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Questions tagged [interactions]

Particle interactions are changes in the nature, number, or state of several particles, usually at a specific space-time point, underlying dynamics. They are represented by special "field interaction terms" in quantum field theory and normally entail interchanges of energy, momentum, and sundry quantum numbers. They include scattering, and particle creation and annihilation.

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Why does a non-linear system lead to interaction and frequency mixing between input's?

When we have a system that is nonlinear and we apply a sum of two different frequency sine waves as an input, we see the output of this system has components that are at the sum frequency of the two ...
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Gravitons and self-interaction

In the book quantum field theory and standard model by Schwartz, there is a problem 9.4 that says by considering lorentz invariency of compton scattering, you can prove that for spin 1 field there ...
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How should two-photon transitions be modelled? Is second-order perturbation theory required? Or are sequential first-order processes sufficient?

For example, I want to consider the following situation: photon transit from $m$ energy level to $m+2$ after absorption of two phonons with frequency $\Omega$. I want to calculate a transition rate ...
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Why can't two real photon, gluon, graviton, and $W$ and $Z$ fields interact by means of their virtual counterparts (the mediators of the process)?

It is a fact that two real (massless) photons, gluons, or gravitons can't react by means of their virtual counterparts (for example, two external photons that interact via one of these massless ...
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How do interaction terms appear in the Lagrangian?

How does forcing the Lagrangian to be invariant under $U(1)$ group give rise to the electromagnetic interaction term?
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How to determine which force mediates an interaction?

I have been given an assignement gives me a number of interactions, asks me to determine which, among these interactions are permitted under conservation laws (Charge, Baryon number, Leptonic numbers, ...
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Are force carrying particles always virtual particles?

Of course we have real (i.e. non-virtual) photons, but when photons play the role of "force carrier" are they virtual? Same thing for gluons. Real gluons have been detected, but when playing the role ...
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What is meant by “Since the exchange of (mesons) occurs frequently, the nuclear force is stronger than the electromagnetic force”?

This is from a book written by Y. Nambu. What role does the mass of mesons (versus mass of photons) have in making the strong force strong? How does this relate to "frequent exchanges" of mesons? ...
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EM waves cancelling each other - do they interact? Does this mean photons interact?

Just watched a video explaining how oil film causes color patterns - first wave is reflected from the outer surface (air-oil), then another wave is reflected from the lower surface (oil-water). ...
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Feynman rules for a general Lagrangian

How do I find the Feynman rules for a general Lagrangian density? For example the Lagrangian $$L = \partial_\mu \psi \partial^\mu \psi +a \psi\partial_\mu \psi \partial^\mu \psi+b \psi^2 \partial_\mu ...
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Quantum field theory with only 3-point vertexes

Given an arbitrary quantum field theory, can I always write it in terms of another (different) quantum field theory containing only operators with 3 fields? (i.e. vertexes with 3 legs) I guess that ...
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Why coupling constants with negative mass dimensions lead to non-renormalizable theories?

can somebody explain or point to the relating mathematics showing Why coupling constants with negative mass dimensions lead to non-renormalizable theories?
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What if interactions such as gravity faded with respect to the cube of distance, rather than square? [duplicate]

It's generally understood that anything that "spreads out" in 3D space dissipates at a rate proportional to the square of distance. Light, sound, gravitational force, etc. I know this is a pretty ...
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Why do we go beyond two-body interaction?

Actually, my question is why do we study many-body interactions. I have just started working in Fractional quantum Hall systems. There we have Coulomb interactions between electrons, which we know is ...
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Why in the iron based superconductor interband interaction is dominating?

What is the mechanism of intraband and interband interaction in the iron based superconductor? and Why interband interaction is dominating?
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Why is it that forces like gravity and electricity (approximately) basically act between pairs of bodies only?

In classical mechanics, with the limit of little movement (so no relativity, waves, and/or "magnetic" effects), we can see that gravitation and electricity can both be described as "two-body" forces, ...
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Can all fields contribute to the potential energy that appears in QM Hamiltonian?

Most importantly: can that potential energy in the QM Hamiltonian able to describe the motion of a single particle in an external electromagnetic field? Background: In non-relativistic QM using the ...
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Why do we observe the nuclear force only in scatterings and decays?

Why, at first glance, are the only forces we perceive to be gravity without quantizing, electromagnetism and nuclear forces only in disintegrations?
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Can someone explain the $ttH$ Coupling?

IS the $ttH$ an actual particle? If so, how can it form? I though the life of the top quark was way to short to form a particle. If it is not a particle, what is meant by coupling.
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Is the direction of the Coulomb force at short distances indeterministic?

Suppose we have two electrons that apply a Coulomb force on each other. At large distances, we can consider the two electrons as point charges and the direction of the Coulomb force would be on the ...
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Deriving effective interactions, e.g. phonon-mediated electron-electron interaction

Upshot of the question: how can I derive the effective electron-electron interaction brought about by the electron-phonon interaction? I've read derivations of the electron-phonon interaction and ...
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Are there anti virtual particles (mediator bosons)?

I have read these questions: Can bosons have anti-particles? Is there a possibility for discovery of anti-graviton, i.e. the graviton antiparticle? Antiparticle for Higgs boson? According to ...
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Equation of state of Lennard-Jones spheres

One way of accounting for van der Waals interactions in fluids is to use the Lennard-Jones potential [*], which has a repulsive term that dominates at short distances which mimics the hard-core ...
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Variance of an interacting quantum field in its vacuum state

A non-interacting quantum field $\hat{\phi}(x)$ can be decomposed into $a_{\textbf{k}}$ and $a_{\textbf{k}}^\dagger$. This enables us to calculate the variance of a free field. For example, the ...
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Is there a way for light to be reflected out of the usual plane of incident?

Is it possible for use to apply a potential or magnetic field to the surface of the media, so that the light being reflected out of the plane of incident? i.e. Compare to an initial "vertical" plane ...
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Does observation in quantum theories always imply interaction (affecting quantum system with photons, electromagnetic fields, etc.)?

The term observation is obscure. As I see so far, observation is always done by means of affecting (!) the quantum system by some means - often photons or electromagnetic waves or whatever else. ...
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Free Electron-Bond Electron Interactions

I am reading an introductory textbook on electronics: Practical Electronics for Inventors by Paul Scherz and Simon Monk. In a section discussing the motion of electrons in circuits, the textbook ...
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Interacting conformal field theories in spacetime dimensions higher 6?

Are there any papers which directly tackle the question of whether or not there exists interacting CFTs in spacetime dimensions higher than 6? It has been proven that there do not exist any ...
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What is the difference between an interaction and a decay in particle physics?

While studying secondary cosmic rays, I have encountered sentences like: Charged pions and kaons can either initiate further interactions or decay. or Because of the low area density at large ...
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Non-minimal coupling between a neutral atom and the EM field

Let us say that I have neutral bosonic atoms interacting with an EM field. I can write down the Lagrangian as \begin{align} \mathcal{L}=\eta^{\mu\nu}\partial_{\mu}\phi\partial_{\nu}\phi^{\dagger}-m^2|...
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Layperson can not get his head around Force Carriers

I read all I can but, this is vexing me. It realtes to Quantum-Mechanics, and I believe Quantum Field Theory, and maybe even Quantum Chromodynamics. Before the Higgs the Model was 16 instead of 17 we ...
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Feynman Rules from Lagrangian with charge conjugation matrix

I'm dealing with a doubly charged scalar singlet that interacts only with the right-handed muon as follows, $$\mathcal{L} = \lambda \psi_{R}C\psi_{R} \phi^{++},$$ where $\lambda$ is the coupling, $\...
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In particle colliders, according to QM, how are two particles able to “collide”?

According to QM, we know that The act of measurement forces a particle to acquire a definite (up to experimental errors) position, so in a particle collider, like the one in CERN, by which means ...
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Is a magnetic force caused by a curvature of something? [duplicate]

If gravity is not a force, but a manifestation of spacetime curvature, what about other forces? What about magnetic force (or Lorenz force)? Is it not a force, but a manifestation of the curvature of ...
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Van der Waals constant $b$ (real gas) chemical form. only

http://www2.ucdsb.on.ca/tiss/stretton/database/van_der_waals_constants.html I don't understand how to calculate exact constant - b, given only the chemical formula and nothing else. Q: T || F --- ...
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What happens in the Hartree and Fock diagrams?

I am trying to understand the Hartree and Fock diagram shown in the picture. To understand it a assume there is an electron entering and leaving at the tail of the tadpole (Hartree diagram) and an ...
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What is the definition of “force” in quantum field theory?

In quantum field theory, there are certain interactions that we seem to associate with the action of "forces." For example, the exchange of a gauge boson between two matter particles is associated ...
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Doesn't dark matter imply a new force?

Given that every particle that we have experimental confirmation of is an oscillation of its field (from what we know), and given dark matter is thought to be a particle yet undiscovered according to ...
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How to know what type of diagram contributes to a two-to-two process?

There are 3 types of diagrams that can contribute to a two-to-two process; the $s$-channel, $u$-channel and $t$-channel. How do I know what diagrams can contribute to a process? I know that in QED, ...
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Can light radiation pressure change its own path

Well i have seen that light creates radiation pressure Can two light waves coming from two different direction change each others path?
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What causes Gravity/Gravitation? [duplicate]

What causes Gravity/Gravitation? We know that mass causes gravitational force which is a non-contact force but is there anything physical that leads gravitational pull. Here, by physical I mean ...
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1answer
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Can the EM field self-interact in the Standard Model?

Since the EM field is a linear combination of the electroweak $U(1)$ gauge field and one of the $SU(2)$ gauge fields, does this mean that it has self-interaction terms carried over from the ...
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Propagators in interaction with derivatives

Given a Lagrangian density containing an interaction with derivates, it's easy how to guess the Feynman rules for vertexes. However i was wondering about propagators: in S-matrix expansion it's ...
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How are photons absorbed by electrons?

I study physics in high school and I was told about the Photoelectric Effect and Compton Effect, and there is something that seems strange to me: How does a photon physically absorbed an electron and ...
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QFT Why do in and out states have a non-trivial overlap?

Im trying to follow chapter 4 about interacting fields in Peskin and Schröder. They define the S matrix by $_{out}<p_1 p_2 | k_a k_b>_{in} = <p_1 p_2 | S | k_a k_b>$, where $S = \lim_{T\...
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Why is the photoionization cross section smaller at higher photon energies?

I read that the photoionization of a ground state hydrogen atom for photon energies $h\nu > I_H$ (where $I_H$ is the ionization energy of $H$ at $13.6 \, \mathrm{eV}$ has a smaller cross section ...
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Derivation of the complete Bethe Bloch formula

I can not find anywhere the explicit calculation for the complete Bethe Bloch formula including all the correction factors. I would like to have at least one idea of ​​the calculation that is actually ...
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Gravity on par with the other forces

Gravity is said to be the weakest of the fundamental interactions, of which the other three, the electromagnetic and the strong/weak nuclear forces, are the more powerful. My question is simply, what ...
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Basics of Compton scatter interactions? Forces behind it?

What is the force that governs compton scattering interaction? Also how is it that we are able to approximate that compton scattering probability is proportional to the mass density of the target ...
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$U(1)_V$ invariance

I'm working with an interaction Lagrangian of the form: $${\cal L}_{int} = \bar{\psi}\Theta\chi \tag1$$ Where $\Theta$ contains other operators, coupling constants, etc. I'm trying to unveil if ...