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In [[quantum field theory]], and especially in [[quantum electrodynamics]], the interacting theory leads to infinite quantities that have to be absorbed in a [[renormalization]] procedure, in order to be able to predict measurable quantities. The renormalization scheme can depend on the type of particles that are being considered. For particles that can travel asymptotically large distances, or for low energy processes, the '''on-shell scheme''', also known as the physical scheme, is appropriate. If these conditions are not fulfilled, one can turn to other schemes, like the [[Minimal subtraction scheme]].
== Rescaling of the QED Lagrangrian ==
:<math> \mathcal L = -\frac{1}{4} F_{\mu \nu} F^{\mu \nu} + \bar{\psi}(i \partial - m )\psi + e \bar{\psi} \gamma^\mu \psi A_{\mu} </math>
:<math>
\psi = \sqrt{Z_2} \psi_r \;\;\;\;\;
A = \sqrt{Z_3} A_r \;\;\;\;\;
m = m_r + \frac{\delta m}{Z_2} \;\;\;\;\;
e = \frac{Z_1}{Z_2 \sqrt{Z_3}} e_r
</math>
== Fermion propagator in the interacting theory ==
Knowing the different [[propagator (Quantum Theory)|propagators]] is the basis for being able to calculate [[Feynman diagram|Feynman diagrams]] which are useful tools to predict, for example, the result of scattering experiments. In a theory where the only field is the [[Fermionic field|Dirac field]], the Feynman propagator reads
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==References==
* M. Peskin and D. Schroeder, ''An Introduction to Quantum Field Theory'' Addison-Weasley, Reading, 1995
* M. Srednicki, ''Quantum Field Theory'', [http://www.physics.ucsb.edu/~mark/qft.html http://www.physics.ucsb.edu/~mark/qft.html]
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