Abstract
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 (mV0 < 200 keV/c2) and 2.1 × 10−5 (mV0 < 2me = 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 (mV0 < 200 keV/c2) and 1.7 × 10−5 (mV0 < 2me). As a result, this mode was definitely excluded as the source of the reported discrepancy in the o-Ps decay rate.