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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S-duality of Einstein-Maxwell-Dilaton theory

Consider theory with action $$S = \int d^D x \sqrt{-g} (R - \frac{1}{2} \partial_\mu \phi \partial^\mu \phi - \frac{1}{2k!} e^{a \phi} F^2 _{[k]} ) $$ where $\phi$ is dilaton and $F_{[k]}$ is ...
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54 views

Correlation functions in a zero dimensional QFT?

I would like to ask about correlation functions in a 0-dimensional matrix model QFT. What information do these correlators give? I know only of correlators between two different spatial positions.
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53 views

Majorana modes in Kitaev chain [duplicate]

I am reading paper about Kitaev chain of electrons, which can exhibit famous Majorana fermions at ends of wire. The Hamiltonian (his Eq. (6)) reads $H = \frac{i}{2} \sum_j - \mu c_{2j-1}c_{2j} ...
3
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67 views

Why is quantising gravity so difficult? [duplicate]

Since gravity is so similar with the Yang-Mills theory, the Christoffel connection is the gauge potential, the Riemann curvature is the field strength, then why is quantising gravity so difficult when ...
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1answer
40 views

What is the easiest way to prepare a Glauber coherent state? [closed]

Without using a laser source. Can you, for example, create a coherent state by filtering another light source (incandescent light bulb, LED, etc)?
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25 views

Majorana neutrino masses from left-handed neutrino condensate?

Let us consider a model with only left-handed neutrinos and with a new-physics interaction between these neutrinos, which leads to their condensation below a certain energy scale. Can we in principle ...
4
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64 views

What should I think of a diverging beta function (in Renormalisation Group flow)?

I have written a set of RG flow equations using Functional Renormalisation Group methods. I am looking at the flow of a well known problem with an additional original coupling. I did not do anything ...
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2answers
246 views

What would have been the story of the Universe if there was no mechanism to produce massive fundamental baryonic particles? [duplicate]

Thanks for those of you who took their time answering my problem but it seems that there is a misunderstanding between us. Most answers are based on the assumption of Electroweak symmetry breaking ...
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24 views

If an anyon picks up a phase upon particle exchange, how can you exchange them twice, isn't that a contradiction if the phase squared is not 1? [duplicate]

I'm trying to understand anyons, as stated on wikipedia, the interchange operator gives a phase https://en.wikipedia.org/wiki/Anyon $|\psi_1\psi_2>=e^{i\theta}|\psi_2\psi_1>$ So when I ...
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65 views

Number operator in interacting quantum field theory

When treating a quantum field, say the real scalar field, it's totally clear to me how to define a (global) number operator: $$\hat N = \intop \text d ^3 \mathbf p \hat a ^\dagger (\mathbf p ...
3
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72 views

Uses of effective action and effective potential

Effective potential allows us to answer the question that whether there will be spontaneous symmetry breaking induced by quantum corrections. Is there any other information that can be extracted from ...
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52 views

Reference request: QFT and AdS/CFT for information theorists

There is a lot of buzz recently about connections between quantum information theory and quantum field theory/string theory. I would like to understand in particular how quantum information methods ...
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73 views

Meaning of counterterms and quantum corrections

When one talks about the “classical Lagrangian” of a field, does one mean the tree-level Lagrangian with physical masses and physical couplings? If yes, does it therefore mean that the bare ...
3
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1answer
82 views

Why can you make $V$ stationary with respect to a parameter of the field in Derrick's theorem?

I'm going over Coleman's derivation of Derrick's theorem for real scalar fields in the chapter Classical lumps and their quantum descendants from Aspects of Symmetry (page 194). Theorem: Let ...
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95 views

An intuition on the Rindler modes

When we are solving the Klein-Gordon equation for the quantization of a massive scalar field on the Minkowski spacetime, we may use the global coordinates and obtain the usual quantization with plane ...
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1answer
43 views

Will an atom emit a polarized photon the same as the polarization of the incident photon?

So say you have a vertically polarized single photon impinging on an atom. The atom absorbs the photon and re-emits it. Does the re-emitted photon have the same polarization (vertical) as the ...
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18 views

Increase in carrier concentration on account of passing a current

I have been told that the carrier concentration in semiconductors increases while a current is passing through it. Is this true? And if it is true, why is it so? Does current passing through a ...
3
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1answer
47 views

Why can the propagation of neutrino mass eignenstates be described by plane wave solutions?

I don't understand why the propagation of neutrino mass eigenstates are given by planewave solutions as expressed in this Wikipedia article. In addition to not being used to thinking in the ...
3
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1answer
68 views

Dependence on UV cut off of some $\phi^4$ diagrams

Consider the one loop corrections to the propagator and the vertex in $\phi^4$-theory:                 ...
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50 views

the density matrix in QFT on a cylinder

My question regards the density matrix in quantum field theory on a cylinder. The partition function is given by $Z=\text{Tr} e^{-\beta H}$. The elements of this thermal density matrix become ...
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1answer
89 views

Superposition of distinct vevs in spontaneous symmetry breaking

The simplest account of spontaneous symmetry breaking goes like this. Take a potential $V(\phi)$ with symmetric minima that are not at $\phi = 0$, like the Mexican hat potential shown in this site's ...
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28 views

Parity and Time reversal when the number of space or time dimensions is even

There's a side remark in the middle of section 2.6 of Weinberg I that I find a bit unclear. Suppose that $L(p)$ is a boost that carries the four momentum $k^\mu=(0,0,0,M)$ to $p^\mu$, and that ${\bf ...
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170 views

How does higher spin theory evade Weinberg's and the Coleman-Mandula no-go theorem?

Recently I heard some seminar on higher spin gauge theory, and got some interest. I know there are some no-go theorems in quantum field theories: Weinberg: Massless higher spin amplitudes are ...
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77 views

Should there be a QFT evolution equation which has different orders of time and space derivatives?

I have seen many QFT equations like Klein-Gordon, Dirac, Weyl, Proca etc. But they order of the time derivative is always the same as the order of the space derivative. Shouldn't there be an equation ...
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Re: The $T$-matrix, Feynman amplitudes, and getting the scattering corrections from the interaction Hamiltonian

I'm running in circles about something in Scattering Theory at the moment. Let me summarize. In quantum theories we are interested in finding experimentally measurable quantities such as scattering ...
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124 views

How is it possible to change the direction of a spin by boosting?

In Weinberg QFT section 2.5.5, he defines the states of momentum $p$ by $$\Psi_{p,\sigma}=U\bigl(L(p)\bigr)\Psi_{k,\sigma}$$ up to some irrelevant normalisation, and $L(p)$ is the Lorentz ...
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What is the meaning of $\mathrm{d}^4k$ in this integral?

From Gerardus 't Hooft's Nobel Lecture, December 8, 1999, he states the following equation (2.1): $$ \int \mathrm{d}^4k \frac{\operatorname{Pol}(k_{\mu})}{(k^2+m^2)\bigl((k+q)^2+m^2\bigr)} = \infty ...
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1answer
86 views

Does Operator Product Expansion form an algebra?

The operator product algebra in CFT is defined as $$\mathcal{O}_i(z,\bar{z})\mathcal{O}_j(\omega,\bar{\omega}) = \sum_{k} C^k_{ij}(z-\omega,\bar{z}-\bar{\omega})\mathcal{O}_k(\omega,\bar{\omega}).$$ ...
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17 views

Stochastic properties of correlation functions in Minkowski vacuum

In the article of Sciama, Candelas and Deutsch "Quantum field theory, horizons and thermodynamics" DOI:10.1080/00018738100101457, on page 333, I don't understand the following passage (the italicized ...
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41 views

What do different supercharges corrrespond to? Is creation mathematically close to fock space?

We may have $I =1,2,..N$ where it is said that this corresponds to $N$ supercharges, $Q^I$. By the supersymmetry algebra, $$[M_{\mu\nu},{\overline{Q}}^{I\dot{\alpha}}] = ...
2
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1answer
89 views

How much the notion of force is discarded in modern physics? [closed]

Suppose that we are not allowed to use the notion of force to describe nature. My understanding of general relativity says that won't be a problem because the Einstein field equation doesn't involve ...
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51 views

Partial derivative vs Total derivative

This is essentially a follow up to my question here since I seem to have some difficulties regarding the differences between partial and total derivatives. Consider a Lagrangian density ...
3
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1answer
44 views

Can glueball be created by electron-positron colliders?

Since electrons and positrons are leptons, which don't experience strong interaction, and glueballs are unadulterated entities of quantum chromodynamics. Does that mean hadron colliders are better at ...
4
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1answer
48 views

Time-dependence of free and interacting Hamiltonians

Consider an interacting field theory with Hamiltonian $$H=H_0+V$$ where $H_0$ is the Hamiltonian of the free theory and $V$ is the added interaction. Now, I know the full Hamiltonian $H$ should be ...
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1answer
142 views

Is space really empty? [duplicate]

This guy here says space is not empty.. It explains how space inside nucleus is Full of fields. Does it mean to say gluons are present everywhere, even outside nucleus? Consider a region without any ...
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1answer
49 views

Coleman Mandula theorem and translations

I don't know what Coleman Mandula theorem is, however if I were forced to say something about it, I will say it is a statement that suggests that internal and spatial symmetries have no unique ...
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1answer
73 views

Dimensional Regularization for $\phi 4$ theory

When using dimensional Regularization for $\phi 4$ theory to calculate the running of the mass, why there is no quadratic divergence as expected? For details, cf ...
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31 views

How do we know that A is a pseudoscalar (CP-odd) Higgs?

Starting from a model with two complex Higgs doublets (as e.g. in the MSSM) we arrive at 5 physical Higgs bosons (instead of 1 as in the Standard Model), 2 of which are charged and 3 are neutral. One ...
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1answer
90 views

How does a particle's spin z component changed under lorentz group [closed]

I am reading weinberg's QFT, on the page 104, exercise 1, He said an observer $O $ see a W-boson (spin one and mass m) with momentum $p $ in the y direction and spin z-component $\sigma$ . a second ...
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55 views

Bogoliubov coefficients and eigenvectors

Suppose you have diagonalized Hamiltonian $H$ in second quantization using Bogoliubov transformation. If Hamiltonian is $N \times N$ matrix, the Bogoliubov transformation will have $N$ coefficients. ...
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29 views

Are there any video lectures that teach Weinberg's QFT books? [duplicate]

I am reading Weinberg's QFT books. I wonder is there any course can help me with it.
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36 views

Surreal Trajectories in Bohmian Mechanics [duplicate]

In February of this year, articles started popping up describing the results reported in this paper by Mahler et al: http://advances.sciencemag.org/content/2/2/e1501466.abstract Taken from this ...
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1answer
126 views

What is “the scale at which a theory is defined”?

I'm trying to learn the renormalization group, but I am confused about renormalization schemes. The general idea of RG is that physical predictions are independent of "the scale at which a theory is ...
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18 views

A few positrons collide with a solid body at rest; what can happen?

Suppose we have a macroscopic solid object. Now we have a beam of Positrons that is injected into this solid Body at vacuum. What can happen? There will take place a pair Annihilation of electrons ...
2
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1answer
63 views

Canonical field momentum in quantum field theory

In the context of the second quantization and the use of fields in the canonical quantization, the canonical momentum of the field is defined as the derivative of the field by the time coordinate. But ...
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1answer
199 views

Restoration of spontaneously broken symmetry at high energy

It is common to find books saying that above a certain energy, a certain symmetry in particle physics is restored, e.g. the $SU(2)\times U(1)$ electroweak symmetry was unbroken between $10^{-36}$ to ...
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225 views

Is the “number of photons” of a system a Lorentz invariant?

I'm wondering whether the number of photons of a system is a Lorentz invariant. Google returns a paper that seems to indicate that yes it's invariant at least when the system is a superconducting ...
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57 views

Wave function of the universe - universes similar to ours [closed]

According to this image, the first peak in the graph corresponds to our universe while the other peaks correspond to other, less probable universes. But what do points slightly right (or left, ...
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1answer
52 views

Addtional QFT Book synergetic to Srednicki. Differences $\phi^4$ and $\phi^3$ [duplicate]

I currently hear a course to basic QFT in path integral formulation. Focus is on few and elementary particles, not on many body systems. The lecturer follows the book of Srednicki, which therefore ...
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49 views

About the non-locality of gravitational energy 2

Gravitational energy is non-local which is essentially because of the equivalence principle. The equivalence principle says that you can always transform your frame so that you feel like in a ...