Positronium decay and C-odd bosons

Triplet-state orthopositronium (o-Ps) decay rate measurements are in marked disagreement by 0.17(2)% with QED theoretical calculations done up to, but not including order {alpha}{sup 2} radiative and relativistic corrections. The existence of a rare exotic decay branch o-Ps {r_arrow} 2{gamma} + X{sup o} (where X{sup o} is a neutral, massive C-odd boson) could resolve the discrepancy. The authors have searched for, but have not found evidence of the X{sup o} in two recent experiments. Stringent limits are placed on the exotic decay branching ratio e{sup +} + e{sup {minus}} {r_arrow} X{sup o} + {gamma} as a function of the X{sup o} mass and lifetime. The results are interpreted for two distinct models of X{sup o} interactions and related to o-Ps decay as well as other experiments, including singlet-state parapositronium decay rate measurements.