Questions tagged [quantum-chromodynamics]
Quantum-ChromoDynamics (QCD) is the quantum field theory believed to describe the strong nuclear force.
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Is there an equation for the strong nuclear force?
The equation describing the force due to gravity is $$F = G \frac{m_1 m_2}{r^2}.$$ Similarly the force due to the electrostatic force is $$F = k \frac{q_1 q_2}{r^2}.$$
Is there a similar equation ...
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Is it pions or gluons that mediate the strong force between nucleons?
From my recent experience teaching high school students I've found that they are taught that the strong force between nucleons is mediated by virtual-pion exchange, whereas between quarks it's gluons. ...
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Why have our eyes not evolved to see "gluons"? [closed]
The photons are the propagators for QED, and we rely on photons to see the world around us.
The gluon is the propagator in QCD. Why have our eyes not evolved to see gluons (either on top of being ...
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What does it mean that there is no mathematical proof for confinement?
I see this all the time* that there still doesn't exist a mathematical proof for confinement. What does this really mean and how would a sketch of a proof look like?
What I mean by that second ...
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What exactly is a pomeron?
The term 'pomeron' was apparently important in the early stages of QCD. I can't find any reference to it in modern QFT books, but older resources sometimes refer to it offhand, and I've yet to find ...
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What is the significance of the QCD scale parameter $\Lambda$?
I see that it appears as a constant in the relation for the running of the strong coupling constant. What is its significance? Does it have to be established by experiment? Is it somehow a scale for ...
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What is the experimental evidence that the nucleons are made up of three quarks?
What is the experimental evidence that the nucleons are made up of three quarks? What is the point of saying that nucleons are made of quarks when there are also gluons inside it?
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Mathematically, what is color charge?
A similar question was asked here, but the answer didn't address the following, at least not in a way that I could understand.
Electric charge is simple - it's just a real scalar quantity. Ignoring ...
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How (or when) do gluons change the color of a quark?
I know a baryon is only stable when it contains a quark of each color. And as far as I know, the gluon essentially changes the color of a quark and moves onto the next, and this is what holds the ...
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Why we don't have macroscopic fields of Higgs bosons or gluons?
Why is it that there exists a classical macroscopic field of photons and gravitons but not that of $Z, W^{\pm}$ bosons, gluons or Higgs boson?
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The divergence in QCD Series-- How many are they, and what do they mean?
I am referring to this question, and especially this answer.
In addition, QCD has - like all field theories - only an asymptotic
perturbation series, which means that the series itself will also
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Is there a strong force analog to magnetic fields?
In special relativity, magnetism can be re-interpreted as an aspect of how electric charges interact when viewed from different inertial frames.
Color charge is more complex than electric charge, but ...
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What is the status of Witten's and Vafa's argument that the QCD vacuum energy is a minimum for zero $\theta$ angle?
The argument, which I reproduce here from Ramond's `Journies BSM', is originally by Witten and Vafa in ($\it{Phys}$. $\it{Rev}$. $\it{Lett}$. 53, 535(1984)). The argument is that for $\theta = 0 $ (...
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Trace and adjoint representation of $SU(N)$
In the adjoint representation of $SU(N)$, the generators $t^a_G$ are chosen as
$$ (t^a_G)_{bc}=-if^{abc} $$
The following identity can be found in Taizo Muta's book "Foundations of Quantum ...
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What are the details around the origin of the string theory?
It is well-known even among the lay public (thanks to popular books) that string theory first arose in the field of strong interactions where certain scattering amplitudes had properties that could be ...
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How to Perform Wick Rotation in the Lagrangian of a Gauge Theory (like QCD)?
I'm studying Lattice QCD and got stuck in understanding the process of going from a Minkowski space-time to an Euclidean space-time. My procedure is the following:
I considered the Wick rotation in ...
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Why are the dineutron and diproton unbound?
It is known that there are no diproton or dineutron nuclei.
Does this mean that two protons or neutrons are not actually attracted to each other? Even if the attraction was weak, wouldn't it cause ...
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What is a mass gap?
I read that Quantum Chromodynamics is a theory with a mass gap. What is a mass gap in laymen terms? Why some theories have it? Which theories does not have it?
Note: I searched for mass gap before ...
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Inverse square rule for strong forces
Most of the forces induced by a point particle follows the $1/r^2$ rule. Then why does the strong force not obey it?
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Do strong and weak interactions have classical force fields as their limits?
Electromagnetic interaction has classical electromagnetism as its classical limit. Is it possible to similarly describe strong and weak interactions classically?
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How far can we separate two bound quarks?
If we try to separate two quarks bound into a meson or a hadron, the energy in the gluon field eventually will be large enough to spawn a quark-antiquark pair.
How far can we stretch that gluon ...
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What's the real resolution of the $U(1)_A$ problem?
To recap the problem, consider QCD with three massless quark flavors. There is a symmetry
$$SU(3)_L \times SU(3)_R \times U(1)_L \times U(1)_R$$
corresponding to independent rotations of the left-...
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What's the deepest reason why QCD bound states have integer charge?
What's the deepest reason why QCD bound states have integer electric charge, i.e. equal to an integer times the electron charge?
Given that the quarks have the fractional electric charges they do, ...
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Why Cronin Effect Happens?
I'm looking for explanation on Cronin effect but unfortunately there's no Wikipedia entry or self-contained paper to start from. The statement of this effect is that:
"At leading order, multiple ...
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Why is the strong CP term $ \theta \frac{g^2}{32 \pi^2} G_{\mu \nu}^a \tilde{G}^{a, \mu \nu}$ never considered for $SU(2)$ or $U(1)$ interactions?
The Lagrangian one would write down naively for QCD is invariant under CP, which is in agreement with all experiments.
Nevertheless, if we add the term
\begin{equation}
\theta \frac{g^2}{32 \pi^2} ...
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Why doesn't the color force between two quarks have an inverse square law?
From what I understand the color force between two quarks doesn't decrease with distance. Why is it that the color force doesn't decrease with the square of the distance if there are three dimensions ...
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Is forward scattering = no scattering?
What is forward scattering? If it is equivalent to no scattering, then why not call it "no scattering"?
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Are "confinement" and "asymptotic freedom" two sides of the same coin?
On Wikipedia it says that the two peculiar properties of quantum chromodynamics (QCD) are: confinement and asymptotic freedom.
Asymptotic freedom is the idea that at low energies we cannot use ...
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What IS Color Charge?
This question has been asked twice already, with very detailed answers. After reading those answers, I am left with one more question: what is color charge?
It has nothing to do with colored light, ...
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QCD and QED with unlimited computational power - how precise are they going to be?
My question is about quantum algorithms for QED (quantum electrodynamics) computations related to the fine structure constants. Such computations (as explained to me) amounts to computing Taylor-like ...
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