimenting with orthopositronium (o - Ps) in vacuum the most exciting one is probably related to the search for the dark matter of a mirror-type. The mirror matter is predicted to exist if parity is an unbroken symmetry of the vacuum. The existence of the mirror matter, which in addition to gravity communicates with our world through photon-mirror photon mixing, would result in orthopositronium (o - Ps) to mirror orthopositronium (o - Ps') oscillations. The experimental signature of this effect is the invisible decay of o - Ps in vacuum.We discuss an experiment to search for o - Ps --> invisible decay in vacuum with a sensitivity in the branching ratio Br(o - Ps --> invisible) similar or equal to 10(-7), which is an order of magnitude better than the present limit on this decay mode from the Big Bang Nucleosynthesis. Details of the design are presented. The effect of various environments on o - Ps --> o - Ps' oscillation probability is also discussed.