The feynman-diagram tag has no wiki summary.
32
votes
2answers
2k views
Why did Feynman's thesis almost work?
A bit of background helps frame this question. The question itself is in the last sentence.
For his PhD thesis, Richard Feynman and his thesis adviser John Archibald Wheeler devised an astonishingly ...
13
votes
2answers
158 views
Applications of the Feynman-Vernon Influence Functional
I am looking for a reference where the Feynman-Vernon influence functional was defined and used in the context of relativistic quantum field theory. This functional is one method to describe ...
10
votes
1answer
260 views
Any practical results yet from 'Twistor Uprising'?
In Nima's lectures on the 'Twistor Uprising' (for example here), he gushes about about new powerful techniques for calculating amplitudes, such as summing Feynman diagrams using 'BCFW recursion.' He ...
8
votes
2answers
90 views
In what sense are loop diagrams quantum corrections?
What's so not-quantum about tree-level diagrams?
8
votes
2answers
167 views
How to prove equivalence of RG flow of QFT coupling constant and diagrammatic resummation at fixed renormalization scale?
QFT books say that solving the RG equation $\frac {dg} {d\textbf{ln} \mu}=\beta(g)$, using the one-loop beta function, is to the "leading log" approximation equivalent to resumming infinitely many ...
7
votes
3answers
321 views
MVH amplitudes and the unitarity method
In the last 5 years there has been a silent revolution in QFT called the unitarity method and the Maximum Violating Helicity (MVH) Amplitudes that basically consist an alternative way to obtain the ...
6
votes
3answers
335 views
What makes a Feynman diagram real or virtual?
Simple question: as the title says, what makes a real Feynman diagram real, and what makes a virtual diagram virtual? Or in other words, how do I tell whether any given diagram is real or virtual? ...
6
votes
3answers
195 views
Is there a simple way to compute some physical constant from Feynman diagram statistics?
I've been playing around writing some software to generate Feynman diagrams for QED, respecting the vertex "rules" described here, and avoiding creating isomorphic duplicates.
So from a starter ...
6
votes
1answer
43 views
Why is there no double counting of $s$- and $t$-channels in string theory?
In string theory for the four particle tree diagram exchange, why is there some mysterious crossing duality between the $s$- and $t$- and $u$-channels? Why isn't there a double counting in the Feynman ...
6
votes
1answer
161 views
SUSY, ways to boost Chargino/Neutralino production?
Does anybody know a good reference that works out the equations for the Chargino/Neutralino production cross section in SUSY? I'm trying to understand if there are any tricks for boosting the ...
5
votes
2answers
497 views
Gentle introduction to twistors
When reading about the twistor uprising or trying to follow a corresponding Nima talk, it always annoys me that I have no clue about how twistor space, the twistor formalism, or twistor theory works. ...
5
votes
1answer
139 views
Two-loop regularization
Working out some quantum field theory computations, I have to find out the value of the two-loop Feynman integral
$$
...
5
votes
2answers
411 views
How do we distinguish between virtual particle exchange and particle decay?
The difference between virtual particles and unstable particles is discussed at length in this question (namely, virtual particles correspond to internal lines in Feynman diagrams and are not ...
5
votes
2answers
363 views
Feynman rules with helicity states.
Whenever Feynman rules are stated they are always without any mention of the helicities - this I find to be very confusing. How does one introduce and account for that?
Is there an intuitive/simple ...
5
votes
1answer
212 views
Can a photon see ghosts?
Does it make sense to introduce Faddeev–Popov ghost fields for abelian gauge field theories?
Wikipedia says the coupling term in the Lagrangian "doesn't have any effect", but I don't really know ...
5
votes
0answers
137 views
An use of the Schwinger-Dyson equation
I was confused as to how the equation 10 on page 7 or equation 21 on page 8 of this paper http://arxiv.org/abs/1211.1866 was derived. Can someone explain from where does this come and what do the ...
4
votes
1answer
260 views
Interpretation of derivative interaction term in QFT
I am trying to understand what a term like $$ \mathcal{L}_{int} = (\partial^{\mu}A )^2 B^2 $$ with $A$ and $B$ being scalar fields for instance means. I understand how to draw an interaction term in ...
4
votes
1answer
109 views
Convergence of quantum effective action to finite loop order
Consider the quantum effective action of a fixed QFT. If we compute it perturbatively to finite loop order $\ell$, we get a sum over an infinite number of Feynman diagrams. For example, the 1-loop ...
4
votes
1answer
144 views
One-loop $\phi^4$ theory in $d = 3$
I'm trying to calculate the 1 loop correction to the propagator in massless $\phi^4$ theory, in $d = 3$, just for fun. The diagram just looks like a straight line with a circle touching tangently to ...
4
votes
0answers
121 views
How fat is Feynman’s photon?
According to my calculations, it is a lot skinnier than Airy’s photon, but still a whole lot fatter than a straight line.
So, how does a photon get from point A to Point B?
The ray optics ...
3
votes
3answers
304 views
What does this “Witten's Dog” Feynman diagram in Futurama episode mean? [closed]
In an old episode ("Mars University") of Futurama which is a TV show, a character named Professor Farnsworth was trying to lecture "Superdupersymmetric String Theory" and "Witten's Dog" to some ...
3
votes
3answers
1k views
Electron Positron annihilation Feynman Diagram
I am having some trouble understanding this fenyman diagram, it seems to indicate that the electron produces the positron, as the arrow of the positron is pointing from the electron.
Additionally ...
3
votes
1answer
60 views
Four-gauge-boson vertex in non-Abelian gauge theories
In Peskin & Schroeder's book page 524, the following diagram is calculated for the gauge boson self-energy in order $g^2$:
In dimensional regularization, its contribution is given by
...
3
votes
1answer
213 views
Wick Rotation, interpretation of $\bar{p}^2$ vs the usual $p^2=m^2$
Suppose we use the metric $(+,-,-,-)$ thus the momentum squared is
$p^2 = p_0^2-\vec{p}^2 = m^2>0$
Defining $p_E:=\mathrm{i}\cdot p_0$ and $\bar{p}:=(\,p_E,\vec{p})$ with Euclidean norm ...
3
votes
1answer
250 views
Feynman rules for SUSY
This might be an incredibly naive question, but I'm wondering if there are a set of rules for "translating" between Standard Model and SUSY. For instance, if I want want to go from a Standard Model ...
3
votes
0answers
96 views
exercise books for Feynman diagrams [duplicate]
I know QFT at graduate level but I'll like to master the skill of working with Feynman diagrams. I'm looking for a book of solved exercises on this topic.
Specifically, I'm looking for the kind of ...
2
votes
1answer
95 views
Getting rid of double delta function in Feynman rules
[1]
A very simple example of feynman rule for scalar fields.
After computing the diagram i have got the following:
$$
-i(2\pi)^4g^2\int d^4q \frac{i}{q^2 -m^2c^2}\delta^{(4)}(p_1 - p_3 -q)
...
2
votes
1answer
268 views
What does the graviton-matter Feynman scattering diagram look like?
The question is in the title really. I know gravity + QFT is on shaky ground but I'm not looking for Feynman rules, just a schematic, if that is possible.
Edit 1: added possible diagram for ...
2
votes
1answer
201 views
4th order feynman-diagrams for $e^- + \mu^+ \to \nu_e + \bar{\nu}_\mu$
The task is to draw the two 4th-order Feynman diagrams of:
$$e^- + \mu^+ \to \nu_e + \bar{\nu}_\mu $$
I drew the first one as (time left->right):
...
2
votes
1answer
152 views
How do you properly define a line in a Feynman diagram?
I've been reading Tony Zee's "Quantum Field Theory" and I'm really enjoying it. However, on p. 45 I came across what I think is an inconsistency. The sentences in question are:
The rules go ...
2
votes
1answer
149 views
Scattering Processes in Scalar Yukawa Theory
I'm trying to compute nucleon-nucleon scattering in scalar Yukawa theory. Here we view a nucleon as a complex scalar field $\psi$ and a meson as a real scalar field $\phi$. They interact through ...
2
votes
1answer
175 views
Scalar Field Theory Decay/Scattering
I have a few questions related to the following interaction Lagrangian (no use of crossing symmetry in the following) involving the uncharged scalar $\chi$ and the charged scalar $\phi$:
...
2
votes
1answer
250 views
Feynman diagrams and Hartree-Fock
I am puzzled by some lines I read in Mattuck's book on Feynman diagrams in many-body problems ( http://www.amazon.com/Feynman-Diagrams-Many-Body-Problem-Physics/dp/0486670473 )
Page 21 (1.14) for ...
2
votes
1answer
98 views
Phase space suppression in loop integrals
Often (if not always) when calculating loop integrals in QFT one encounters extra 2 $\pi$'s that serve to suppress higher order corrections more so than the most naive guess would give. This happens ...
2
votes
0answers
37 views
What are the rainbow and ladder approximations in a solid state physics context?
All references I find talk about quarks and gluons, where I have only very limited knowledge about.
From it's name (rainbow) I guess it applies to fermions coupled to bosons and we're interested in ...
2
votes
0answers
50 views
Chirality when moving around legs in Feynman diagrams
Assuming one has the following term in a Lagrangian:
$$
g (\overline{A_R} B_L)(\overline{C_R}D_L)
$$
where A,B,C,D correspond to spin 1/2 Dirac particles and the subscripts $R$ and $L$ denote left- ...
2
votes
1answer
132 views
Photon-photon scattering matrix element
I have to write the matrix element $i\mathcal{M}$ given the Feynman rules of QED for the $\gamma \gamma$ scattering as the example above. Suppose that time is the vertical. There are two times, $t, ...
2
votes
1answer
263 views
Feynman rule 4-point vertex WW -> ZZ
I am looking for the Feynman rule of the 4-point gauge boson interaction of W+ W- -> Z Z. I am guessing it looks like the Yang Mills 4-point vertex for gluons, but with helicity included.
Equation ...
2
votes
1answer
148 views
Feynman Diagram for Bragg Equation
Did Feynman ever derive the Bragg equation as a particle representation using Feynman diagrams? And where is it available? I spoke to Freeman Dyson and he couldn't recall.
1
vote
1answer
141 views
Feynman diagrams in three dimensions
Is there such a thing as an extension of Feynman diagrams into three dimensions?
Canonical Feynman diagrams use one space and one time dimension to visualize processes involving elementary particles ...
1
vote
1answer
101 views
Two-photon scattering: colours
Is there a particular conservation principle that necessitates that the outcoming photon pair has the same frequencies as the incoming photon pair?
I'm thinking in particular of these Feynman-like ...
1
vote
2answers
163 views
Is single photon annihination of electron-positron pair prohibited by Feynman diagram analysis?
It is obvious that electron-positron pair cannot annihilate to a single photon which will violate the momentum conservation. My question is can we get this knowledge from Feynman diagram or ...
1
vote
1answer
77 views
Degree of divergence of a Feynman diagram
I am studying the degrees of divergence of Feynman diagrams. I feel that I miss something but I don't really understand what. Please apologize if this question is silly. Anyway.
As an introduction to ...
1
vote
1answer
297 views
Feynman Rules for massive vector boson interactions
I am stuck at the beginning of a problem where I am given an interaction term that modifies the regular QED Lagrangian. It involves the interaction between a photon field and a massive vector boson: ...
1
vote
1answer
139 views
Diagram-like perturbation theory in quantum mechanics
There seems to be a formalism of quantum mechanics perturbation that involve something like Feynman diagrams. The advantage is that contrary to the complicated formulas in standard texts, this ...
1
vote
1answer
179 views
Does path integral and loop integral in a Feynman diagram violate special relativity?
Consider a correlation function between two points A(x1,t1) and B(x2,t2), we need to integrate over paths which could be ...
1
vote
0answers
54 views
Feynman Diagram for Interaction of Deuteron with Oxygen-16
I have to draw a Feynman diagram for the interaction of a deuteron with Oxygen-16.
The interaction is as follows:
$ d + {}^{16}O \rightarrow p + {}^{17}O . $
I am generally ok with Feynman ...
0
votes
3answers
649 views
Conservation of electric charge in Feynman diagram
Here is a Feynman diagram showing the mutual annihilation of a bound state electron positron pair into two photons:
Is the electric charge conserved at the point A (or B)? What is the "charge" of ...
0
votes
1answer
111 views
weak bosons and feynman-stueckelberg interpretation
from Wiki "The W bosons have a positive and negative electric charge of 1 elementary charge respectively and are each other's antiparticle."
Q:If each is the other's antiparticle then which is ...
