Direct search for two-photon decay modes of orthopositronium

A direct search for γ rays from the forbidden decay o-Ps→2γ has been performed using the same apparatus that recently measured an anomalously high o-Ps decay rate. Using a high-resolution Ge γ-ray detector, a 233-ppm limit is set on the branching ratio to a pair of 511-keV γ rays, and a 200-ppm limit is set on the branching ratio pair of unequal-energy γ rays that sum to 1022 keV. Hence, these 2γ modes cannot be responsible for the o-Ps decay-rate discrepancy between theory and experiment of 1400±230 ppm