Why should there be one-particle states in an interacting quantum field theory? I'm a mathematician trying to learn quantum field theory. This question has two parts: first, I want to double check that I'm thinking about the surrounding issues correctly, after that I'll ask my main question.

It's taken me a while to understand how I should think about particle states in an interacting quantum field theory and how they relate to the field operators. Here's what I think is true; please tell me if I'm wrong!


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*When we pass from a free to an interacting theory, no one has a description of the state space that's as nice as the Fock space picture. Instead, we simply posit that the combined spectrum of the momentum operators $P^\mu$ contains some isolated hyperboloids $p^2=m^2$. (This is the famous "mass gap".) We decide to call the corresponding eigenstates "one-particle states with mass $m$". This name is somewhat sensible since, in particular, these states will be eigenstates of the Hamiltonian $H=P^0$, so they are stable and have the sorts of momenta we would expect from a particle of mass $m$. These states have very little to do with any one-particle states for any free theory.

*For a field operator $\phi$, look at the Källén-Lehmann representation of the corresponding two-point function. If $|p\rangle$ is a one-particle state for which $\langle p|\phi(0)|\Omega\rangle\ne 0$, it turns out there will be corresponding pole in the two-point function and vice versa. (When this condition is met, $\phi$ is an "interpolating field" for $|p\rangle$.) We can use this to construct a one-particle state out of an appropriately smeared version of $\phi$ applied to the vacuum; this cruicially requires the isolatedness of the eigenvalue mentioned above.

*If the interpolating field is one of the "basic" fields appearing in the Lagrangian, the state we build in this way is an "elementary" particle, and if it's some more complicated combination of products of basic fields it's a "composite" particle. For a given one-particle state, there are in general lots of different fields you could use for this purpose, but we always assume there is at least one.

*We can build $n$-particle states by doing a similar smearing trick with well-separated smearing functions and letting $t\to\pm\infty$; in particular, unlike the one-particle states, these multiple-particle states are only really meaningful in this asymptotic limit. But once we have these, we can define the S-matrix in terms of their overlaps, and we can write down the LSZ formula and a bunch of Feynman diagrams and we're off to the races.

My main question (aside from "is everything I just said correct?") is why should we expect these assumptions to hold? In particular, the parts that are the least obvious to me are (a) why should the spectrum of the $P^\mu$'s contain these isolated hyperboloids, and (b) why should I assume that the corresponding states all have interpolating fields as in (2) above?
I'm aware that there are some problems extracting a completely rigorous version of this whole story and that massless particles, gauge symmetries, and so on introduce complications; that's not what I'm asking about. I'm asking for intuition: right now these assumptions --- the mass gap and the existence of interpolating fields --- feel arbitrary to me, and I'm wondering if anyone has a way of thinking about the logical structure that makes it feel more natural.
 A: An interacting quantum field theory on Minkowski space should have a representation of the Poincare group as unitary operators, and the irreducible representations of the Poincare group are exactly the stable one-particle states. This point of view is due to Wigner. There can be both massive and massless irreducible representations, the mass gap isn't crucial for the idea. There is also no fundamental distinction between elementary and composite one-particle states.
A general state will break up into irreducible components under the action of the Poincare group. This is manifestation of the idea that you can describe an interacting state by a collection of asymptotic one particle states. It depended on there being a representation of the Poincare group, which in turn depends on the Minkowski space setting.
If we have this Hilbert space structure with products of irreducible representations, we can define raising and lowering operators and then we can in turn collect these into fields with appropriate transformation properties. This is the point of view of Weinberg's QFT volume 1, which you might be interested in.
But in practice we simply assume that an operator (which might be a combination of 'elementary' fields) has a non-vanishing matrix element between the vacuum and any state which is consistent with its transformation properties. Although they necessarily exist, it is rather hard to choose an operator by accident which violates this assumption (it's like guessing the exact wave functions of an anharmonic oscillator).
