This experiment was motivated by the long standing discrepancy between theory and experiment in the lifetime of ortho-positronium. A possible contribution of an exotic channel could have explained what at that time was still considered a  puzzle (the new measurements of the Michigan and Tokyo groups in 2003 are now agreement). 

Exotic decays have been searched extensively (for a review see for example (Mod. Phys. Lett. A Vol. 17, No. 26 (2002) 1713), one of the remaining possibilities which was not yet excluded was the possibility of a decay of o-Ps in a photon and two weakly interacting particles. The goal of our experiment was to search for it  with the sensitivity necessary to exclude it as a possible origin of the discrepancy. Our result have been published (Badertscher et al., Phys Lett. B543:29-34, 2002),

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