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.

Filter by
Sorted by
Tagged with
0
votes
0answers
26 views

Implications of momenta sign conventions in QCD Feynman rules

In QCD, the Feynman rules for the vertices are usually presented with all momenta pointing inward. For the total amplitude of a given process, does this also affect the signs of the momenta taken in ...
0
votes
0answers
23 views

How to put the terms in correct order when deriving amplitudes in QED from the S-Matrix?

I am trying to derive the amplitude for the Møller scattering process $e^-+e^-\rightarrow e^-+e^-$ from "first principles", that is, by working out the calculation with the relevant S-matrix terms ...
1
vote
1answer
49 views

Could $\nu_e+p\rightarrow e^-+\pi^++p$ occur via the weak interaction? If so, how would be the Feynman diagram?

I checked if the reaction conservate baryon number, charge and leptonic number and it seems like it does. But I tried to draw the Feynman diagram and I don't understand how the proton interact, so ...
2
votes
1answer
55 views

Normalization of the integration measure of the Feynman's formula to combine denominators

In Mark Srednicki "Quantum field theory", section 14 -Loop corrections to the propagator-, it is presented the Feynman's formula to combine denominators: $\frac{1}{A_1 ... A_n} = \int dF_n (x_1 A_1 + ....
2
votes
0answers
65 views

Can we do one-loop integrals in the unitary gauge?

$\hspace{5cm}$ Imagine we want ot compute one of the diagrams for the self-energy of the quark $u$, with external momentum $p$. Inside the loop, we would have a $W^+$ and a $d$-quark propagator, with ...
7
votes
1answer
446 views

Why does charmonium (and phi mesons) not decay via quark and antiquark annihilation?

The decay of heavy quark/antiquark pairs (say $c\bar{c}$, $s\bar{s}$) is supposedly 'suppressed because of the Zweig/OZI rule', see for instance Phi meson. And they certainly have a longer lifetime ...
1
vote
0answers
44 views

Feynman rules for a general Lagrangian

How do I find the Feynman rules for a general Lagrangian density? For example the Lagrangian $$L = \partial_\mu \psi \partial^\mu \psi +a \psi\partial_\mu \psi \partial^\mu \psi+b \psi^2 \partial_\mu ...
0
votes
2answers
56 views

How to figure out whether a force mediator is $W^+$ or $W^-$ in Feynman diagram?

How to figure out whether a force mediator is w+ or w- in feynman diagram? I always make sure each vertex is zero but sometimes I get my w+ or w- worng.
2
votes
1answer
78 views

Feynman loop diagrams

I am learning about Feynman diagrams, and I have trouble interpreting loop diagrams. The picture below shows a loop-induced decay of the neutral pion. $\pi^0 \rightarrow e^+ + e^-$. I understand that ...
1
vote
0answers
76 views

Manybody theory: cancellation contributions to proper polarization

I don't understand why the first order diagrams (c) e (d) give null contribute to the correlation energy. We are considering systems of fermions uniform in space and time (we are in momentum space) ...
0
votes
0answers
39 views

Spinless $e^{-}\gamma\rightarrow e^{-}$ Cross section

I was trying to figure out the cross section $\frac{d\sigma}{d\Omega}$ for spinless $e^{-}\gamma\rightarrow e^{-}$ scattering. First I wrote the terms associated with each component. Vertex: $$ie(...
0
votes
0answers
113 views

Feynman diagrams of phi-meson decay

I am learning how to make Feynman diagrams, and I have a couple of questions about two possible decays of the phi-meson. The first question relates to the reaction $\phi \rightarrow\text{K}^+ + \...
1
vote
1answer
38 views

How is the charge conserved at the up to anti-down vertex?

I can see how the charge is conserved in the overall diagram but I can not understand how it is conserved at the up to anti-down vertex.
0
votes
0answers
26 views

Literature result for generic one-loop triangle integral

I've never really asked a literature based question here before, but was wondering if anyone knows where I may find a reliable source for symbolic expressions for generic one-loop triangle diagrams ...
1
vote
1answer
60 views

LSZ reduction, momentum diagram, QFT

I was initially confused about which way to choose the sign of the momentum, since it gives rise to different exponential momentum combinations and thus different deltas for momentum conservation. I ...
4
votes
1answer
140 views

Why does a triangle anomaly appear in a gauge theory?

I have read that when we construct a theory with abelian gauge symmetry there will appear some anomalies when we do the quantum correction to the theory. In 4D space such anomalies are explained by ...
0
votes
1answer
34 views

What parameters determine the probability of virtual photon emission/absorption?

Suppose an electron is producing an electric field by emission of virtual photons and interacting with other particles. What parameters determine the probability that it will emit at least one virtual ...
1
vote
0answers
68 views

Higher point anomalies cancellation from trace

I had asked this question within this one before, but having also made other 2 independent enough questions there, decided to ask this one by itself here. So, it is a well known fact that the ...
0
votes
0answers
39 views

B violation and electric charge

Within SM you can prove that despite we have baryon number conservation respect to Noether theorem, at quantum level baryon (and lepton) number is violated as $$ \Delta B = 3·\Delta n_{CS}, \quad n_{...
1
vote
1answer
62 views

Orbital angular momentum for single particle in particle physics

I am taking an introductory course in particle physics and am quite confused about conservation of angular momentum at a vertex. This trouble arose specifically when considering the decay $\pi^0 \...
2
votes
1answer
75 views

Arrow convention for neutral particles in Feynman diagram

In a Feynman diagram, the arrows on the charged particles are along the direction of negative charge flow. As a result, the arrow is not always directed from the vertex of creation and not always ...
1
vote
1answer
67 views

Why do the diagrams in $\Gamma[\Phi]$ differ from those in $\Phi\Gamma^{\rm int}_{\Phi}[\Phi]$ only by numerical prefactors?

Suppose $W$ is the generator of connected Feynman diagrams in $\Phi^4$ theory. We define $$\Gamma[\Phi]=W[j]-W_jJ,\tag{13.37}$$ where $$W_jJ=\int{dxW_j(x)j(x)}\tag{13.38}$$ and $$ \Phi\equiv\frac{\...
1
vote
0answers
26 views

Substitution of propagator to a product in Zinn-Justin

I'm reading Quantum Field Theory and Critical Phenomena, 4th ed., by Zinn-Justin and on page 127 he defines an action $$S_{\epsilon}[\phi]=\int{dxdy} \phi(x)\phi(y)[K(x,y) + \epsilon ] + V(\phi).$$ ...
0
votes
1answer
59 views

How does absorption of virtual photons “smear” a particle?

I have read that the electron self-energy correction, a process in which the electron absorbs a virtual photon, changes the position of the electron (which leads to the lamb shift in Hydrogen-like ...
2
votes
0answers
56 views

What is the Quantum Mechanical analogue of the Bethe-Salpeter equation?

For studying the bound states of quantum fields theories (e.g. studying excitons or mesons), the Bethe-Salpeter equation is often used as the starting point. Quoting Wikipedia the equation is: $$\...
0
votes
0answers
37 views

Any connected diagram is a tree of full propagators

In P. Etingof, Geometry & QFT, MIT 2002 online lecture notes; Lemma 3.11 (https://physics.stackexchange.com/users/7266/abdelmalek-abdesselam).) He says that any connected diagram is a tree of ...
2
votes
1answer
68 views

Feynman diagram and vertices in a hadron decay

This question is about Feynman diagram and vertices in a hadron decay, which comes from Problem 89.5 in Srednicki's textbook Quantum Field Theory. The reaction involved is \begin{equation} d\...
3
votes
1answer
156 views

Combinatorics geometric series two-point function

In this answer Proof of geometric series two-point function it is said: Now what about the coefficients in front of each Feynman diagram? Due to the combinatorics/factorization involved it ...
0
votes
0answers
33 views

Euclidean Feynman Rules derivation using Wick Theorem

When studying perturbation theory and Feynman rules, the standard derivation seems to start from the S-Matrix element in the interaction picture, and expands it into some series, after which Wick's ...
0
votes
0answers
22 views

Feynman diagram combinator and multiplier value

Dear experts, I try to learn by self feynman diagram from examples of x^3 and x^4 for first and second order consolidated from different reading material. For x^3, combinator values of 3 * 3 for n=1 ...
1
vote
0answers
56 views

Neutrino scattering cross-section in high energy limit

I've come across a problem in out particle physics lecture notes that I have been unable to resolve on my own. I've paraphrased the bits of the notes that are giving me so much trouble: The notes ...
0
votes
0answers
19 views

How to evaluate the Feynman gauge photon propogator in coordinate space?

In coordinate space the Feynman gauge looks like: $$\frac{g_{\mu\nu}}{\partial^2} .$$ Without fourier transforming, how does this operator act on currents? In general for inverse derivative operators ...
4
votes
0answers
63 views

How to calculate $n$-point functions of interacting fields in curved spacetime (Schwarzschild metric)?

How to renormalize quantum field theory in curved spacetime? (or in Schwarzschild spacetime?)  I want to calculate n-point functions $$<0|Tφ(x_1)...φ(x_n)|0>$$ in massless $φ^4$ theory in ...
2
votes
3answers
109 views

Why do we demand that the counterterms in $\varphi^3$ theory be $O(g^2)$?

In Srednicki's QFT book, section 9, he introduces the $\varphi^3$ lagrangian: $$\mathcal{L}= -\frac{1}{2}Z_\varphi(\partial_\mu\varphi)(\partial^\mu\varphi) -\frac{1}{2}Z_mm^2\varphi^2 +\frac{1}{6}...
1
vote
0answers
43 views

Failing to show $\xi$-gauge-independence in an abelian Spontaneously Broken Gauge Theories (SBGT)

I am studying the following paper: Appelquist, Carazzone, Goldman & Quinn, Renormalization and Gauge Independence in Spontaneously Broken Gauge Theories, https://doi.org/10.1103/PhysRevD.8.1747 ...
0
votes
1answer
42 views

Lorentz decomposition of electromagnetic current Cheng&Li $\mu\rightarrow e\gamma$ p.421

I've been struggeling with this for a while, should be very easy probably.... In Cheng and Li p. 421 they consider the transition $$T(\mu \rightarrow e\gamma) = \epsilon^\lambda<e|J^{em}_\lambda|\...
0
votes
1answer
59 views

$\phi^3$ 2D 1-loop diagram disambiguation

I would like to calculate the 1-loop 1-PI correction to the propagator for $\phi^3$ scalar theory in 2 dimensions, where the integral is finite. Performing the usual procedure (Feynman trick, Wick ...
6
votes
0answers
222 views

Ward identity for 'general' operator and current diagrams

This is actually about two related doubts and I hope is appropriate for a single question (if not, I will happily divide it). So, my problems are related to the analysis and calculation of chiral ...
0
votes
0answers
33 views

Drawing lowest-order Feynman diagrams for $\rho^0 \rightarrow \pi^- \pi^+$ decay [duplicate]

I am trying to draw the lowest order Feynman diagrams for $\rho^0 \rightarrow \pi^- \pi^+$ for a homework question. I came up with the following answers: In the answer key given, only the diagram ...
2
votes
0answers
68 views

Error in proof of Wick's theorem

I am having a little trouble proving wick's theorem. I'll start from the last step that I know is correct. We define $$\left\langle \prod_{j=1}^{2m}x_{i_j}\right\rangle:=\left.\frac{\partial^{2m}}{\...
1
vote
1answer
94 views

Feynman diagrams in Gaussian integrals

I am looking for suggestions for material regarding Feynman diagrams for gaussian integrals. I am looking for something of the sort of: Pedro Vieira, Statistical Physics Applied to Quantum Field ...
0
votes
0answers
34 views

Is there any Feynman diagram for Baryon/Hyperon decay with missing energy?

I am reading the hyperon decays with missing energy this paper tells for the prospects of baryon decays with missing energy I want to know some of the Feynman diagrams for hyperon decays. Can anyone ...
0
votes
1answer
85 views

Feynman diagram for semileptonic decay of neutral kaon

I am unsure how to draw a feynman diagram for a reaction that occurs as follows $$ K^0 --> l^+\nu_l\pi^- $$ Any tips would be helpful.
0
votes
0answers
43 views

IR divergence Feynman diagram topology query

I am trying to calculate the superficial degree of Infrared divergence. To do this I am reading section 12 of this source. It seems you can calculate it by a method involving the 'shrinking' of ...
1
vote
0answers
49 views

Reference for Feynman diagram technique(position space) in Thermal Field Theory

I am trying to study perturbative expansion of Sachdev-Ye-Kitaev model, where I know that the dominant terms are the Melonic diagrams. I am interested in seeing how perturbative corrections affect the ...
0
votes
1answer
39 views

Why do we always choose centre-of-mass frame to study particle physics problems (e.g. Feynman diagrams)? Even in non-relativistic regimes (v<0.3c)

Example: electron positron annihilation forbids a single photon production, and searching through this forum (Why can't a single photon produce an electron-positron pair?) and a few books, they ...
2
votes
1answer
82 views

Lepton flavor violating process on loop level $\mu \rightarrow e \gamma$ via Higgs, Divergent integral

I am stuck with calculating the proces $\mu \rightarrow e \gamma$ as in this diagram: I wrote down the matrix element like this: $$ \mathcal{M}=\bar{u}(p-q)\left[\int \frac{d^4k}{(2\pi)^4}A\frac{(k+...
2
votes
0answers
48 views

Computation of Feynman integrals in terms of Chen Iterated integrals

One of the techniques used to compute Feynman integrals in dim-reg is the so-called "differential equations method". Exploiting integration-by-parts identities (linear functional relations between ...
0
votes
0answers
39 views

Cross section for 2 particles that decay independently

This maybe is a naive basic question but I want to be sure. If I want to calculate the cross section of the process $$p\bar{p} \rightarrow W^+HX \rightarrow e^+\nu_e b\bar{b} X$$ ($H$ is a Higgs ...
1
vote
1answer
103 views

EW theory vertices

I'm trying to undertand the following vertex: Initial state of up and anti-down quarks with finalk state made of $W^+$ boson. Does it go with left or right projector? I think that from Lagrangian it ...