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

## Main Question or Discussion Point

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.

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.