# Is there a way to compute (trivalent) Feynman integrals inductively from smaller diagrams?

+ 3 like - 0 dislike
327 views

Suppose that I would like to compute the Feynman integral associated to the trivalent graph One can argue that this diagram comes from two copies of the smaller diagram glued together at the external vertices.

We could argue even further that it really comes from two copies of the following diagram where we instead cut one of the edges on each diagram, yielding the second, and then gluing them together.

Suppose that I know how to compute the integral associated to either this second (or preferably third) diagram. Is there some sort of algorithm for reducing the integral of the first diagram to some combination of integrals for the second and third diagrams?

I should remark perhaps that I am being deliberately vague about what the context of this computation is. I'm mostly hoping that there is some formal way to combine the smaller integrals to produce the larger ones, something that would look like, say, a multiplicative structure on an $R$-algebra freely generated by diagrams, for some ring $R$. Or something like that.

Edit: I feel that I should note that one hope that I have (which may prove ill-founded) is that this will in some sense be computable in a manner akin to the gluing or other type formulae in Gromov-Witten theory. It'd be really nice if we could write something like $$I_{\Gamma_g} = \sum_{g_1+g_2 = g} C_{g_1,g_2} I_{\Gamma_{g_1}}I_{\Gamma_{g_2}}$$ or something.

This post imported from StackExchange Physics at 2014-05-04 11:26 (UCT), posted by SE-user Simon Rose

+ 1 like - 0 dislike

I'm not sure of the level at which you're looking for answers. Two avenues (which are continuing to be researched actively) to check out:

1. Work on amplitudes by Zvi Bern and co, starting in the 90s.
2. BCFW recursion relations for gauge theory amplitudes recursively generate all diagrams from 3 point amplitudes.

In general, if a theory "fundamentally" has a 4-valent vertex, it is not clear if one could break all Feynman diagrams into 3-point amplitudes. However, in some interesting classes of theories, one can.

Generally useful resources on progress in amplitude techniques:

You might want to read up on the optical theorem and unitarity cuts.

This post imported from StackExchange Physics at 2014-05-04 11:26 (UCT), posted by SE-user Siva
answered Apr 29, 2014 by (720 points)
My theory should not have a 4-valent vertex, since it is coming from a cubic action.

This post imported from StackExchange Physics at 2014-05-04 11:26 (UCT), posted by SE-user Simon Rose
For the diagrams you've drawn, sure. Fwiw, I tried to keep my comments fairly general.

This post imported from StackExchange Physics at 2014-05-04 11:26 (UCT), posted by SE-user Siva
I've downloaded the two references you suggested; I'll take a look through them. Thanks!

This post imported from StackExchange Physics at 2014-05-04 11:26 (UCT), posted by SE-user Simon Rose

 Please use answers only to (at least partly) answer questions. To comment, discuss, or ask for clarification, leave a comment instead. To mask links under text, please type your text, highlight it, and click the "link" button. You can then enter your link URL. Please consult the FAQ for as to how to format your post. This is the answer box; if you want to write a comment instead, please use the 'add comment' button. Live preview (may slow down editor)   Preview Your name to display (optional): Email me at this address if my answer is selected or commented on: Privacy: Your email address will only be used for sending these notifications. Anti-spam verification: If you are a human please identify the position of the character covered by the symbol $\varnothing$ in the following word:p$\hbar\varnothing$sicsOverflowThen drag the red bullet below over the corresponding character of our banner. When you drop it there, the bullet changes to green (on slow internet connections after a few seconds). To avoid this verification in future, please log in or register.