A. Zee, Quantum Field Theory in a Nutshell · Problem I.7.4

Field Theory Redux

Physics — QFT

Problem

By Lorentz invariance we can always take

k_1 + k_2 = (E, \vec{0})

in (23). The integral can be studied as a function of

E

. Show that for both internal particles to become real

E

must be greater than

2m

. Interpret physically what is happening.