All Questions
Tagged with quantum-field-theory s-matrix-theory
153
questions with no upvoted or accepted answers
9
votes
0
answers
768
views
Weinberg's S-matrix and split into free and interacting Hamiltonian
TL;DR: How can states of an interacting QFT asymptotically follow the trajectories governed by the free Hamiltonian, when, say, the free and interacting groundstates are different, and the states look ...
- 935
8
votes
0
answers
226
views
QFT's bound states references
At a graduate level, QFT courses teach very well how to perform perturbative calculations using LSZ or even the background field method. Plenty of books are suggested to go into the details of this ...
8
votes
0
answers
263
views
Is S-matrix theory a subset of modern quantum field theory?
As a graduate student of physics in the early 2000's, our particle physics classes started with quantum field theory, since QCD had long been established as a good model of the nuclear strong force. ...
- 278
8
votes
0
answers
148
views
Self-energy that does not obey sum rule
Analytically, I calculated a self-energy $\Sigma(\omega)$, for which I verified that
1) $\text{Im}\big[\Sigma(\omega)\big] \leq 0$ for all $\omega$ and specifically $\text{Im}\big[\Sigma(0)\big] = 0$,...
- 2,910
7
votes
0
answers
369
views
LSZ Reduction Formula and Wavepackets in Peskin
In Section 7.2 of Peskin and Schroeder, the LSZ Reduction formula is discussed. I can follow the discussion leading up to the equation after (7.41) [at the bottom of Pg. 225]:
$$\sum_\lambda \int\frac{...
- 996
6
votes
0
answers
182
views
What is the physical interpretation of the automorphism on bounded operators induced by an S matrix?
In a QFT, the S-matrix $S$ is a unitary operator, that fixes the vacuum and commutes with the unitary operators implementing the action of the Poincare group on an appropriate Hilbert space $H$.
...
- 1,151
5
votes
0
answers
98
views
Form of the optical theorem in non-Abelian theory
I am studying chapter 16.3 from Peskin & Schroeder and I am trying to follow through the argument where we include contributions from ghosts to satisfy the Ward identity in non-Abelian gauge ...
- 2,035
5
votes
0
answers
159
views
Analytic continuation of Feynman amplitudes seems ill-defined
I was reading Peskin & Schroeder's book on Quantum Field Theory and on chapter 7, "The optical theorem for Feynman diagrams" (page 232) they extend analytically the Feynman amplitude $i \...
5
votes
0
answers
243
views
LSZ formula in Srednicki, normalization issue
In the Ch.5 of his book, Srednicki says LSZ formula is valid provided the following conditions hold:
$$
\langle 0|\phi(x)|0\rangle = 0, \langle p|\phi(x)|0\rangle = 1
$$
To achieve these conditions, ...
- 51
4
votes
0
answers
70
views
Redefinition of fields and interpretation of the particle content
Suppose I have some Lagrangian $\mathcal L_1$ involving multiple fields $\phi_i$ with interactions. I can reparametrize the Lagrangian in terms of new fields $\psi_i$ by inserting some ...
- 153
4
votes
0
answers
97
views
Is crossing symmetry reliant on using the Feynman propagator?
If one used the advanced or retarded propagator instead of the Feynman propagator would crossing symmetry necessarily be violated in some scattering amplitudes?
- 2,270
4
votes
0
answers
116
views
Has it been proven that causality means the S-matrix is analytic?
In books like 'the analytic S-matrix' they give justification that causality implies analyticity however they also said it hasn't been explicitly proven.
This book was written a while ago, has it been ...
- 1,933
4
votes
0
answers
83
views
Time-independent source and quantum field theory
Can anyone explain the fundamental reason of why time-independent sources cannot emit or absorb energy. Does it have to do with time-translation symmetry and Noether's theorem?
I was studying the ...
- 369
4
votes
0
answers
389
views
Scattering amplitudes and LSZ formula for off-shell renormalization scheme
TLDR: The question: Does it make sense to calculate scattering amplitudes using an off-shell renormalization scheme?
I expand a bit by using a theory of a single self interacting massive scalar. I ...
- 1,129
4
votes
0
answers
364
views
Spinor Helicity Formalism: Reference Spinors $q$ in Compton Scattering
My question is rather straight forward, but the setup in order to pose the question is a little lengthy; please bear with me!
I am trying to calculate the average over initial states and sum over ...
- 321