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

In doing my [tex]\phi^{3}[/tex] 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"?
 

vanesch

Staff Emeritus
Science Advisor
Gold Member
5,007
16
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

Science Advisor
Homework Helper
Insights Author
12,927
504
It's nonphysical [itex]\lambda \phi^{3} [/itex].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 [itex] \phi [/itex].

Daniel.

---------------------------------------------------
[1] A.Zee,"Quantum Field Theory in a Nutshell",PUP,2003.
 
dextercioby said:
It's nonphysical [itex]\lambda \phi^{3} [/itex].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 [itex] \phi [/itex].
Do you mean my thread was moved because [itex]\lambda \phi^{3} [/itex] is not a physical theory? :smile:
 

dextercioby

Science Advisor
Homework Helper
Insights Author
12,927
504
No,no,you had put in the wrong forum.It's Quantum Physics related and it should have been posted here.:smile: General Physics is not for such specific problems.

Daniel.
 

selfAdjoint

Staff Emeritus
Gold Member
Dearly Missed
6,764
5
Dexter is this a general rule that QFT math questions go in quantum mechanics? Why not particles?
 
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.
 

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Hot Threads

Top