Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Self Energy for N flavor phi^4 theory

  1. Dec 23, 2014 #1
    I am reading the fifth chapter on perturbation theory of Condensed Matter Field Theory by Altland and Simons. This question is about the section starting on page 223.

    To discuss self energy, they introduced a vector field ##\phi = \{ \phi^a \}, a = 1, \cdots , N##. The action of the field is given by
    S[\phi] = \int d^dx (\frac{1}{2} \partial \phi \cdot \partial \phi + \frac{r}{2} \phi \cdot \phi + \frac{g}{4 N} (\phi \cdot \phi)^2)
    The goal is to compute the perturbation expansion of the Green function
    G^{ab}(x-y)=\langle \phi^a (x) \phi^b (y)\rangle
    using the self energy operator ##\Sigma_p##.

    In momentum space, the Green function is given by
    G^{ab}_{p} = [(p^2+r- \hat \Sigma_p)^{-1}]^{ab},
    where the diagrams for ## \Sigma_p ## is shown in the figure
    The text claims that represented in terms of the Green functions, the first order contribution to the self-energy operator is given by
    [\Sigma^{(1)}_{\mathbf{p}}]^{ab} = - \delta^{ab} \frac{g}{L^d} (\frac{1}{N} \sum_{\mathbf{p'}} G_{0,\mathbf{p'}} + \sum_{\mathbf{p'}} G_{0,\mathbf{p-p'}}),
    where the first (second) term in the parenthesis corresponds to the first (second) diagram in the figure. I am having trouble reproducing this result. Specifically,
    (1) Where does the overall minus sign come from?
    (2) Since the interaction strength is given by ##g/4N##, from the result, the first diagram has a contribution of ##4## and the second diagram has a contribution of ##4N##. How do I get these factors?
    (3) How do I derive the Feynman rules for these diagrams?

  2. jcsd
  3. Dec 28, 2014 #2
    Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook