Search for very weakly interacting, short-lived, C-odd bosons and the orthopositronium decay rate problem

The discrepancy between theory and experiment for the orthopositronium (o-Ps) decay rate could be resolved by an exotic decay branch o-Ps-->X(0) gamma gamma, where X(0) is a low-mass C-odd boson. An experimental search for X(0) is reported in which Compton-like interactions with electrons or decay gamma rays (X(0)-->3 gamma) could be detected. None are observed and sensitive, but model-dependent limits are set (for two specific models). One model, testable by this method, allows the good agreement between theory and experiment for the parapositronium decay rate to be reconciled with the disagreement for the o-Ps decay rate.