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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 is the photopeak at a higher energy than the compton edge?

Why does the photoeffect deposit more energy than interactions via Compton scattering? Or the other way around: Why is the photopeak right (at a higher energy) than the Compton edge? https://en....
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Four fundamental forces to explain metabolism

Apologies if this question does not make sense. I have tried looking for an answer but couldn't find one and have zero knowledge I was watching this video last night which I found fascinating. In it,...
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Do neutrons interact with electro-magnetic fields? [closed]

Does neutrons interact with electromagnetic fields? If yes, what kind of interactions would they undergo?
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Where does the fine structure constant come from?

I have this question: Where does the fine structure constant come from? Is it derived? Is it assumed? I will be most thankful if you will also include other detailed info that you think is also good ...
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2answers
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Why are derivatives in interaction terms treated differently from derivatives in the kinetic term?

I know that derivative couplings in a Lagrangian interaction, such as $$\mathcal{L}_{int} = \lambda \phi (\partial_{\mu}\phi)(\partial^{\mu}\phi)$$ bring down two momentum factors into the matrix ...
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1answer
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Shouldn't dark energy be considered a fifth fundamental force?

As everyone knows from the Standard Model, there are 4 fundamental forces that describe the Universe. But isn't the dark energy, the force that makes the universe expand, different from them? Maybe ...
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3answers
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How bosons do not violate conservation of energy?

As far as I understand bosons are energy packets which carry forces: e.g. Higgs bosons carry gravity. What I don't understand is, for example if we have an isolated object which constantly releases ...
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Are radiation interactions pressure, temperature dependent?

I read somewhere that gamma radiation interact with matter mostly via Compton scattering. I guess there are other type of interactions as well and probably a constant ratio between them. Does this ...
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1answer
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Understanding particle scattering vs interaction

I have two questions to clarify what I am actually trying to ask through this question: What is the difference between "interaction" and "scattering" in particle physics? Does scattering of two ...
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1answer
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Boson fermion interaction

Does there exists a process of boson-fermion interaction through which a virtual boson that carries force on fermionic matter turns into two (or more) fermions (with the correct conservations)? If not,...
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Visual simulation of elementary particles and interactions thererof?

Pardon me for a very stupid question, but knowing that SM predicts so much about sub-atomic particles, their interactions and whatnot, means that we have formalized these into some sorts of equations,...
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Why don't there seem to be any dimensionless fields in nature?

Scalar fields have dimension 1, spinor fields dimension 3/2, and vector bosons like the photon dimension 1. According to the principles of renormalizability (along with others), this restricts the ...
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1answer
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Are interacting and free field operators equal in the Schroedinger picture?

Taking for example a scalar hermitian field (which in the free case would obey to the Klein-Gordon equation), is it true that in the Schroedinger picture the following expression hold true both in a ...
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Speed of electromagnetic interactions

We know that electromagnetic waves fly with the speed of light, but my question not about waves. Consider a very strong electromagnet that creates a substantial field 3 meters away. Then we send a ...
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1answer
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Are there elementary forces acting in directions than 0 or 90 relative to their fields?

Some forces act in the same direction as their field orientation, like a gravitation. Other forces, for instance the force acting on a charged particle in a magnetic field, are perpendicular to that ...
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String interactions: intuitive vs worldsheet pictures

I am trying to understand precisely the interactions in open/closed string theories from several (interconnected) aspects: relation between the "intuitive" picture (by that I mean spatial sections of ...
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1answer
84 views

Why charged particle interaction cross section increases as the particle's energy decreases?

I'm tying to gain a little more understanding of the dose deposition profile of high energy (~100MV) protons in matter (depth dose curve - medical physics). I understand that the Bragg peak occurs ...
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1answer
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Strength of gravitational waves vs. electromagnetic waves

If the recent gravitational wave's energy had reached us as visible light, how bright would it have been? Stackexchange complains about the form and brevity of the question so i add something... if it ...
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How can I see where this formula for a general vertex factor comes from?

I have been reading Srednicki from the beginning and doing all the exercises, and I hit a big roadblock at Q10.4, as I can't seem to figure out what Srednicki is doing in his solution. Luckily, I ...
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1answer
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Force via “exchange particles” or “via field”

More or less I have come across two concepts to explain non contact forces: FIELD CONCEPT: modification of space by the source which in turn produces force on the other (That is in my classroom ...
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2answers
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QFT: Range of 'collision'

If two particles approach each other, they can [provided that their properties add to those of other particle(s)] interact and go from, say, $$e + \bar e \to \gamma + \gamma$$ My question is how ...
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Quantum Mechanics: Pictures description

This is a question related to the Schrodinger and Heisenberg picture. Consider a physical system. There are two states- initial and final. Now this is the explanation from Schrodinger and Heisenberg ...
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6answers
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For collision, physical contact is not a necessary condition. Why?

In my textbook, it is written that "For collision, physical contact is not a necessary condition". How can collision occur without physical contact? If there is no physical contact, then there ...
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3answers
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Gravity and its effect on electrons [duplicate]

If gravity pulls everything downward why do electrons in a vertically held wire with a voltage across it not experience a downward pull?
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Status of particles in interacting QFT

From my readings in QFT and answers such as this, I've read that the concept of particles and particle-number in interacting systems becomes ill-defined in QFT. Of course, in the real world, a number ...
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Covalent and electrostatics interactions

I was reading one of the notes on protein folding today, and suddenly my eyes lit up when I saw one data. It was comparing different types of interactions with respect to their order of energy. It ...
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1answer
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Question about quantum interaction [closed]

Here are some of my doubts: (there's quite a number which I couldn't find satisfying ans to, you can choose any qn to ans, thanks in advance) How close must two wave functions be to be considered an ...
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Matsubara Green's function

The RKKY coupling has the form of $\chi_{i,j}(r,r')= \frac{-2}{\pi} Im \int_{-\infty}^{\varepsilon_F} d\varepsilon \; Tr[\sigma_i G(r,r',\varepsilon) \sigma_j G(r',r,\varepsilon)]$. where i and j ...
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Why are we using the interaction picture?

I know the interaction picture states and operators: \begin{align} \lvert\psi_I(t)\rangle &=e^{i\hat{H}_0t}\lvert\psi_S(t)\rangle,\\ \hat{O}_I(t) &=e^{i\hat{H}_0t}\hat{O}_Se^{-i\hat{H}_0t},\\ \...
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Why is it necessary for Fields to explain a Force?

We all know that objects create fields and force is about what happens when an alien object is influenced by fields. Why is it necessary to use the concept of a field influencing objects to explain ...
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2answers
692 views

Does gravity have anything to do with Van Der Waal's forces?

Does gravity have anything to do with Van Der Waal's forces? Just throwing this out there, I was wondering if they do because gravity is such a weak force and the VdW forces at a molecular level could ...
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0answers
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Finite-Temperature $\phi^{4}$ theory - Why is the massless $T\neq 0$ contribution diverging?

I'm following Chapter 3 of Kapusta and Gale's Finite-Temperature Field Theory here. I'm considering the following integral (the unrenormalized self-energy evaluated at zero-four momentum): $$ \...
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241 views

Repulsive part of interatomic/intermolecular potentials: Pauli, Coulomb or both?

We know that if we take two atoms/molecules, their interaction energy shows a short-range attractive part and a medium-range attractive part. One popular way to schematize this interaction is to ...
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Do solids, liquids and gases resist compression due to the same reason?

In crystalline solids, the constituent atoms sit close to each other in their equilibrium positions. The solid is not compressible because as the pressure is increased, the atomic orbitals tend to ...
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1answer
508 views

When referring to Strong Nuclear Force and Weak Nuclear Force, is it all right to call them Nuclear Forces?

Nuclear Force seems to refer to Residual Strong Force. If so, how could we refer to Strong Nuclear Force and Weak Nuclear Force?
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1answer
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Calculating vertex factor for scalar field theory

I am practising basic QFT and am having some trouble with calculating the vertex factor of an interacting theory involving two real scalar fields, $\phi_{1}$ and $\phi_{2}$. If I create a generic ...
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2answers
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S-matrix derived directly in terms of the interaction picture

Consider a quantum mechanical system with Hamiltonian $$H=H_0+H_{\text{int}}.$$ Consider $H_0$ to be time-independent, so that its associated time-evolution operator is $U_0(t,t_0)=e^{-i(t-t_0)H_0}$....
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1answer
661 views

Computing a Gaussian Path Integral (with integral in exponent)

I have been studying the path integral approach to QFT and set myself the challenge of starting from a $\phi^3$ interaction Lagrangian and following the method through to completion. I started with: $$...
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2answers
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How does the Earth know when to send a graviton to a newly born particle?

Pardon if this is a silly queston, but I'm reading this for the first time. It says that the force we perceive between two objects is an effect of the exchange of the force carrier particles. Even ...
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1answer
239 views

Vacuum bubbles and LSZ reduction

Let me preface this by saying that I don't have an issue with this: $$ \langle\Omega|T\phi_H\cdots\phi_H|\Omega\rangle = \frac{\langle 0|T\phi_I\cdots\phi_IS|0\rangle}{\langle 0|S|0 \rangle}, $$ ...
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1answer
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QFT and its non-rigorous assumptions

I have been trying to figure out all the non-rigorous assumptions of QFT (as performed in an operator theory) that allow it to function as it currently is. So far, the three big candidates I found are ...
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3answers
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What is the issue with interactions in QFT?

I've started studying QFT this year and in trying to find a more rigorous approach to the subject I ended up find out lots of people saying that "there is no way known yet to make QFT rigorous when ...
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$\phi^{4}$ theory Feynman rules

One of the momentum space Feynman rules in $\phi^{4}$ theory (for correlation functions) is that for an external point with 4-momentum $p$ (with direction headed towards the external point), we need a ...
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1answer
373 views

Computing S-Matrix Elements from Feynman Diagrams

In Peskin and Schroeder (PS), the Feynman rules for calculating correlation functions are first presented. Only terms involving all field contractions need to be considered. In Section 4.6, this is ...
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3answers
588 views

$\phi^{4}$ theory

Consider a scalar field theory with a $\phi^{4}$ interaction term $$\mathcal{L}=\frac{1}{2}(\partial_{\mu}\phi)^{2}-\frac{1}{2}m^{2}\phi^{2}-\frac{\lambda}{4!}\phi^{4},$$ where $\lambda\ll 1$. I am ...
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1answer
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Physical problem with logarithmic potential

Could someone tell me please examples of physical situations (if there are) where there might appear a potential of the form $$V(r)=V_{0}\ln(r/\xi),$$ being $r>0$ the radial coordinate in ...
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1answer
454 views

How do $\phi^4$ terms result in an interacting (classical) field?

It was my understanding that a scalar field, $\phi(t;\vec{x})$, varying in time and parametrised by position, was 'non-interacting' if the field evaluated at one point affected the field evaluated at ...
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1answer
324 views

Photon propagator counterterm in QED

The lagrangian for QED including counterterms is $$\mathcal{L} = -\frac{1}{4}F_{\mu \nu}^{2}+i\bar{\psi}\gamma^{\mu}{\partial_{\mu}} \psi-m_R \bar{\psi}\psi-e_R \bar{\psi}\gamma^{\mu}A_{\mu}\psi- \...
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Interaction Lagrangian with derivatives

I have a problem with derivatives in the interaction part of the Lagrangian. Considering the Lagrangian $${\cal L}=\frac{1}{2}\partial_{\mu}\phi\partial^{\mu}\phi+\frac{1}{2}\partial_{\mu}\psi\...