When is a propagator on shell?

[nolanp2]

Hey all,

just a simple question that's confusing me about amplitudes for feynman diagrams. How do i know whether a system needs to be considered as being on shell, and hence has an imaginary component included in the denominator of the propagator?

If i have one propagator which is required to be on shell, does that mean all propagators in the diagram must now be on shell?

thanks

 

[RedX]

heh I'm not sure. I do know that after a Wick rotation you ought to be able to neglect the imaginary component in the demoninator I think. maybe there are cases where even with a Wick rotation you can't neglect it. no wait after you take care of the demoninator with Feynman parameterization then after Wick rotation you should be able to ignore the imaginary part of the denominator.