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The understanding of the origin of dark matter has great importance forcosmology and particle physics. Several interesting extensions of the standardmodel dealing with solution of this problem motivate the concept of hiddensectors consisting of SU(3)xSU(2)_LxU(1)_Y singlet fields. Among these models,the mirror matter model is certainly one of the most interesting. The modelexplains the origin of parity violation in weak interactions, it could alsoexplain the baryon asymmetry of the Universe and provide a natural ground forthe explanation of dark matter. The mirror matter could have a portal to ourworld through photon-mirror photon mixing (epsilon). This mixing would lead toorthopositronium (o-Ps) to mirror orthopositronium oscillations, theexperimental signature of which is the apparently invisible decay of o-Ps. Inthis paper, we describe an experiment to search for the decay o-Ps -> invisiblein vacuum by using a pulsed slow positron beam and a massive 4pi BGO crystalcalorimeter. The developed high efficiency positron tagging system, the lowcalorimeter energy threshold and high hermiticity allow the expectedsensitivity in mixing strength to be epsilon about 10^-9, which is more thanone order of magnitude below the current Big Bang Nucleosynthesis limit and ina region of parameter space of great theoretical and phenomenological interest.The vacuum experiment with such sensitivity is particularly timely in light ofthe recent DAMA/LIBRA observations of the annual modulation signal consistentwith a mirror type dark matter interpretation.

Positronium Portal into Hidden Sector: A new Experiment to Search for Mirror Dark Matter