Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

Please see this question for a clear background of the notation I use. My issue is that I want to use Wick's theorem to calculate the amplitude of meson $\psi(p_1)\psi(p_2)\rightarrow\psi(p_1')\psi(p_2')$ scattering. I can quickly get to the point where I need to evaluate:

$\langle p_1',p_2'|:\psi_1^\dagger\psi_1\psi_2^\dagger\psi_2:|p_1,p_2\rangle$

In my notes, the next step is

$\langle p_1',p_2'|:\psi_1^\dagger\psi_1\psi_2^\dagger\psi_2:|p_1,p_2\rangle=\langle p_1',p_2'|\psi_1^\dagger\psi_2^\dagger|0\rangle\langle0|\psi_1\psi_2|p_1,p_2\rangle$

where he seems to have changed orders of the fields and inserted $|0\rangle\langle0|$, $|0\rangle$ being the vacuum of the free theory. Can anyone explain what this particular step is? From thereafter the derivation is pretty clear to me

share|improve this question
add comment

1 Answer

You have to put the first term (which in not in normal order, hence the dots) in normal order. The vacuum mapping is the identity matrix so you simple CAN put it.

share|improve this answer
3  
Hi John, can you expand on your answer a bit? –  Brandon Enright Dec 15 '13 at 23:54
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.