Questions tagged [quantum-chromodynamics]
Quantum-ChromoDynamics (QCD) is the quantum field theory believed to describe the strong nuclear force.
1,102
questions
13
votes
2
answers
3k
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 ...
- 131
13
votes
1
answer
480
views
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 ...
- 6,601
13
votes
1
answer
1k
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) = -\frac{...
- 4,619
13
votes
1
answer
202
views
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 ...
- 8,995
12
votes
3
answers
4k
views
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 ...
- 131
12
votes
4
answers
4k
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 ...
- 393
12
votes
2
answers
2k
views
$SU(3)$ vs $SO(3)$ color gauge
I have kind of a dumb question: what would happen if the color gauge group is $SO(3)$ instead of $SU(3)$, assuming there are still colors and physical states are still color singlets? Will we e.g. get ...
- 175
12
votes
1
answer
1k
views
How many colors really are there in QCD?
In abelian gauge theory (electrodynamics), the matter fields transform like (please correct me if I am wrong)
$$
|\psi\rangle\rightarrow e^{in\theta(x)}|\psi\rangle\tag{1}
$$
under a gauge ...
- 133
12
votes
3
answers
4k
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 ...
- 7,398
12
votes
1
answer
2k
views
Non-existence of gluon singlets: Any recent theoretical progress?
An unanswered question from last year (2012) on gluon singlets asked whether there is any theoretical explanation for the experimental absence of the ninth or colorless (singlet) gluon. This is the ...
- 22.1k
12
votes
1
answer
2k
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
$$\mu\frac{dg_\text{YM}}{d\mu}=-\beta_0N_\text{c}g_\text{YM}^3-\beta_1N_\text{c}^...
- 133
12
votes
1
answer
397
views
Why does strong interaction increase with distance?
I read numerous times that strong interaction increases with distance. But how can one actually derive the force-distance relation from the lagrangian (quark field + gluon field + gauge coupling)? ...
- 644
11
votes
1
answer
5k
views
What is meant by “factorization scale factor” in QCD calculations?
What is “factorization scale factor” $\mu_F$ and how is it different from “renormalization scale factor” $\mu_R$ in QCD calculations?
When are both of them equal, such that $\mu_R=\mu_F=\mu_0=\sqrt{m^...
- 573
11
votes
1
answer
2k
views
Do color-neutral gluons exist?
If I'm correct a quark can change color by emitting a gluon. For example a blue up quark $u_b$ can change into a red up quark by emitting a gluon:
$$u_b \longrightarrow u_r + g_{b\overline{r}}$$
(Here,...
- 392
11
votes
1
answer
3k
views
Polarization Sums in QCD for the calculation of parton model splitting functions
Before i state the actual problem, here's a premise. In the case of a Spin 1 massive particle it's possible to demonstrate that $$\sum_{\lambda=0,\pm1}\epsilon_{\lambda}^{* \ \mu}\epsilon_{\lambda}^{\...
- 2,263