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Quantum Field Theory (QFT) is the theoretical framework describing the quantisation of classical fields which allows a Lorentz-invariant formulation of quantum mechanics. QFT is used both in high energy physics as well as condensed matter physics and closely related to statistical field theory. Use this tag for many-body quantum-mechanical problems and the theory of [tag:particle-physics]. Don’t combine with [tag:quantum-mechanics].

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Just an outline: $\partial^0\phi$ is the canonical momentum because that's defined as $$\pi\sim\frac{\delta S}{\delta \dot\phi}\,,$$ as in classical mechanics. The definition of $P^\mu$ basically re …
answered Nov 15 '18 by Toffomat
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The basic point is that in the path integral $Z=\int \text{D}\phi e^{\text{i}S[\phi]/\hbar}$, the value of the action is relevant, whereas in the classical theory the action is just there to be extrem …
answered Aug 4 '17 by Toffomat
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You say you know about Feynman diagrams, at least at tree level. So you might have seen loop diagrams, i.e. diagrams that contain a loop of internal lines (see e.g. https://en.wikipedia.org/wiki/One-l …
answered Apr 30 '18 by Toffomat
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Gauge symmetries are redundancies of the theory, i.e. they remove degrees of freedom -- consider QED, where the four components of the gauge field give rise to only two degrees of freedom. Hence, gaug …
answered Jul 1 by Toffomat
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Consider a single chiral multiplet with scalar and spinor components $\phi$ and $\xi$. Schematically, the SUSY transformations are $$ \begin{split}\delta \phi &\sim \bar \xi\epsilon\\\delta \xi&\sim \ …
answered Jun 26 '18 by Toffomat