# Questions tagged [quantum-field-theory]

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 particle physics. Don’t combine with the [quantum-mechanics] tag.

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### How to determine the sign of coupling constant in quantum field theory?

Experimentally, we determine the value of the coupling constant by measuring the scattering cross-section and compare it with the results calculated by the scattering amplitude. However, in the ...
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### Does this is an onshell propagator?

This image is from here (page 6-7). My question refers to Eq. 26 and 27 The denominator changed from $(p_1 -p_3)^2 - m^2$ to $p_3^2 - 2p_3p_1$. So they used $p_1^2 = m^2$. But I thought, this relation ...
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### How to calculate the path integral (or partition function) of a set of phonons with continuous frequencies?

I have a system of phonons with continuous frequencies so that the Hamiltonian of this system is $H=\int_0^{k_0} h(k) a_k^\dagger a_k \mathrm{d}k$. How do I calculate the partition function of this ...
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### In/out states in scattering theory, Weinberg vol 1

Chapter 3 of volume 1 of Weinberg's QFT says the following regarding in and out states in scattering theory: Implicit in the definition of the states is a choice of the inertial frame from which the ...
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### Uncertainty principle from QFT

Is it possible to derive uncertainty principle from QFT? Which kinds of perturbation (particles, monopoles, ...) are rule out by this principle?
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### $1+1D$ $U(1)$ gauge theory is a quantum mechanical system

In article Exotic Symmetries, Duality, and Fractons in 2+1-Dimensional Quantum Field Theory there is statement (page 13): Ordinary $1 + 1$-dimensional $U(1)$ gauge theory is effectively a quantum ...
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### Stability and topological charge of kink (anti-kink) solutions (soliton)

I am reading the book << Gauge theory of elementary particle physics >>. In chapter 15, it presents a model having finite-energy solution. First, we have a $1+1D$ spacetime model \begin{...
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### Solving the Poisson-Schrodinger equations numerically

I need to find the solution to the Poisson- Schrodinger equation in the newtonian approximation, which are basically coupled differential equations given by: \begin{equation} \nabla^2 V=8\pi G M^{2}\...
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### How to measure gauge boson?

note that it's taught that in standard model particles that mediated weak interaction force are gauge bosons: vector particles, spin 1, take one charge. I'm wondering how to prove these properties in ...
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### Mass terms for scalar lagrangians?

First off, a pre-question: if I got this wrong, then probably the whole reasoning is wrong as well. Studying the lagrangian for a two-particle scalar field with a quartic interaction in the context of ...
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### Scattering in QM/NRQM vs QFT

As discussed in this question: How does a wave packet get scattered? And shown in, for example, this video: https://www.youtube.com/watch?v=iq4lGVznr_8 In quantum mechanics particles are scattered due ...
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### How does the pion decay constant $f_{\pi}$ depend on the number of flavors $N_f$?

I know that for standard QCD with quarks, chiral symmetry breaking occurs for around 4 - 5 light/active quarks - i.e. pions, or generally pseudoscalar goldstone bosons, definitely appear in the ...
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### Singularities in gauge-fixing conditions and topological defects

I am studying the 't Hooft's paper "Topology-of-the-gauge-condition-and-new-confinement" https://doi.org/10.1016/0550-3213(81)90442-9 and there are several points which I would like to ...
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### ${\cal N}=2$ (Poincaré) supergravity in three dimensions

I cannot find any references where the off-shell algebra of ${\cal N}=2$ supergravity in $D=3$ dimensions is given. From the study of the (massless) representations of ${\cal N}=1$ supersymmetry, I ...
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### Recombination phenomena in CFT

Now I study very interesting lectures Superconformal symmetry and representations and I face some statements, which are unclear to me. In unitary CFT there are unitary bounds for dimensions of ...
As far as I've understood, instantons like the sphaleron can give rise to processes that violate $B+L$ but conserve $B-L$, where the baryon and lepton number can only change by a multiple of three. ...