Quantum-ChromoDynamics (QCD) is the quantum field theory believed to describe the strong nuclear force.
14
votes
5answers
4k views
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 ...
11
votes
0answers
126 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 $ ...
10
votes
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 ...
9
votes
4answers
436 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 ...
9
votes
3answers
231 views
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 ...
9
votes
1answer
314 views
QCD phase diagram in the large N limit
I am sending a couple of questions which seem a bit more specific than others on this site, partially to probe if there is a point in doing so. Not sure what is the range of expertise here, and no way ...
9
votes
1answer
174 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
1k 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. ...
8
votes
1answer
188 views
Why is it common to plot $xG(x,Q^2)$ and not simply $G(x,Q^2)$?
I'm trying to understand the modern description of high-energy scattering processes involving hadrons in the initial states. The phenomenological parton distributions functions play a central role, ...
8
votes
2answers
861 views
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 ...
8
votes
2answers
249 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, ...
8
votes
1answer
119 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 ...
7
votes
2answers
334 views
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 ...
7
votes
1answer
50 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 ...
7
votes
2answers
327 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?
7
votes
1answer
26 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 ...
6
votes
4answers
612 views
What's the distinctions between Yang-Mills theory and QCD?
So Yang-Mills theory is a non-abelian gauge theory, and we used a lot in QCD calculation.
But what are the distinctions between Yang-Mills theory and QCD?
And distinctions between supersymmetric ...
6
votes
2answers
59 views
Neutral pions and chromodynamics
$\pi^0$ particles are either up-antiup or down-antidown (or strange-antistrange?) They must be opposite colors to preserve neutrality. Why don't the opposite quarks annihilate?
6
votes
1answer
252 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 ...
6
votes
1answer
266 views
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 ...
6
votes
1answer
90 views
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?
6
votes
1answer
204 views
Reference for the ${\cal N}=3$ Chern-Simons Lagrangian at general $N_c$, $N_f$
I was wondering if someone could give me a reference where someone has explicitly written the Lagrangian for ${\cal N}=3$ $SU(N_c)$ Chern-Simons theory coupled to $N_f$ fundamental hypermultiplets.
...
6
votes
1answer
1k views
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 ...
6
votes
1answer
291 views
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?
...
6
votes
1answer
91 views
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 ...
6
votes
1answer
267 views
Why are the Higgs phase and the confinement phase identical in Yang-Mills-Higgs systems?
If we couple a Yang-Mills theory with a Higgs field and some quarks in the fundamental representation, we can have a Higgs phase and a confining phase. However, they are indistinguishable. The Wilson ...
6
votes
0answers
100 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
...
6
votes
0answers
65 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
votes
2answers
214 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 ...
5
votes
2answers
218 views
What happens to a single quark in lattice QCD simulations?
I understand that if a pair quark/antiquark, out of the vacuum, tries to separate then the energy increases, another pair is produced, and we finish with two mesons or generically two hadron jets. But ...
5
votes
2answers
103 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.
...
5
votes
4answers
281 views
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?
5
votes
2answers
372 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 ...
5
votes
1answer
189 views
Why are the even and odd Regge trajectories degenerate?
This is an old classic which I don't think ever got a clear answer. The Gribov-Froissart projection that gives the relativistic version of Regge trajectories treats even angular momentum differently ...
5
votes
2answers
119 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 ...
5
votes
2answers
704 views
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 ...
5
votes
1answer
113 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 ...
5
votes
1answer
99 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 ...
5
votes
1answer
43 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 ...
5
votes
1answer
162 views
Why are Wilson lines the effective degrees of freedom for high energy partons?
I've read that Wilson lines are the effective degrees of freedom for high-energy partons, when you consider collinear gluon emission. But I'm struggling to find a readable account of this topic. Does ...
5
votes
0answers
161 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 ...
4
votes
1answer
91 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" ...
4
votes
2answers
58 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?
4
votes
1answer
54 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 ...
4
votes
1answer
86 views
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 ...
4
votes
2answers
97 views
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'. ...
4
votes
1answer
299 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 ...
4
votes
1answer
299 views
What's jet multiplicity?
That's a basic thing, but, surprisingly, it is very difficult to find concise explanation of:
What is the definition of jet multiplicity?
Why is it interesting?
4
votes
1answer
205 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 ...
4
votes
1answer
118 views
Is the color gauge group spontaneously broken in QCD by the Savvidy vacuum?
One model for confinement in quantum chromodynamics is the Savvidy vacuum. This is a spontaneous symmetry breaking of color gauge symmetry by the gauge fields. The vacuum is divided into Savvidy ...
