Measurement of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="italic">e</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="italic">e</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="normal">−</mml:mi></mml:mrow></mml:msup></mml:mrow></mml:math>annihilation at rest into four γ rays

We have measured for the first time the decay rate for ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}4\ensuremath{\gamma} from rest, by using a multi-\ensuremath{\gamma}-ray spectrometer. The branching ratio R=${\ensuremath{\lambda}}_{\mathit{S}}^{4\ensuremath{\gamma}}$/${\ensuremath{\lambda}}_{\mathit{S}}^{2\ensuremath{\gamma}}$, where ${\ensuremath{\lambda}}_{\mathit{S}}^{4\ensuremath{\gamma}}$ and ${\ensuremath{\lambda}}_{\mathit{S}}^{2\ensuremath{\gamma}}$ are the 4\ensuremath{\gamma} and 2\ensuremath{\gamma} rates, respectively, was measured to be R=[1.30\ifmmode\pm\else\textpm\fi{}0.26(stat)\ifmmode\pm\else\textpm\fi{}0.16(system)]\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$, consistent with the lowest-order QED calculation.