## QED

In this chapter, Quantum Electrodynamics (QED) is discussed which is
the best tested quantum field theory.

One of the most fundamental concepts in particle physics is the
Gauge Principle which allows to introduce
fundamental interactions by requiring invariance under (to be specified) local
phase transformations. This is discussed in quite some detail for the case
of QED.

Next, the complete form of the QED Lagrangian is discussed.

From the equation of motion for a Dirac field interacting with an external
electromagnetic field the Feynman Rules are
motivated.

The chapter closes with calculations of QED processes using the Feynman Rules.