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Questions tagged [feynman-diagrams]

A diagrammatic technique introduced by Richard Feynman to describe the quantum behaviour of subatomic particles and their interactions. Do not use for general questions on diagrams that are not of the Feynman kind.

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Calculating a Feynman diagram with the helicity basis

In the book by Peskin and Schroeder, they calculate the leading order diagram for the process $e^- e^+ \to \mu^- \mu^+$ (see page 136 for the labelling of the momenta). They do this in two ways: using ...
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Understanding Feynman Diagrams in Loop Corrections to the propagator $\phi ^3 $ theory [closed]

I found other posts talking about the same chapter in the same book, but none of them were exactly about what I am asking here. In Srednicki's chapter 14 (Loop corrections to the propagator), we are ...
Fernando Garcia Cortez's user avatar
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2PI Effective Action from Double Legendre Transformation

This answer (https://physics.stackexchange.com/q/348673) provides good intuition for why Legendre transformation induces 1-particle irreducible graphs: It mainly tries to convey the idea that the ...
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Feynman rule for scalar QED vertex

A popular problem in QFT textbooks and courses is to derive the Feynman rules for scalar QED. Usually, this theory is presented via the following Lagrangian density: $$\mathcal{L} = (D_\mu\phi)^\...
Rafael Grossi's user avatar
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Making Feynman Diagrams for a given process

I do not have much experience in field theory so please do suggest a text or source where I can find detailed explanation for my question. My question is, how to draw feynman diagram for a given ...
Eviciium's user avatar
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Proper time of two particles being the same when they are under a tree-level interaction

I have a question: I have a figure like this: The interpretation of the diagram is this: a particle is emitted at $x_2$, interacts with the field at $x_1$ and from $x_1$ another particle is emitted ...
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Determining if Self-Energy is Complex from Action Alone?

My understanding of quantum field theory is that an interacting two-point function of spin-0 bosons will have the form: $ \frac{i}{p^2-m_0^2-\Sigma(p)}$, where $\Sigma(p)$ is the self-energy, the sum ...
JudahReynolds's user avatar
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How can I calculate the cross section in general?

How can I calculate the cross-section of a process with three possible Feynman diagrams? I usually see examples with only the scattering amplitude defined by the $t$ channel, but if the scattering ...
LittleBlue's user avatar
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What does it mean for the electron to be absorbed in electron capture?

In electron capture, when looking at the Feynmann diagram, I'd interpret the interaction as the electron emitting a W boson which interacts with the proton and turning into a neutrino. However, as the ...
Flamethrower's user avatar
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Why does an all connected diagram contribute to two-point function?

I am recently reading E.Witten's review for $1/N$ expansion of QCD. In there, considering the main contribution of quark bilinears like $\bar{q}q$, then He mentions that in free field theory there is ...
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Feynman diagrams in Yukawa interaction

I want to understand drawing Feynman Diagrams better, therefore I wanted to draw some for the Lagrangian with a Yukawa interaction term: $$L = \bar{\psi}(i \partial\!\!\!/ - m)\psi - g \bar{\psi}\phi \...
WoistdasNiveau's user avatar
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Confused with the product of Feynman propagators [closed]

While reading Thomas Rauh's 'Introduction to Feynman integrals and multiloop techniques' I came across the following identity $$\frac{1}{A^aB^b}=K\int^1_0 \frac{dx x^{a-1}(1-x)^{b-1}}{[xA+(1-x)B]^{a+b}...
PRAKHAR MATHUR's user avatar
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How to derive momentum-space Feynman rules from position-space ones?

I was reading Peskin-Schroeder's QFT text (P-S) and came across Equation (4.47) stating the vertex factor when four lines meet. P-S says the $z$-dependent factors of the diagram is: $$ \int d^4z\,e^{-...
math-physicist's user avatar
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Feynman rule from dynamical Chern-Simons

Consider the following action that \begin{equation} S = \int d^4x\sqrt{-g}\left(-\frac{1}{2}(\partial\phi)^2 + V(\phi) + \frac{2R}{\kappa^2} - \frac{\phi}{4f}{}^*RR\right) \end{equation} where \...
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Feynman diagrams in string theory

I am beginning to study string theory, I have a beginner level doubt: If we consider a Feynman torus diagram in string theory, it is a worldsheet. What does it represent? Does it actually mean that in ...
SX849's user avatar
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Exact definition of topological non-identical diagrams

It is often said that Feynman diagrams for fermions do not have symmetry factors. Consider I have a spinless fermionic quantum many-body system with Hamiltonian: $$H=\int_{r}\psi^{\dagger}(r)\frac{\...
John 's user avatar
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Textbook for Feynman Diagrams

I need to get an understanding of Feynman diagrams, something I currently have basically no knowledge of, and I am looking for a good textbook/place to start. I have finished my 3rd year undergrad, ...
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How to interpret a branching fraction diagram?

I'm confused as to how can I (student in master 1 class of particle physics) interpret such diagram: Where $q^2 = m^2(l^+l^-)$. I understand that the y axis represents the variation of the ...
Ducktective's user avatar
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Why is $F_1(0) = 1$? (form factor in QED)

In section 6.2 in Peskin and Schroeder (P&S) on vertex corrections, it is shown (eq 6.34) that the physical charge of the electron is given by $eF_1(0)$ (as shown by comparing the nonrelativistic ...
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Relation between time-ordered propagator in condensed matter and Feynman propagator

In particle physics I am used to the Feynman propagator being decomposed into positive and negative frequency Wightman functions. For example, this is the representation used in Eq. (6.2.13) of ...
ds283's user avatar
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Yukawa Interaction with Chiral Fermions

I understand that an interaction like $$g\overline\psi_L\psi_R\phi$$ is allowed in the SM (for the sake of this discussion lets ignore gauge charges and focus only on Lorentz invariance). I want to ...
SamuelFGC's user avatar
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What is a correlation function of renormalized currents in QED?

The correlation function $$ \langle\beta|\mathcal{T}J(x_1)\cdots J(x_n)|\alpha\rangle,\tag{1} $$ where $J^\mu=e\bar{\psi}\gamma^\mu\psi$ is the QED current, is usually interpreted as the diagram of ...
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What goes wrong with strongly coupled theories?

Let $\lambda$ be the coupling constant of a quantum field theory. It is said that Perturbation theory is only valid when the theory is weakly coupled ($\lambda \ll 1$). In most cases, the series of ...
CBBAM's user avatar
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In a scalar QFT, how are Feynman diagrams drawn if the interaction Lagrangian has multiple terms?

In my introductory course for QFT we have covered many different interaction Lagrangians using scalar fields, for example $\phi^3$ theory. However, so far we've only covered Lagrangians with a single ...
agaminon's user avatar
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Power-series expansion in coupling/Planck constant

By using Feynman rules of the interacting theory, one obtains the scattering amplitude $$\mathcal{M} = \mathcal{M}_0 + \mathcal{M}_1 + \cdots = \sum^{\infty}_{i = 0}\mathcal{M}_i\tag{1}$$ Where $\...
user avatar
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A question on IR divergence in Peskin-Schroeder chapter 6

In equation 6.64 of Peskin Schroeder, it computes $f_{\text{IR}}(q^2)$ in the limit $-q^2\to\infty$. Now, if we try to simplify the integral: \begin{align} f_{\text{IR}}(q^2) &=\int_0^1d\xi\;\frac{...
Soumyajit Datta's user avatar
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3 answers
111 views

How do vacuum bubbles "dress" terms in the $S$-matrix numerator?

I am self-studying QFT using the book "A modern introduction to quantum field theory" by Maggiore. On page 124-125 he's doing the calculation in the interaction picture for a process with ...
Andrea's user avatar
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Some integrals in QED Renormalisation

I am currently leaning the renormalisation of QED and I have met some tricky integral that seems unsolvable. The integrals are shown in Quantum Field Theory and the Standard Model by Schwartz, page ...
quantumology's user avatar
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Possible decays of a scalar particle, Feynman diagrams and calculation of branching ratio - Explenation of solution required

Consider the Lagrangian describing two Dirac fields of masses $m_1$ and $m_2$ and a scalar neutral field of mass $M$, which in addition to the free terms contains the following interaction term \begin{...
Camthalian's user avatar
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How does crossing symmetry affect the value of an electroweak vertex (e.g. $u+W^-\to d$ vs. $W^-\to\pi^-$)?

Two of the weak charged current vertices I often encounter in my first foray into particle physics are $u+W^-\to d$ and $W^-\to\pi^-$, shown in the following Feynman diagrams: These feel like the ...
DanDan面's user avatar
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When a proton and neutron fuse in Big Bang Nucleosynthesis, do they emit a virtual neutral pion that emits two photons?

In Big Bang Nucleosynthesis, when a proton and neutron fuse to form a deuteron, how does the binding energy get shed? The binding is remanent strong nuclear force so does it emit a virtual neutral ...
Mongrav's user avatar
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How can I count diagrams (in this scalar QED example) at a particular order without drawing all the Feynman diagrams?

Here's a 3-loop diagram for light-by-light scattering in scalar QED (from Schwartz textbook question 9.2): The question 9.2 asks approximately how many other diagrams contribute at the same order in ...
IGY's user avatar
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Yukawa Theory with Spinor-Helicity Formalism

I am trying to learn spinor-helicity formalism, and I am attempting to calculate the all-minus amplitude of 4-fermion scattering at tree level in Yukawa theory, but I have no way to check if my ...
MathZilla's user avatar
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General formula for a $f1\to f2$ gamma processes (e.g. $\mu \longrightarrow e\gamma$), understanding the detailed calculation in the paper by Lavoura

I want to work out in detail the paper by Lavoura to get some real practice as a beginning student. I have stumbled upon several problems as I am moving along with the arguments. Firstly, I am not yet ...
Hubert John's user avatar
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Residue theorem application

First I want to provide a little bit of context: I finished my undergrad degree in physics in 2008 and after that I moved into strategic consulting and into the financial world. Right now, at 41 years ...
ateixeira82's user avatar
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1 answer
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Divergences in tree-level diagrams?

Consider the Feynman diagram in $\phi^4$ theory where there are three incoming momenta ($p_1$, $p_2$, and $p_3$), three outgoing momenta ($q_1$, $q_2$, and $q_3$), and one internal line so that this ...
CBBAM's user avatar
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Why do correlation functions involving composite fields require special analysis?

For simplicity I will be considering $\phi^4$ theory. To analyze correlation functions of the form $$\langle \phi(x_1)\phi(x_2)\ldots\phi(x_n)$$ with $$x_1 \neq x_2 \neq \cdots \neq x_n \tag{1}$$ we ...
CBBAM's user avatar
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1 vote
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How does renormalization affect divergent subdiagrams?

Suppose we have a theory that is super-renormalizable and let $\Gamma^n$ denote the sum of all 1PI diagrams of this theory with $n$ amputated external legs. In such theories, for all $n$ sufficiently ...
CBBAM's user avatar
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2 votes
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Understanding $W^{(n)}$, $\Gamma^{(n)}$, and $\Sigma$ in Feynman diagrams

In quantum field theory (specifically $\phi^4$ theory), $W$ is the sum of all connected Feynman diagrams and the effective action $\Gamma$ is the sum of all 1PI Feynman diagrams. They are related by a ...
CBBAM's user avatar
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Difference between renormalizable and super-renormalizable theories

In $\phi^n$ theory in Peskin & Schroeder the superficial degree of divergence is: $$D = d - V[\lambda] - \big(\frac{d-2}{2}\big)N \tag{10.13}$$ where $d$ is the dimension, $V$ is the number of ...
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Vertex factor type-I seesaw

Consider the following interaction term: \begin{equation} \mathcal{L}_\text{int} = y_{ij}\bar{\ell}_{iL}\tilde{\eta}N_{jR} + \text{h.c.} \end{equation} where \begin{equation} \ell_{iL} = \begin{...
SNC92's user avatar
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Why are tree-level diagrams classical? [duplicate]

I see this statement from time to time. But when I look at scattering in QED or phi-4 theory, I don't understand how classical calculation can reproduce these amplitudes. To what extent is this ...
Bababeluma's user avatar
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Feynman rule: proton-proton-photon versus electron-electron-photon QED vertex

If the Feynman rule for the vertex is written as $$-ieQ\gamma^\mu$$ ($e>0$, charge of positron, $Q$ is the charge of the interacting particle in units of $e$), the electron-electron-photon QED ...
Solidification's user avatar
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1 answer
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Why is $\frac{1}{2}\delta_Z(\partial_\mu \phi_r)^2 - \frac{1}{2}\delta_m \phi_r^2$ treated together in Feynman diagrams?

From P&S consider the $\phi^4$ bare Lagrangian: $$\mathcal{L} = \frac12 (\partial_\mu \phi)^2 - \frac12 m_0^2\phi^2 - \frac{\lambda_0}{4!} \phi^4.\tag{p.323}$$ When using renormalized perturbation ...
CBBAM's user avatar
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Are loops counted twice in Feynman diagrams?

Consider the 2 point function in $\phi^4$ theory which is given as something proportional to $$\int D(x-z) D(y-z) D(z-z) d^4 z,$$ where $D$ is the propagator. The corresponding Feynman diagram looks ...
CBBAM's user avatar
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Furry's theorem in Electroweak and strong interactions

We can think of Furry's theorem as the consequence of $CP$ invariance of $QED$. For parity, the vector bilinear changes sign, hence, under charge conjugation, it should. The vacuum is $C$-invariant, ...
Tanmoy Pati's user avatar
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Is Feynman's Compton scattering diagram the same as the one in most books?

Feynman used the following Compton scattering diagram: Whereas most books use this diagram in their derivation: Is there a way to relate the two? Feynman's diagram makes more sense to me ...
JBatswani's user avatar
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Fourier transform of Feynman Integral

In Nastase, Introduction to AdS/CFT, the first chapter talks a little about the star-triangle duality. In fact, it was claimed that the Fourier Transform of a Feynman-like diagram in position space in ...
LSS's user avatar
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Quark-Gluon vertex 1-loop correction in QCD

I'm trying to calculate the one loop correction to the quark-gluon vertex of QCD using euclidean formalism ($x^0 \rightarrow -ix^4$) and I'm having trouble to compute the integral in the picture below....
Banach Caccioppoli's user avatar
2 votes
1 answer
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Expanding the generating functional $W[J]$ for connected diagrams as a power series in $\hbar$

This is a follow up of a recent post I made (Making sense of stationary phase method for the path integral), but here I will work in Euclidean space, i.e. a Wick rotation has been performed. Let $$Z[J]...
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