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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The anticommutator of $SU(N)$ generators
For the Hermitian and traceless generators $T^A$ of the fundamental representation of the $SU(N)$ algebra the anticommutator can be written as
$$
\{T^A,T^{B}\} = \frac{1}{d}\delta^{AB}\cdot1\!\!1_{d} +...
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Nambu-Goldstone bosons from a quantum anomaly symmetry breaking?
We know that:
Nambu-Goldstone bosons come from Goldstone theorem: a spontaneous (continuous)-symmetry breaking of the system leads to massless scalar modes.
quantum anomaly: is the anomalous ...
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Is a 6-quark particle viable?
It is my understanding (which may be flawed) that protons and neutrons are stable because the 3 (R, G, and B) quarks form a "white" color singlet. Wouldn't 6 quarks or even 9 quarks create ...
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Why aren't all quarks clumped together in one giant hadron?
As far as I am aware, the strong interaction is attractive only, and its carrier, the gluon, is massless meaning it has unlimited range. If this is the case, how come we only observe quarks in pairs ...
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Why Does Lattice QCD Use Heavier Than Physical Masses For Calculations?
It is common place (e.g. here), for Lattice QCD calculations to be computed using reference masses (such as the pion mass) which are greater that the physical values of those quantities.
Sometimes, ...
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How does the strong force increase in attraction as particles move farther away?
I have heard that the strong force can be imagined similar to a rubber band, where the farther you extend the ends, the harder they pull themselves back together. My question is how? From what it ...
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Why $SU(3)$ and not $U(3)$?
Is there a good reason not to pick $U(3)$ as the colour group? Is there any experiment or intrinsic reason that would ruled out $U(3)$ as colour group instead?
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Bound states in QCD: Why only bound states of 2 or 3 quarks and not more?
Why when people/textbooks talk about strong interaction, they talk only about bound states of 2 or 3 quarks to form baryons and mesons?
Does the strong interaction allow bound states of more than 3 ...
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Physical meaning of the chiral condensate in QCD
Considering the QCD Lagrangian in the chiral limit, where all the quarks masses are set to zero. Then the Lagrangian has the following chiral symmetry:
$$
SU(L)_{V} \times SU(L)_{A} \times U(1)_{V} \...
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Topological susceptibility
In QCD we have strong CP violation (and hence a $\theta$-dependence of the theory) only if the topological susceptibility of the vacuum is nonzero:
$$\langle F\tilde{F},F\tilde{F}\rangle_{q \...
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If the classical Maxwell theory describes E&M fairly, how well-done is the classical Yang-Mills theory for chromodynamics?
If the classical Maxwell theory describes electrodynamics (electromagnetism E&M) fairly well, how suitable would the classical Yang-Mills theory (say SU(3) color) describe the chromodynamics, and ...
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Why are baryons equivalent to Skyrmions?
Baryons in quantum chromodynamics can be described as a bound state of three quarks. But they can also be described as a topological soliton of the pion field. How can both descriptions be equivalent?
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Gauge invariant but not gauge covariant regularization
I'm not sure if someone's already asked this before, but I was wondering, in field theory,
when we say that a certain field is gauge invariant but not gauge covariant, what does this mean? In ...
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Can massive fermions have zero modes?
I'm confused about fermion zero modes in relation to instantons. I understand that instantons can create fermion zero modes, but it's not clear to me when a fermion has a zero mode.
For example, ...
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Theta parameter of QCD: Very small, periodic in $2\pi$ or both?
I've been learning about the theta parameter of QCD and I'm confused about the fact that it's supposed to be very small but at the same time some sources say that the Yang-Mills theory should be ...
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The contribution to mass from the dynamical breaking of chiral symmetry
The claim is often made that the discovery of the Higgs boson will give us information about the origin of mass. However, the bare masses of the up and down quarks are only around 5 MeV, quite a bit ...
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Could the universe have non-vanishing net colour charge?
I've heard that the strong force doesn't decrease in strength with increasing distance, and that's why quarks must be confined within hadrons. But could there be, say, a single quark out there, so ...
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Derivation of the effective potential between a quark and an anti-quark
Typically in particle physics books (not in QFT books!) I have often seen this statement that the potential between a heavy quark and its anti-quark can be "empirically" represented as $V(r) = -\frac{...
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Probability of forming mesons vs baryons
When a heavy quark hadronizes it has some probability of forming a meson vs forming a baryon. I suspect there is a well known branching ratio for each type of hadron. Does anyone know what the ...
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If free quarks can't exist, how did the universe form?
As I understand, the Big Bang started with a photon gas that then created the other particles. Thus obviously there would be some free quarks in the early Universe unless quarks are always created in ...
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