A. Zee, Quantum Field Theory in a Nutshell · Problem N.3.2
Field Theory Redux
Physics — QFT
Problem
In appendix 1 we recursed by complexifying the momenta of two external lines with helicity + and −. In the derivation of the recursion relation (3) we could have picked any two external lines to complexify. Determine the amplitude calculated directly in chapter N.2, namely A(1−,2−,3+,4+), by complexifying lines 1 and 2. This is an example of the self-consistency argument sketched in the text.
Particle physics experimentalists are fond of saying that yesterday's spectacular discovery is today's calibration and tomorrow's annoying background. The canonical example is the Nobel-winning discovery of the CP-violating decay of the KL meson into two pions. In theoretical physics, yesterday's discovery is today's homework exercise and tomorrow's trivium.