Griffiths Introduction to Elementary Particles · Problem 8.3
Quantum Chromodynamics — David J. Griffiths
Physics — QFT · ratio-R-definition
Problem
Why don't we use σ(e+e−→e+e−) in the denominator, to define R [eq. (8.7)]? [Answer: For one thing, we would have to include the crossed diagram and the kinematic factors would no longer cancel.]