The tag has no usage guidance.

learn more… | top users | synonyms

10
votes
2answers
315 views

How to count the number of cubic tree-level Feynman diagrams at $n$ points?

I'm following this paper: arXiv:0805.3993 [hep-ph] where it's said that the total number of distinct tree-level diagrams at $n$-points with cubic vertices only is $(2n-5)!!$ I want to know where this ...
1
vote
1answer
127 views

Feynman-like diagrams for photon/electron interaction and electron recollision

Feynman diagram is a useful and powerful tool to represent path integrals. But, most of its applications seem to be focused on relativistic particule creation/annihilation. As far as I can ...
0
votes
2answers
75 views

Quantum field theory why is it a trace?

So in perskin when computing the amplitude of $e^+e^-\rightarrow\mu^+\mu^-$ summing up over spin they say \begin{align}\sum_{s,s'}\bar{v}^{s'}_a(p_2)\gamma^\mu_{ab}u^s_b(p_1)\bar{u}^{s}_c(p_1)\gamma^...
6
votes
0answers
91 views

In QED/Yang Mills, why do fermions contribute 4 times as much as scalars to vacuum polarization?

Consider a Yang-Mills theory in $4D$ over a gauge group $G$ $$ \mathcal{L} = - \frac{1}{4} F^{a\mu\nu}F_{\mu\nu}^a + \bar \psi i D_\mu \gamma^\mu \psi + (D_\mu \phi)^\dagger D^\mu \phi $$ where $\...
2
votes
1answer
120 views

Order of Feynman diagrams for electroweak processes?

I want to compare two Feynman diagrams and be able to say which one describes a process that is more likely to happen. As far as I understand, this is done by considering the order of the diagram. ...
2
votes
1answer
748 views

QCD color factors from quark gluon vertices

The color factors in QCD tell us the relative strength of the coupling of a quark emitting a gluon, a gluon emitting a quark-antiquark pair or a gluon emitting two gluons. To calculate let them we ...
2
votes
1answer
136 views

What are skeleton diagrams and what is their use in qft and many-body physics?

How does one construct skeleton diagrams from specific Feynman diagrams (e.g. for the electronic Green function in QED and in many-body gases, for the polarization function, for the vertex function, ...
3
votes
1answer
55 views

Feynman graph of induced current

I'm self studying QFT from Peskin and Schroeder. In chapter 19 of this book, page 653 (Perturbation theory anomalies) the expectation value of the induced current is calculated. I'm confused with the ...
3
votes
1answer
59 views

Physical interpretations of the generating functions $Z[J]$ and $W[J]$ (or $E[J]$)

In quantum field theory, the generator of all Green's functions $Z[J]$ and that of the connected Green's functions $E[J]$ are related as $$Z[J]=\exp[-iE[J]]=\int D\phi\exp[i\int d^4x(\mathcal{L}(\phi)+...
2
votes
1answer
126 views

On shell and off shell simultaneously?

I am considering the following one loop virtual correction in the DIS process: where I have a quark of momentum $p$ coming in, emitting a gluon before interacting with a photon of momentum $q$ to ...
0
votes
0answers
36 views

Two Loop QED Feynman Diagram

I'm trying to find the momentum space integral representation of the below Feynman Diagram, but am having troubles with the fact that there are two loops within the system. The main question I have ...
1
vote
2answers
60 views

Where does $W$ boson mass come from in neutron decay?

Here's a diagram of neutron decay. Up and down quarks have rest masses of 2-4 MeV. The $W$ boson has a rest mass of 80 GeV. Where has this extra mass come from?
0
votes
0answers
28 views

color structure of a QCD Feynman diagram

I am trying to find a color structure of a Feynman diagram in QCD. I understand that for the diagram shown (left) we read from the double line notation diagram (right) , that the color structure is $ \...
7
votes
2answers
613 views

Is there a way to calculate the photoelectric effect in QED via a Feynman diagram?

The photoelectric effect is the historic origin of the quantum particle description of light. From it we learn that when light is shone onto a metal single photons interact with single electrons in ...
2
votes
3answers
310 views

Is Bohmian Mechanics incompatible with loop corrections?

For those who continue to be unsatisfied with Quantum Mechanics (QM), Bohmian Mechanics (BM) is an alternative worth considering. It is sometimes claimed that BM is equivalent to QM, but Lubos Motl ...
0
votes
0answers
23 views

Elementary particles interaction time (in LHC, for example)

Feynman description of an interaction contains diagrams with different total time steps (that contribute only a little to the amplitude, I guess). Is there a calculation, for a given interaction, what ...
0
votes
0answers
41 views

Feynman graph, proportional charge of a vertex

It is defined that the contribution of a vertex in a Feynman Graph towards the probability amplitude $ M_{fi} $ is proportional to a charge $Q_f$. Yet I seem to cannot find any precise explanation how ...
12
votes
4answers
2k views

Do virtual particles actually physically exist?

I have heard virtual particles pop in and out of existence all the time, most notable being the pairs that pop out beside black holes and while one gets pulled away. But wouldn't this actually violate ...
0
votes
2answers
81 views

What particles do these Feynman diagram lines represent?

What particles could be represented by A and B (labels I have added myself - this is not a homework question)? For A, it looks like it's supposed to be a photon, but I was under the impression that ...
5
votes
1answer
84 views

Why is the $D^0$ oscillation so different from the $K^0$ and $B^0$?

I have looked for this answer into many articles and books but I am not able to figure out why $D^0\to\bar{D}^0$ is so highly suppressed if compared to the $B^0 \to \bar{B}^0$ and $K^0 \to \bar{K}^0$ ...
0
votes
0answers
34 views

Feynman Diagram of $\nu_\mu + p \rightarrow \nu_\mu + p + \pi^0$

According to my class notes, the following interaction is possible $$\nu_\mu + p \rightarrow \nu_\mu + p + \pi^0$$ I tried drawing a Feynman diagram and got the below, but don't know what happens ...
-1
votes
1answer
30 views

For processes with multiple Feynman diagrams is the cross section increased?

For example in electron, positron annihilation $e^- + e^+ \rightarrow 2\gamma$ has 2 diagrams, whereas, $e^- + e^+ \rightarrow 3\gamma$ has 6 possible diagrams. This suggests, $\frac{\sigma_{2\gamma}}...
3
votes
1answer
441 views

Do we need virtual particles?

I understand the $\Delta t \cdot \Delta E \geq \hbar / 2$ relationship and the idea behind them. However, I don't understand why do we need them at all. I'm a physics undergraduate. As far as I know, ...
0
votes
0answers
51 views

How to draw Feynman diagram for K^- + p -> Sigma^0 + pi^0 — superposition in diagram

I'm trying to draw the Feynman diagram for the reaction $$K^- +p \rightarrow \Sigma^0 + \pi^0$$ I have two problems: How to draw the $\pi^0 = (u\bar u + d\bar d)/\sqrt 2$, a superposition of ...
0
votes
0answers
33 views

Superficial degree of divergence for scalar theories

I have a few questions regarding the derivation of the degree of divergence for feynman diagrams. The result is $$D = [g_E] - \sum_{n=3}^{\infty} V_n [g_n]$$ (following notation in Srednicki, $P118$) ...
2
votes
2answers
246 views

Sign of Feynman rules with derivative couplings

Feynman rules for derivative couplings always make me confused. For example, the derivative in $gV^\mu\phi^+\partial_\mu\phi^-$ will give you $\pm ip_{-\mu}$, where $\pm$ depends on whether the ...
0
votes
0answers
35 views

Wick contraction in proton-pion production

Proton-pion production $\gamma + p \rightarrow \pi^0 + p$ occurs through the interaction hamiltonian $$\mathcal H_{int} = ig \bar \psi^{(p)} \gamma_5 \psi^{(p)} \phi + e \bar \psi^{(p)} \gamma_{\mu} \...
1
vote
0answers
49 views

Conventions for propagators in Feynman diagrams [closed]

So far, I picked up the following rules for the propagators: Scalars: Dashed Fermions: Solid Abelian gauge boson: Wavy Non-abelian boson: springy Ghost: Dotted This made much sense to me until I ...
8
votes
2answers
222 views

How do derivative couplings affect canonical quantization?

Consider a Lagrangian for a scalar field $\phi$ with an interaction term $$\mathcal{L}_{int} = (\partial^2 \phi)^2 \phi.$$ Here I'm suppressing all indices for brevity. Now, this is just a three-...
0
votes
0answers
54 views

Pion-pion scattering amplitude with an effective Lagrangian

I have to compute the scattering amplitude at tree level for the process $$ \pi^{a}\pi^{b} \rightarrow \pi^{c}\pi^{d} $$ with an interaction given by what is written in the square brackets of the ...
2
votes
1answer
37 views

Computation of theta-term from triangle diagram

The chiral $U(1)$ anomaly in QCD can be calculated exactly by one-loop Feynman diagrams, for example by the famous triangle diagram. I am currently performing the computation to get a better ...
3
votes
1answer
531 views

Scalar two loop diagram in $\varphi^4$ theory

Could someone explain how, or at least show me a link that explicitly shows the calculation of a two-loop corrections to scalar’s two-point function in $\varphi^4$ theory in the massless limit.
0
votes
0answers
49 views

Coherent Addition of Feynman Diagrams

I've performed a scattering cross section calculation for a tree level Feynman diagram. My adviser wants me to perform the second order calculation which involves a loop in the Feynman diagram. His ...
3
votes
1answer
91 views

Interpreting the Cross Section Ratio R

Below is experimental data for the ratio $$R=\frac{\sigma(e^+e^-\rightarrow hadrons)}{\sigma(e^+e^-\rightarrow\mu^+\mu^-)}$$ as a function of the centre of mass energy $\sqrt s$. I am interested in ...
0
votes
0answers
23 views

1-loop integration for self-energy

I am trying to calculate the following integral that corresponds to 1-loop contribution to electron self-energy for a specific component of the momentum: $$ \Delta_i(q) = \int \frac{dp_1 dp_2 \ldots ...
3
votes
2answers
68 views

How does SUSY avoid to create non-Lorentz interactions?

A three-legs fermion interaction or boson absorbing a fermion are things we do not see in QFT because the corresponding terms in the Lagrangian are not Lorentz invariant. But in susy, naively, such ...
0
votes
0answers
36 views

Divergent diagrams in QED

I was reading about how to choose divergent diagrams in QED by using the concept of Superficial degree of divergence. We have an empirical relation $$ D= 4-E_b -\frac{3}{2}E_f $$ where $E_b$ is number ...
1
vote
1answer
103 views

How does Bremsstrahlung occur in a vacuumized particle accelerator?

In our electrodynamics course we have learned that accelerating a charged particle will lead to a loss of energy in the particle due to Bremsstrahlung. The exact amount of power radiated away is given ...
1
vote
1answer
98 views

Does massive particle really move at speed of light? [closed]

According to this answer I understood that particles with mass also move at speed of light but interaction with higgs field make this movement zigzag. So average speed is below speed of light. But I ...
0
votes
1answer
31 views

How to determine the direction of arrow on Feynman diagram for $W$ boson line?

I am somewhat confused. Looking through these slides (especially the 11th), which show Feynman diagrams involving $W$-bosons, I can't figure out which way to draw the arrow near the $W$ boson? How do ...
0
votes
1answer
40 views

Gluon to quark scattering

I am trying to calculate the $g g\to q \bar q$ amplitude but am finding it intractably complex. I have determined that there are only two independent polarizations that contribute non-zero ...
1
vote
1answer
67 views

Cutoff-dependent “inverse propagator” for renormalization

In Zee's QFT in a Nutshell, when introducing mass renormalization, he calculates the "inverse propagator" for a $\phi^4$ scalar field theory to order $\lambda^2$ by considering the two diagrams shown: ...
1
vote
1answer
75 views

Tree Level Feynman Diagrams of Electron Positron Interaction

Consider the interaction $$e^-+e^- \rightarrow e^-+e^-$$ The following is a tree level Feynman diagram for this: We can also make the paths of the two electrons on the right hand side cross over and ...
2
votes
0answers
76 views

Feynman rules from interaction Lagrangian with electromagnetic tensor (vertex)

I am currently studying for my QFT exam and in particular learning the methods of reading the Feynman rules directly off the Lagrangian. However, I'm still a bit uncertain how to deal with ...
0
votes
0answers
33 views

Obtaining the $s,t,u$ Feynman diagrams by Wick contraction

Consider a real scalar field described through the following lagrangian $$\mathcal L = \frac{1}{2} \partial_{\mu} \phi \partial^{\mu} \phi - \frac{1}{2}m^2 \phi^2 - \frac{g}{3!}\phi^3$$ The second ...
0
votes
1answer
45 views

What interaction shown in this Feynman diagram and is it possible?

I saw the following Feynman diagram in my textbook, not sure if a typo or something I don't know: Just to clarify, the "e" on the upper left does not have any superscript. I am not sure if such a ...
2
votes
0answers
49 views

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 ...
11
votes
5answers
362 views

Can Feynman diagrams be used to represent any perturbation theory?

In Quantum Field Theory and Particle Physics we use Feynman diagrams. But e.g. in Schwartz's textbook and here it is shown that it applies to more general cases like general perturbation theory for ...
7
votes
1answer
136 views

The process $\mu^+\mu^-\rightarrow hh$

I am doing some calculations in the Standard Model. I have a question that seems rather simple but makes me think a lot. I want to compute the cross section of the following process at the leading ...
4
votes
1answer
269 views

Computing box diagrams with non-vanishing external momenta

I'm trying to explicitly compute the following box diagram in the Feynman-t'Hooft gauge: If I neglect the impulsion of the $s$ quark, then the final amplitude is given by $$\mathcal{A} \propto [\...