# Feynman Diagrams: Polarization question

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1. Sep 27, 2014

### Breo

Hello folks,

I've just started to read "An Introduction to Quantum Field Theory by Peskin and Schroeder" and at the end of 6th page I could read:

"... Since H_I should conserve angular momentum, the photon to which these particles couple must have the correct polarization vector to give it this same angular momentum: $$\epsilon^\mu = (0, 1, i, 0)$$ ..."

Why those components? I lack of knowledge about polarization and dont know were it comes and what it means.

Thank you in beforehand

2. Sep 27, 2014

### Staff: Mentor

i has a phase shift of pi/2 relative to 1. That is exactly the condition for a circular polarization in z-direction. I don't have the book, so I don't know the exact context where this is used.

You could also use (0,i,-1,0) or similar as the absolute phase does not matter.

3. Sep 27, 2014

### Breo

Oh, I get it. I was blind as I'm new with 4-vectors and not used to plarizations concepts.

Thank you.

4. Sep 28, 2014

### ChrisVer

by new to 4-vectors what do you mean? Don't read any QFT book if you are new to 4-vectors, but try instead some Special Relativity... Roughly said QFT= SR+ QM, so you need both QM and SR understanding.