All Questions
Tagged with quantum-chromodynamics quantum-field-theory
60
questions
19
votes
3
answers
5k
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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?
35
votes
2
answers
5k
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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 ...
- 5,569
19
votes
4
answers
2k
views
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, ...
- 13.1k
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
22
votes
4
answers
14k
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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 ...
- 1,701
19
votes
2
answers
3k
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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 ...
- 1,707
16
votes
1
answer
2k
views
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 ...
- 5,228
13
votes
2
answers
1k
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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, ...
- 457
6
votes
2
answers
445
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Measuring and calculating free quark masses
Particle data book contains masses of the free quarks. I wonder, how do experimentalists determine the masses of the free quarks even though they are trapped inside hadrons (except perhaps in quark-...
- 26.8k
5
votes
2
answers
530
views
What is relevant for Yang-Mills theory below $d = 4$?
There are two ways to write the Lagrangian for Yang-Mills, differing by the scaling of the Yang-Mills field. Fancy theorists tend to write
$$S = \int d^dx \, \frac{1}{4e^2} \, \text{tr}(F^2)$$
while ...
- 103k
4
votes
1
answer
331
views
What was above and below the QCD chiral symmetry breaking temperature?
Above a critical temperature in the Universe, there was probably a phase of unbroken approximate QCD chiral symmetry. Mathematically, the symmetry breaking is triggered when the operators $\bar{\psi}{\...
- 11.8k
4
votes
1
answer
547
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Glueball mass in non-abelian Yang Mills theory
How can the glueball mass be calculated in Yang-Mills theory?
- 327
1
vote
1
answer
265
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Why there is only $t$-channel for $u+\bar d\rightarrow u+\bar d$ scattering?
I am reading Griffith's textbook on particle physics about the $u+\bar d\rightarrow u+\bar d$ scattering for QCD. On page 285, it says there is only one possible channel ($t$-channel):
Why there are ...
- 1,783
26
votes
1
answer
2k
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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
...
- 2,566
19
votes
1
answer
2k
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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-...
- 103k