# Problem with Feynman Rule for Vertices

1. Dec 22, 2006

### Neitrino

Dear PF,
I would like to thank you for existence of Physics forums since it appears very helpful.

Dear PF,
I was reading one paper and trying to derive Feynman rules for some vertices, but I could not get the same result as given in paper, as a guideline I use ‘t Hooft’s diagrammar and the same is given in appendices of Cheng and Lee – Gauge Theory of elementary particle physics. I attach extract from one Arxiv paper, extract from diagrammar and what I was trying to write.
Could you please have a look and advise me what I do wrong?

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2. Jan 2, 2007

### Neitrino

Dear all,
Could anyone pls give some feedback... May be my question senseless...

3. Jan 5, 2007

### AlphaNumeric

For instance, suppose you have the term $$ie\gamma^{\mu}\bar{\psi}\psi A_{\mu}$$, then taking the functional derivatives would leave you with $$ie\gamma^{\mu}$$, that's the Feynman rule. Just remember that if you've something like $$e\phi^{2}\psi$$ you'll pick up the factor of 2 from differentiating $$\phi^{2}$$ twice. I know it sounds obvious but you won't believe how often slip ups like that throw a spanner in the works!