Limit on an exotic three-body decay of orthopositronium

An exotic annihilation of e⁺e^- at rest into a single γ and a weakly interacting neutral vector boson V⁰ was searched for by measuring the related γ energy under the condition that no additional γ was detected by a hermetic detector. No evidence of such an annihilation was observed, resulting in the upper limits on the branching ratio of 4.2 × 10^-6 (m^vo < 200 keV/c²) and 2.1 × 10^-5 (m_vo < 2m_e = 1022 keV/c²). These upper limits were converted to those on the branching ratio of an exotic three-body decay (γγV⁰) of orthopositronium (o-Ps), i.e. 6.2 × 10^-6 (m_vo < 200 keV/c²) and 1.7 × 10^-5 (m_vo < 2m_e). As a result, this mode was definitely excluded as the source of the reported discrepancy in the o-Ps decay rate.