Sudakov Limits

Abstract

In this chapter we investigate the Sudakov limit and related limits. We expand diagrams contributing to the quark or electron form factor and four-point amplitudes — see Fig. 8.1. These diagrams are relevant to a large number of physical processes. In the Sudakov limit for the form factor, the square of the momentum transfer Q ² ≡ −(p ₁ − p ₂)² is large compared with the masses, and, for the four-point amplitudes, the Mandelstam variables s = (p ₁ + p ₂)², t = (p ₁ − p ₃)² are larger than the masses. In the latter case, one has t = −s(1 − cosθ)/2 in the centre-of-mass frame, so that this limit is connected with scattering at fixed angles.