Measurement of five-photon decay in orthopositronium

Abstract

We have measured five-photon decay of orthopositronium (o-Ps) and obtained the branching ratio of o-Ps\ensuremath{\rightarrow}5\ensuremath{\gamma} to o-Ps\ensuremath{\rightarrow}3\ensuremath{\gamma} as [2.${2}_{\mathrm{\ensuremath{-}}1.6}^{+2.6}$(stat.)\ifmmode\pm\else\textpm\fi{}0.5(syst.)]\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$, being consistent with the lowest-order QED calculation. Hence the five-photon decay process does not account for the discrepancy between the experimentally obtained decay rate for o-Ps and the theoretically calculated decay rate for o-Ps\ensuremath{\rightarrow}3\ensuremath{\gamma}. \textcopyright{} 1996 The American Physical Society.