Search for an exotic three-body decay of orthopositronium

We report on a direct search for a three-body decay of the orthopositronium into a photon and two penetrating particles, o-Ps→ γ+X ₁+X ₂. The existence of this decay could explain the discrepancy between the measured and the predicted values of the orthopositronium decay rate. From the analysis of the collected data a single candidate event is found, consistent with the expected background. This allows to set an upper limit on the branching ratio B(o-Ps→ γ+X ₁+X ₂)<4.4×10 ^-5 (at the 90% confidence level), for the photon energy in the range from 40 keV < E_γ< 400 keV and for mass values in the kinematical range 0⩽ m_{X 1}+ m_{X 2}⩽900 keV. This result unambiguously excludes the o-Ps→ γ+X ₁+X ₂ decay mode as the origin of the discrepancy.