What is a loop (in Feynman Diagrams)?

[Sekonda]

Hey,

I was wondering what exactly a loop is of this kind :

I understand the initial & final four momentums to be the same and so there is no boundary on the possible allowed momentums in the loop.

My professor said the loop is equal to (or can be modeled as) the self-interaction by mass with factor -im^2.

My question is what does this loop represent? I think it is just a self-interaction which can take any momentum by means of 'off-shell' or virtual particles, does this mean all self-interactions regardless of their momenta are virtual particles when represented by a loop or only ones constrained to some set of momenta's?

Thanks,
SK

 

[Vanadium 50]

That particular loop doesn't make much sense, since it gets absorbed in the definition of a proton. Where it matters is when you have additional external lines on the loop. These represent new physical processes.

 

[andrien]

perhaps it is not drawn correctly,so may be it is vacuum polarization ,
http://en.wikipedia.org/wiki/Vacuum_polarization