Limit on an exotic three-body decay of orthopositronium

An exotic annihilation of ${\rm e}^{+}{\rm e}^{-}$ at rest into a single γ and a weakly interacting neutral vector boson ${\rm V}^0$ 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 $4.2 \times 10^{-6}$ ($m_{{\rm V}^0} < 200$ keV/c2) and $2.1 \times 10^{-5}$ ($m_{{\rm V}^0} < 2m_{\rm e} = 1022$ keV/c2). These upper limits were converted to those on the branching ratio of an exotic three-body decay ($\gamma\gamma{\rm V}^0$) of orthopositronium (o-Ps), i.e. $6.2 \times 10^{-6}$ ($ m_{{\rm V}^0}< 200$ keV/c2) and $1.7 \times 10^{-5}$ ($m_{{\rm V}^0}< 2m_{\rm e}$). As a result, this mode was definitely excluded as the source of the reported discrepancy in the o-Ps decay rate.