# Annihilation of Electron by Positron to Produce at least 2 Gamma Rays

 Sci Advisor HW Helper P: 11,956 This is the simplest exercise in quantum field theory, showing that the vertex of QED can't have all 3 particles on the mass shell. You only need the energy-momentum equation describing the mass shell. $$p^2 = m^2 \, \mbox{for the electron/positron and} \, p^2=0 \, \mbox{for the photon}$$