Sign of theπ0→γγDecay Amplitude

Abstract

The experimentally observed constructive interference between the amplitude for the Primakoff effect and the other amplitudes involved in ${\ensuremath{\pi}}^{0}$ photoproduction is interpreted as showing that the ${\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$ decay amplitude and ${g}_{\ensuremath{\pi}\mathrm{NN}}$ have opposite signs. The experimental ${\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$ amplitude thus agrees in sign and approximately in magnitude with the result of computing the triangle graph in perturbation theory with a single elementary proton going around the fermion loop.