Search for an exotic three-body decay of orthopositronium o-Ps→γ+X1+X2

We carried out a model independent search for an exotic three-body decay of orthopositronium o-Ps --> gamma + X-1 + X-2 into a photon and two weak interacting particles X-1 and X-2. The existence of this decay could explain the discrepancy between the measured and the predicted values of the orthopositronium-decay rate in vacuum. Our result excludes this decay mode as the origin of the discrepancy and an upper limit on the branching ratio has been set B(o-Ps --> gamma + X-1 + X-2) < 4.4 x 10(-5) (at the 90% confidence level), for the photon energy in the range from 40 < E-gamma < 400 keV and for the weak interacting particles X mass range 0 <= m(X1) + m(X2) <= 900 keV.