All Questions
Tagged with quantum-field-theory s-matrix-theory
463
questions
6
votes
2
answers
1k
views
Can scattering amplitudes be simplified with 1PI diagrams?
I have been teaching myself quantum field theory, and need a little help connecting different pieces together. Specifically, I'm rather unsure how to tie in renormalization, functional methods, and ...
- 81
22
votes
2
answers
6k
views
What is the physical interpretation of the S-matrix in QFT?
A few closely related questions regarding the physical interpretation of the S-matrix in QFT: I am interested in both heuristic and mathematically precise answers.
Given a quantum field theory when ...
- 221
16
votes
1
answer
4k
views
When we define the S-matrix, what are "in" and "out" states?
I have seen the scattering matrix defined using initial ("in") and final ("out") eigenstates of the free hamiltonian, with
$$\left| \vec{p}_1 \cdots \vec{p}_n \; \text{out} \right\rangle
=
S^{-1}
\...
- 163
10
votes
1
answer
1k
views
In quantum field theory with a mass gap, why do states in the asymptotic future/past turn out to have a Fock space structure?
In quantum field theory with a mass gap, why do states in the asymptotic future/past turn out to have a Fock space structure? For a free quantum field theory, that's trivial, but why is that the case ...
4
votes
0
answers
772
views
Validity of Cutkosky cutting rules for fermions
It is rather obvious for me that the generalized optical theorem (see e.g. Peskin&Schroeder) must hold for S-matrix elements for fermions as it is directly related to the unitarity of the S-matrix....
- 594
8
votes
1
answer
405
views
Dual Resonance Model: Fermions
I am going through Ramond's 1971 paper Dual Theory for Free Fermions Phys Rev D3 10, 2415 where he first attempts to introduce fermions into the conventional dual resonance model.
I get the 'gist' of ...
- 6,381
6
votes
2
answers
488
views
If the S-matrix has symmetry group $G$, must the fields be representations of $G$?
If the fields in QFT are representations of the Poincare group (or generally speaking the symmetry group of interest), then I think it's a straight forward consequence that the matrix elements and ...
- 8,523
5
votes
3
answers
2k
views
Unitarity of S-matrix in QFT
I am a beginner in QFT, and my question is probably very basic.
As far as I understand, usually in QFT, in particular in QED, one postulates existence of IN and OUT states. Unitarity of the S-matrix ...
- 2,226
9
votes
4
answers
2k
views
Different kinds of S-matrices?
It seems to me that the notion of an "S-matrix" refers to several different objects
One construction you can find in the literature is allowing the coupling constant to adiabatically approach 0 in ...
Squark
5
votes
1
answer
2k
views
Help with Cutkosky cutting rules for fermions
I know that a cut boson propagator is replaced with the mass shell delta function. But what happens when you cut a fermion propagator? Do you just replace the denominator with a mass shell delta ...
- 1,776
187
votes
2
answers
31k
views
Why do we not have spin greater than 2?
It is commonly asserted that no consistent, interacting quantum field theory can be constructed with fields that have spin greater than 2 (possibly with some allusion to renormalization). I've also ...
- 2,861
15
votes
1
answer
2k
views
Does de Sitter space admit an asymptotic S-matrix?
From the Penrose diagram of de Sitter space, we see it has a future and past conformal boundary, and they are both spacelike. So, does de Sitter space admit an asymptotic S-matrix? Sure, in the usual ...
- 245
16
votes
2
answers
2k
views
CFT and the Coleman-Mandula Theorem
The Coleman-Mandula theorem states that under certain seemingly-mild assumptions on the properties of the S-matrix (roughly: one particle states are left invariant and the amplitudes are analytic in ...
- 161