In on-shell (OS) scheme, one of the renormalization conditions is that the propagator, say, a scalar theory
1p2+m2−Σ(p2)−iϵ
must have a unit residue at the pole of physical mass p2=−m2. Some textbooks say this is to make sure the propagator behaves like a free field propagator near the pole. But why?
Answer
The OS condition that ∂Σ∂p2|p2=−m2=0
Suppose that we used a different renormalization scheme in which our counter-terms contain no finite parts (e.g. MS scheme). In the OS scheme, we removed finite parts which were logarithmic in our artificial regularization scale μ. In our new choice, the propagator might have a residue, say R.
This residue manifests itself in an irritating way; the field will be re-normalized such that ϕ=1√RϕB. In the LSZ formula, however, external lines contribute factors R (from the KG equation cancelling the propagators). So external scalar lines contribute a factor √R in the MS scheme.
So, whilst this choice in the OS scheme is somewhat arbitrary, it's convenient, because external scalar lines contribute a factor 1.
I'm trying to learn these points myself, so hopefully someone can expand/correct my answer where necessary...
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