Projection Operators for Energy and Spin
In practical calculations of quantum electrodynamic (QED) processes we will become acquainted with a technique of calculation which allows the simple treatment of complicated expressions; especially the calculation of traces of products of many γ matrices. It is based on a projection procedure, i.e. a method to project a spinor with a given sign of energy and fixed polarization out of a general wave function or a wave packet. The appropriate operators which achieve this are called projection operators.
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