Griffiths Introduction to Elementary Particles · Problem 7.37
Electrodynamics of Quarks and Hadrons — David J. Griffiths
Physics — QFT · positronium-decay-amplitude
Problem
Starting with equation (7.155), calculate ∣M∣2, and use equation (7.103) to sum over photon polarizations. Check that the answer is consistent with equation (7.160), and explain why this method gives the correct answer. [Note that we are now summing over all photon polarizations, whereas in fact the photons must be in the singlet configuration.]