The tag has no wiki summary.

learn more… | top users | synonyms

6
votes
1answer
215 views

Two-loop regularization

Working out some quantum field theory computations, I have to find out the value of the two-loop Feynman integral $$ ...
2
votes
1answer
285 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): ...
6
votes
2answers
914 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 ...
8
votes
3answers
671 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 ...
0
votes
1answer
140 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 ...
2
votes
1answer
225 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, ...
47
votes
2answers
4k 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 ...
10
votes
1answer
346 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 ...
2
votes
1answer
185 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 ...
6
votes
2answers
590 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 ...
2
votes
2answers
283 views

Does path integral and loop integral in a Feynman diagram violate special relativity?

Consider a correlation function between two points $A(x_1,t_1)$ and $B(x_2,t_2)$, we need to integrate over paths which could be infinite long. But the time length $(t_1-t_2)$ is finite, so if $A$ ...
6
votes
1answer
322 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 ...
3
votes
1answer
185 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.
3
votes
1answer
454 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 ...
9
votes
2answers
319 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 ...
0
votes
3answers
1k 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 ...
2
votes
1answer
224 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 ...
8
votes
2answers
291 views

In what sense are loop diagrams quantum corrections?

What's so not-quantum about tree-level diagrams?
5
votes
3answers
286 views

Dimensional Regularization Integral Formula

In the formula $$\int \frac {d^{4-2\epsilon} \ell} {(2\pi)^{4-2\epsilon}} \frac 1 {(\ell^2-\Delta)^2} = \frac i {(4\pi)^{2-\epsilon}} \Gamma(\epsilon) \left(\frac 1 \Delta\right)^\epsilon,$$ how ...
4
votes
1answer
182 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
3answers
2k 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 ...
4
votes
1answer
629 views

Help with Cutkosky cutting rules for fermions

I know that a cut boson propagator is replaced with the mass shell delta function. But what happens when you cut a fermion propagator? Do you just replace the denominator with a mass shell delta ...
6
votes
1answer
108 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 ...
13
votes
2answers
389 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 ...