# Tag Info

### Scattering, Perturbation and asymptotic states in LSZ reduction formula

The first question we have to ask is: what is a one particle state in an interacting theory? It is reasonable to require that they are states that are both momentum eigenstates and energy eigenstates. ...
• 11k
Accepted

### Quantum Field Theory in position space instead of momentum space?

The most important reasons we use momentum space Feynman rules are: In position space, the Feynman rules generate convolutions of propagators. Because of the convolution theorem, the momentum space ...
Accepted

• 172k
Accepted

### The reasoning of the definition of $S$-matrix

As lurscher already said correctly, the idea is to choose an initial and a final time that is (in time) away from the actual time-dependent interaction in order to make sure that the interaction does ...
• 6,571

### Why do we not have spin greater than 2?

There is a fabulous explanation in Schwartz QFT and the standard model, p153. The absence of massless particles with spin > 2 is a consequence of little group invariance and charge conservation. ...
• 1,147
Accepted

### Why this loop carries an integral if there is no undetermined momenta?

The momentum conservation at the vertex is NOT $p_1-p_2-k=0$ but rather $p_1-p_2-k+k=0$ which leaves $k$ completely free. The vertex has two half-edges carrying the momentum $k$ but in opposite ...

### Spin-$J$ Amplitude $A_J(s,t) = - \frac{g^2(-s)^J}{t-M^2}$?

The intuitive proof is straightforward. The group $SU(2)$ is simple, and so any representation is contained in the tensor product of sufficiently many copies of the fundamental (à la Young). More ...
Accepted

### The interpretation of the quantum field

(This answer is written from the perspective of lattice QFT, where everything is mathematically well-defined. Lattice QFT is not fundamental, of course, but most of the QFTs we use in physics are not ...
• 51.4k

### Weinberg, Effective Field Theories

Unitarity says that the sum of the probabilities for all possible out states (for a given in state) must be 1. Since the probability is the square of an amplitude, if any individual amplitude has a ...
• 35.5k
Accepted

### Green's Functions from Gell-Mann and Low Theorem

$\newcommand{\ket}[1]{\left|#1\right\rangle}$ $\newcommand{\bra}[1]{\left\langle#1\right|}$ $\newcommand{\braket}[1]{\left\langle#1\right\rangle}$ After reading from Yeh's notes on Advanced Condensed ...
• 748

### Relation between scattering matrix and an effective Hamiltonian

It's been some time since this question has been asked, but allow me to post a new answer. Its piece of my thesis, where I've put the derivation (since its so hard to find). I hope it'll help anyone, ...
Accepted

### Why does the action have to be hermitian?

The classical action (of particles or fields) has to be real, because this means a real classical Lagrangian. This is needed because (canonical) momenta are obtained (for instance for a particle) from ...
• 17k

### What is meant by open-string tachyon scattering amplitude?

Veneziano amplitude is a 4 tachyon amplitude in bosonic open string theory. Two tachyons are ingoing and two are outgoing. From the stringy point of view, tachyons are present in both closed and open ...
• 4,390

### What actually means to compute things at tree level?

In principle in QFT you want to calculate the interaction picture unitary time evolution operator. Then you can use the operator to evolve quantum states just like you would in normal quantum ...
• 1,018

### How does a perturbation theory make sense in quantum field theory?

A sketch of the philosophy: Step one: Introduce a regulator. Everything is finite. Step two: Set up the perturbative expansion. This is a purely algebraic step, where there is no notion of "this is ...

• 535
Accepted

### Significance of LSZ reduction formula

The premise of the question is that you either use LSZ formula or "directly" compute $\langle f | Te^{-i\int d^4xH_I} |i\rangle$. Implying that these are two unrelated approaches. While the LSZ ...
• 19.3k
Accepted

### Crossing Symmetry for Particles with Spin

General concepts Crossing symmetry is basically the CPT theorem applied in the context of the LSZ formula, using microcausality to re-order the field operators. The role of the CPT theorem is to ...
Accepted

### Why amplitudes are rational functions?

After the OP explained in the comments what exactly they're looking for, I will attempt an answer. There are a few separate facts that need explanation: Tree amplitudes are rational functions of ...
• 686

### QFT: relation between Cutkosky's cutting rules and the optical theorem

The case is somewhat the opposite. The cutting rules are far more general and can be used to prove the optical theorem at each order in perturbation theory. Furthermore, the cutting rules mean that ...
• 7,947
Accepted

### How do we calculate the S-matrix using non-perturbative QFT?

In general, for computing the scattering cross-section it is sufficient to use the pole approximation of the corresponding Green function in Heisenberg representation (details can be found in Weinberg'...
• 8,549