# Tagged Questions

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 ...

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### What is the Green's function of the Klein-Gordon equation with a variable mass?

Usually, the Klein-Gordon equation's propagator is calculated with a constant mass. But what if the mass is a variable? That is, $$(-\partial^2 + m(x)^2)G(x, y) = \delta^4(x-y)$$ where $m(x)$ is a ...
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### Mean free path in QFT

I'm trying to understand the hydrodynamic approximation of a general QFT when the large $k$ and $\omega$ DOF have been integrated out i.e that at highly enough temperature every non-trivial QFT ...
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### Poincare representations for interacting field theory

I was going through Rudolf Haag's memoir http://link.springer.com/article/10.1140%2Fepjh%2Fe2010-10032-4 and came across these lines: '..in quantum ﬁeld theory (or for any system of interacting ...
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### Equation for Electric and Magnetic field from the equation for a “massive photon”

I was reading the Quantum Field Theory book by Maggiore. There he says that in side a superconductor the photon satisfies the equation $$(\Box+m^2)A_\mu=0$$ Then he adds that the electric field and ...
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### Running coupling, effective potential and the stability of vacuum

Consider the potential $$V(\phi)=\frac{1}{2}\mu^2\phi^2+\lambda\phi^4$$ where $\phi=\phi(t,\textbf{x})$ is a real scalar field. Let, $\mu^2<0$ and $\lambda>0$ then the potential is bounded from ...
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### A box loop-integral [closed]

I am trying to evaluate the integrate $$\int\frac{d^Dk}{(2 \pi)^D} \frac{1}{(k^2)^2(k^2-m^2)}$$ using dimensional regularisation ($D=4-2\epsilon$). From various references it appears that it should ...
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### Problem understanding electromagnetic interaction with matter (non-relativistic QED)

I'm having trouble understanding the interaction of radiation with matter in (elementary non-relativistic QED) in Coulomb gauge ($\nabla\cdot\boldsymbol{A}=0$). We saw how to quantize the free ...
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### Photon one (odd) point function in QED

In Peskin's QFT textbook, when discussing the superficial divergence of loops in QED, the book says (page 317): "To analyse the photon one-point function,note that the external photon must be ...
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In AdS/CFT, when the spacetime is a planar AdS black hole with dimension ($d+1$), the corresponding energy of boundary field theory is proportional to the black hole mass parameter. For example when $... 1answer 106 views ### S-matrix element I'm confused with the relation between the fully resummed propagator in a given QFT and the corresponding S-matrix element. According to the LSZ reduction formula ($\phi^4$theory for definiteness ... 0answers 93 views ### How to write the second quantization form of spin-orbit coupling(Dzyaloshinskii-Moriya interaction)? Spin orbit coupling is the single particle term, so the second quantization form can be written like:$\langle \alpha\sigma|s\cdot(\nabla V\times P)|\beta\sigma'\rangle c^{+}_{\alpha\sigma}c_{\beta\...
I know that the use of twistur-momentum variables makes manifest the arising of certain poles in scattering amplitudes: if the sum of external momenta $P_I = p_i + p_{i+1} + ... + p_j$ is going on-...