Questions tagged [quantum-chromodynamics]
Quantum-ChromoDynamics (QCD) is the quantum field theory believed to describe the strong nuclear force.
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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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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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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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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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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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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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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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How much of the proton's mass is due to the Higgs field?
The proton mass is 938 MeV. People often claim that
(A) The proton is a bound state of two up quarks and one down quark, with the three quarks contributing a total rest mass of $2 \times (2.2 \text{...
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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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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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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 is the precise statement of the OZI Rule?
What is the precise statement of the OZI Rule?
I've heard that a diagram is OZI suppressed if it can be "cut in two by cutting only gluon lines", but I don't really understand.
For example, consider ...
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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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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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Do color-neutral gluons exist?
If I'm correct a quark can change color by emitting a gluon. For example a blue up quark $u_b$ can change into a red up quark by emitting a gluon:
$$u_b \longrightarrow u_r + g_{b\overline{r}}$$
(Here,...
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Is there an explanation for the 3:2:1 ratio between the electron, up and down quark electric charges?
I understand that the NNG formula relates $Q$, $I_3$, and $Y$ and can be derived in QCD; does this unambiguously predict the electric charge ratios without making assumptions about the definitions of ...
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What IS Color Charge?
This question has been asked twice already, with very detailed answers. After reading those answers, I am left with one more question: what is color charge?
It has nothing to do with colored light, ...
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Physical meaning of the chiral condensate in QCD
Considering the QCD Lagrangian in the chiral limit, where all the quarks masses are set to zero. Then the Lagrangian has the following chiral symmetry:
$$
SU(L)_{V} \times SU(L)_{A} \times U(1)_{V} \...
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From which dimensionful constants does proton mass arise?
It is well known that the most of the proton (or any other hadron with light quarks) mass is not made up from quark masses, but it is dynamically generated by QCD mess inside. I've also heard that, ...
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Gluon radiation from a nucleon?
Because neutrons and protons consist of quarks, their magnetic moments differ from the so-called nuclear magneton (the natural unit for expressing magnetic dipole moments of nucleons).
A neutron has a ...
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What is the Noether charge associated with the the color $SU(3)$ symmetry of QCD?
A version of the Noether's theorem applies to local gauge symmetries. What is the Noether's charge associated with a non-abelian gauge symmetry such as the color $SU(3)$ and how is that derived? I ...
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Similar masses and lifetimes of the $\Delta$ baryons
Why do the four spin 3/2 $\Delta$ baryons have nearly identical masses and lifetimes despite their very different $u$ and $d$ quark compositions?
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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 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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Why is color conserved in QCD?
According to Noether's theorem, global invariance under $SU(N)$ leads to $N^2-1$ conserved charges. But in QCD gluons are not conserved; color is. There are N colors, not $N^2-1$ colors. Am I ...
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Intuitive explanation of how hadron mass emerges from the strong force
I'm not familiar with QCD, but I'm looking for intuitive explanation of this phenomenon (it could be that easy explanation does not exist).
What I've read is that large part of hadron masses arises ...
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Why do the quarks constantly change colors?
In a hadron the quarks constantly change colors.They,then exchange gluons to remain color neutral.For example a red quark becomes blue by emitting a red and anti blue gluon and then the blue quark ...
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Do the proton PDFs change much with Q?
Specifically, the second moments, do they change much from say 100 to 1000 GeV? Why or why not?
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Where can I get the most accurate measurements of parton distribution functions?
Where would I look to get the most accurate experimental values of parton distribution functions for the proton? I know these functions aren't measured directly, but I'd basically like to find a fit ...
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If quarks can't be isolated in the first place, how did they become confined in the early universe?
On one hand, we know that quarks cannot exist in isolation. This is because the energy required to dissociate a quark-antiquark pair in a meson (or quarks in a hadron) will create a mesons (or hadrons)...
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