If you think that this question is likely to get closed then please do not answer and only say that in the comments since this system doesn't let me delete the question once it has answers.
Everytime I am faced with an analysis of the spectral function it looks like a "new" unintuitive set of jugglery with expectation values and I am unable to see a general picture of what this construction means. I am not sure that I can frame a coherent single question and hence I shall try to put down a set of questions that I have about the idea of spectral functions in QFT.
- I guess in a Dirac field theory one defines the spectral function as follows,
ρab(x−y)=12<0|{ψa(x),ˉψb(y)}|0>
Also I see this other definition in the momentum space as,
SFab(p)=∫∞0dμ2ρab(μ2)p2−μ2+iϵ
Are these two the same things conceptually?
I tried but couldn't prove an equivalence.
(I define the Feynman propagator as SFab=<0|T(ψa(x)ˉψb(y))|0>)
Much of the algebraic complication I see is in being able to handle the quirky "_" sign in the time-ordering of the fermionic fields which is not there in the definition of the Feynman propagator of the Klein-Gordon field (..which apparently is seen by all theories!..) and to see how the expectation values that one gets like <0|ψa(0)|n><n|ˉψb(0)|0> and <0|ˉψb(0)|n><n|ψa(0)|0> and how these are in anyway related to the Dirac operator (iγμpμ+m)ab that will come-up in the far more easily doable calculation of the spectral function for the free Dirac theory.
Is it true that for any QFT given its Feynman propagator SF(p) there will have to exist a positive definite function ρ(p2) such that a relation is satisfied like,
SF(p)=∫∞0dμ2ρ(μ2)p2−μ2+iϵ
So no matter how complicatedly interacting a theory for whatever spin it is, its Feynman propagator will always "see" the Feynman propagator for the Klein-Gordon field at some level? (..all the interaction and spin intricacy being seen by the spectral function weighting it?..)
- One seems to say that it is always possible to split the above integral into two parts heuristically as,
SF(p)=∑(free propagators for the bound states)+∫states((Feynman propagator of the Klein-Gordon field of a certain mass) ×(a spectral function at that mass))
Is this splitting guaranteed irrespective of whether one makes the usual assumption of "adiabatic continuity" as in the LSZ formalism or in scattering theory that there is a bijection between the asymptotic states and the states of the interacting theory - which naively would have seemed to ruled out all bound states?
To put it another way - does the spectral function see the bound states irrespective of or despite the assumption of adiabatic continuity?
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