Quantum-ChromoDynamics (QCD) is the quantum field theory believed to describe the strong nuclear force.
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Some more questions about the BCFW reduction
This question is a continuation of this previous question of mine and I am continuing with the same notation.
One claims that one can actually split this $n$-gluon amplitude such that there is just ...
4
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1answer
77 views
Asymptotic Freedom - Qualitative Explanation
I am doing a (mostly qualitative) course on Particle Physics, and am confused about the concept of asymptotic freedom. The lecture notes basically say that a quark may experience no force/be "unbound" ...
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Are Gribov copies responsible for the observed Mass-gap and color confinement properties of Pure Yang-Mills Theory?
New solutions to the existing problems are presented by the quantum mechanics and Einstein's theory of special relativity when treated together. The creation of particle/anti-particle pairs by quantum ...
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1answer
86 views
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 ...
3
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1answer
60 views
For mesons, or baryons, do sea quarks contribute to the angular momentum of the bound state?
The total angular momentum of a bound state of quarks, such as a meson say, can be done by studying the spin and orbital angular momentum of the 2 valence quarks.
What about the sea quarks why they ...
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2answers
88 views
The status of $SU(3)_C$ symmetry in the Standard Model
In the Standard Model of Particle physics the $SU(2)_{EW}$ symmetry and the $SU(2)$ isospin symmetry are broken. What about $SU(3)_C$? Is it broken too?
if YES, what breaks the symmetry?
If NO, what ...
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4answers
327 views
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 ...
6
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95 views
Beta function of pure $SU(N_\text{c})$ Yang-Mills theory
What is the dependence of the beta function of pure $SU(N_\text{c})$ Yang-Mills theory on the number of colors? I guess
...
8
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2answers
995 views
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. ...
3
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48 views
Does the number of left handed chiral quark superfields always equal half the number of quark flavours?
In Weinberg's "The Quantum Theory of Fields Vol III" page 267 we're told that $n_f = 2N_f$. Where $n_f$ are the number of flavours and $N_f$ is the number of left chiral quark superfields (or the ...
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What is equation of quark confinement, what is the process which called confined? [closed]
Quarks confined within larger particles such as proton, what is properties of quarks confinement?
why quark confined, anyway!?
divine purpose or what physical process?
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64 views
Does quark color contribute to “spin degeneracy” for QGP calculations?
Like the title say, does quark color matter in counting contributions in a early universe plasma (QGP), as when adding up the total plasma energy density, or is it just spin? The book I have (Pathria) ...
6
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2answers
209 views
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 ...
4
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0answers
55 views
No mixing in light cone perturbation theory
In hep-ph/0609090, Triumvirate of Running Couplings in Small-x Evolution, Kovchegov et. al. calculated the running coupling correction to the Jalilian-Marian, Iancu, McLerran, Weigert, Leonidov and ...
3
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0answers
57 views
Final-state baryons in $p \bar p$ collisions in Pythia
I'm trying to simulate inclusive $\chi_c$ production in $p \bar p$ collisions at very low energies (~ 5.5 GeV) using Pythia8 event generator. Leaving aside problems bounded with applicability of ...
5
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0answers
124 views
Decomposing a Tensor Product of $SU(3)$ Representations in Irreps
Can somebody explain in a simple way why, talking about representations, $3\otimes3=3\oplus6$, $3\otimes\bar{3}=1\oplus8$ and $3\otimes3\otimes3=1\oplus8\oplus8\oplus10$?
Here $3$ and $\bar{3}$ are ...
8
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1answer
118 views
What is the Principle of Maximum Conformality?
I'm trying to understand this article about an advance in the theoretical understanding of QCD which centers on the Principal of Maximum Conformality. What is this Principle? In other words, what is ...
5
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2answers
115 views
Why is there a linear relationship between charge and isospin?
So the title basically says it all. Something that's really bothering me is the fact that the Gell-Mann Nishijima relationship can be extended to provide a linear relationship between charge and all ...
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112 views
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 $ ...
6
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1answer
247 views
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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0answers
100 views
Weak isospin and types of weak charge
My understanding is that QCD has three color charges that are conserved as a result of global SU(3) invariance. What about SU(2) weak? Does it have two types of charges? What I'm getting at is:
U(1) ...
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4answers
1k views
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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2answers
54 views
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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229 views
Lattice QCD and the 5th dimension
I was digging into Nielson-Ninomiya Theorem and doubler fermions, as well as solutions to these problems using Domain Wall Fermions and overlap lattice fermions, both of which make effective use of a ...
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1answer
110 views
What are the problems in quark-gluon plasma? [closed]
Someone (T.D.Lee?) said that quark-gluon plasma would (after 1980?) be very important to understand high-energy physics experiments.
I read some description of this new state in wikipedia ...
4
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1answer
201 views
A certain gluon scattering amplitude
I am stuck with this process of calculating the tree-level scattering amplitude of two positive helicity (+) gluons of momentum say $p_1$ and $p_2$ scattering into two gluons of negative (-) helicity ...
7
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1answer
25 views
Low-energy gluodynamics as a string
Does anyone know of a (most likely heuristic) derivation of the use of the string sigma model action to model the soft gluonic interactions between color charges? I'm familiar with the classic ...
2
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0answers
60 views
CP-violation in weak and strong sectors
There is a possible CP-violating term in the strong sector of the standard model proportional to $\theta_\text{QCD}$. In the absence of this term, the strong interactions are CP-invariant. In the ...
5
votes
2answers
363 views
Feynman rules with helicity states.
Whenever Feynman rules are stated they are always without any mention of the helicities - this I find to be very confusing. How does one introduce and account for that?
Is there an intuitive/simple ...
9
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1answer
168 views
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) = ...
8
votes
2answers
239 views
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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1answer
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Is color confinement detected?
I'm a graduate student studying QFT. I'm quite interested that is color confinement detected or proved? (both directly and indirectly) Or it is just an assumption?
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320 views
What is the experimental status of AdS/CFT, AdS/QCD, AdS/CMT, etc?
What experiments have challenged or supported AdS/QCD, AdS/CMT, etc? What experiments should we look forward to do this?
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1answer
130 views
SU(3) gauge invariance in QCD
In QCD, the gauge-invariant lagrangian under the trasformation
$ \psi \to \psi' = e^{ig T^a \theta^a(x)} \psi$
is written as:
$\mathcal{L} = \bar{\psi}(i\gamma^\mu D_\mu - m)\psi - ...
7
votes
1answer
49 views
Why isn't there heterotic holographic QCD?
I think the question speaks for itself... Top-down holographic QCD, like Sakai-Sugimoto, always involves the Type II string. There are one or two papers on hQCD using the Type 0 string. But I can't ...
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2answers
103 views
Is there something like Hawking radiation that makes protons emit component quarks?
If Hawking radiation can escape from black holes, could quarks perhaps become separated from protons despite it being "impossible" for that to happen?
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2answers
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What constitutes a 'reliable' instanton calculation?
In Modern Supersymmetry, John Terning, on pgs 151, and 153 performs a so called 'reliable' instanton calculation when dealing with the ADS superpotential 'since the gauge group is completely broken'. ...
5
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1answer
107 views
Some questions about the BCFW reduction
I am trying to give a fast sketch of what the BCFW reduction does and embed within it some questions at the steps which I don't seem to understand clearly. The first bullet point is sort of a very ...
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1answer
67 views
Colour decomposition of $n-$gluon tree amplitude
I have here a $SU(N_c)$ Yang-Mill's theory and let the index $i$, label the $n$-gluons, and $\{k_i, \lambda_i, a_i\}$ be its momenta, helicity and colour index and $\cal{A}_n^{tree/1-loop}(\{k_i, ...
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1answer
115 views
In a gauge theory, are two states related by a global phase transformation identified?
In a gauge theory (non-abelian for this question), I am told that two states $|\psi\rangle$ and $|\phi\rangle$ are to be identified if they are related by a gauge transformation $U(x)$
...
2
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0answers
54 views
Cancellation of the quadratic divergence in QCD
I am currently reading about QCD in QFT-Peskin&Schroeder. When calculating 1-loop diagrams for QCD and using dimensional regularization, the 3-vertex boson loop, 4-vertex boson loop and ghost loop ...
4
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1answer
53 views
Phase diagram of simplified QCD
Consider QCD with a single generation of massless quarks (u, d). This is probably the simplest variant of QCD which bears some relation to the real world. The theory has the following exact global ...
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1answer
41 views
What does Gribov's last paper tell about coloured states?
In the first days of July 1997, after a long driving effort, crossing all of Europe to come to a meeting in Peñiscola, Vladimir Gribov fell fatally sick and he passed away one month later. His paper ...
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1answer
106 views
Difference between Quark-Gluon Plasma and Color-Glass Condensate?
From the news. Apparently they are very similar, yet different.
5
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2answers
96 views
What maintains quark spin alignments in baryons?
What maintains quark spin alignments in baryons?
The $uud$ proton and $udd$ neutron are both spin 1/2, implying that two of their spin 1/2 quarks are always parallel and the other is always opposed.
...
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0answers
86 views
Macroscopic chromodynamics
Lately I've been reading about gamma ray lasing phenomena, and I've been wondering about the applications of this.
More concretely, the above fantastic question led me to wonder if we could somehow ...
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1answer
64 views
Is There Any Difference Between Strong Quarks Force and Strong Nuclear Force?
the spectrum of quarkonium and the comparison with positronium potential for the strong force is:
$V(r) = - \frac{4}{3} \frac{\alpha_s(r) \hbar c}{r} + kr$
(where the constant $k$ determines the ...
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76 views
Glueball mass in non-abelian Yang Mills theory
How can the glueball mass be calculated in Yang Mills theory?
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1answer
262 views
How does Delta baryon decay conserve angular momentum?
I'm a chemist so bear with me:
I understand the Delta baryons $\Delta^{+}$ and $\Delta^{0}$ to be in some sense spin (and isospin) quartet states of the proton and neutron. These can decay straight ...
3
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2answers
104 views
Playground of Forces
Why is it that the gravitational force acts on large sized objects while the strong and weak nuclear forces act at subatomic levels only? What is that stops each other to enter each others domain?
