How to compute the number of loops in a Feynman diagram?

lonelyphysicist

In doing my $$\phi^{3}$$ theory I didn't know exactly how to count the number of loops in a diagram given the number of vertices, internal and external lines. Is there a general algorithm in doing this? What if we have more than one interaction vertex (e.g. the Standard Model)?

PS. What does it mean when it says my thread has been "moved"?

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vanesch

Staff Emeritus
Gold Member
lonelyphysicist said:
PS. What does it mean when it says my thread has been "moved"?
It means an administrator thought that it better belonged to another subforum. If you talk about the "physical mass" thread, I'd like to know why it was moved, because I don't think it is really a college-level question !

cheers,
Patrick.

dextercioby

Homework Helper
It's nonphysical $\lambda \phi^{3}$.The vertex (the OPI Green function of 4 points in the first order of perturbation theory) is made up of 3 scalar bosons.Self interaction doesn't mean creating/annihilating particles (that means there's no emission,nor absorption of particles),but only pure scattering (identical particles scattering one off another).

Read Zee's [1] argument against this theory and others involving other powers in $\phi$.

Daniel.

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[1] A.Zee,"Quantum Field Theory in a Nutshell",PUP,2003.

lonelyphysicist

dextercioby said:
It's nonphysical $\lambda \phi^{3}$.The vertex (the OPI Green function of 4 points in the first order of perturbation theory) is made up of 3 scalar bosons.Self interaction doesn't mean creating/annihilating particles (that means there's no emission,nor absorption of particles),but only pure scattering (identical particles scattering one off another).

Read Zee's [1] argument against this theory and others involving other powers in $\phi$.
Do you mean my thread was moved because $\lambda \phi^{3}$ is not a physical theory?

dextercioby

Homework Helper
No,no,you had put in the wrong forum.It's Quantum Physics related and it should have been posted here. General Physics is not for such specific problems.

Daniel.

Staff Emeritus
Gold Member
Dearly Missed
Dexter is this a general rule that QFT math questions go in quantum mechanics? Why not particles?

lonelyphysicist

Under the "Quantum Physics" forum link it says "Quantum Mechanics and Field Theory" - that's why I posted my questions here.

Anyone who knows some QFT would know that these questions do form the bulk of the QFT experience. Renormalization, for instance, is all about the technicalities - how many loops, how to get rid of the large logs, what subtraction scheme to use, etc. Feynman diagram computation itself involves lots of machinery too.

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