The second-order diagrams we considered previously are often called “tree” diagrams, since they look like trees with branches. The next higher-order two-particle → two particle scattering processes in QED are given by diagrams shown in Fig. 44.1(a)−(d), which are referred to as “radiative corrections.” As we will discuss below, they all involve loops where, according to the Feynman rules, one must integrate over the internal momenta. The common characteristic of these loops is that they are all divergent in the limit *k* → ∞ , called “ultraviolet” divergence, where *k* is an internal loop momentum.

There are also other divergent radiative corrections. These are the so-called ...

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