A new experiment to search for the invisible decay of the   orthopositronium
  We propose an experiment to search for invisible decays of orthopositronium (o-Ps) with a 90% confidence sensitivity in the branching ratio as low as $10^{-8}$. Evidence for this decay mode would unambigously signal new physics: either the existence of extra--dimensions or fractionally charged particles or new light gauge bosons. The experimental approach and the detector components of the proposed experiment are described. 
A. Badertscher, P. Crivelli, W. Fetscher, S.N. Gninenko, J.P.   Peigneux, A. Rubbia, D. Sillou