Search for exotic decays from low-energye+e−direct annihilation

A new exotic-decay candidate is proposed to resolve the orthopositronium decay rate discrepancy. A search has been performed for the process ${\mathit{e}}^{+}$ ${\mathit{e}}^{\mathrm{-}}$→γ+${\mathit{X}}^0$, where the ${\mathit{X}}^0$ is a long-lived, low-mass boson that interacts very weakly with matter and has a charge conjugation eigenvalue C=-1. No evidence for the ${\mathit{X}}^0$ has been found in this experiment, with essentially the entire ${\mathit{X}}^0$ mass range from 0.1 to 1 MeV being excluded as the cause of the decay rate discrepancy. Previous results from other experiments are analyzed for sensitivity to this ${\mathit{X}}^0$.