All Questions
Tagged with quantum-chromodynamics standard-model
155
questions
26
votes
3
answers
5k
views
How (or when) do gluons change the color of a quark?
I know a baryon is only stable when it contains a quark of each color. And as far as I know, the gluon essentially changes the color of a quark and moves onto the next, and this is what holds the ...
- 508
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?
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
14
votes
5
answers
2k
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Why aren't all quarks clumped together in one giant hadron?
As far as I am aware, the strong interaction is attractive only, and its carrier, the gluon, is massless meaning it has unlimited range. If this is the case, how come we only observe quarks in pairs ...
- 451
14
votes
2
answers
2k
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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?
- 558
14
votes
3
answers
2k
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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 ...
- 17.1k
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
11
votes
1
answer
2k
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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,...
- 392
11
votes
0
answers
445
views
Is the QCD Lagrangian without a $\theta$-term invariant under large gauge transformations?
In his book "Quantum field theory", Kerson Huang states that we need to add the term $$\frac{i\theta}{32\pi^2}G_{\mu\nu}^a \tilde{G}_{\mu\nu}^a$$ to the Lagrangian, to make it invariant under large ...
- 10.1k
9
votes
3
answers
2k
views
Why are gluons massless as their range is finite?
The range of electromagnetic waves and gravitational force is infinity and the particles exchanged during these interactions are photons and gravitons respectively. Both are massless following the ...
9
votes
2
answers
286
views
Why are protons/neutrons spin-$\frac{1}{2}$ when they are made up of three valence quarks (also spin-$\frac{1}{2}$)?
If we take the composite angular momentum of the three valence quarks, we should have a proton/neutron spin angular momentum of,
$$S=\{ \frac{1}{2},\frac{3}{2} \}$$
using the general rule for adding ...
- 941
9
votes
2
answers
2k
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Why is $SU(3)$ chosen as the gauge group in QCD?
Why is $SU(3)$ chosen as the gauge group. Why not $U(3)$? Why does it even have to be unitary?
- 2,147
9
votes
1
answer
2k
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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 ...
- 7,398
9
votes
1
answer
807
views
Spontaneous symmetry breaking of anomalous global abelian symmetries and $U(1)$ of QCD
Let me restate the $U(1)$ problem of QCD:
Let us forget about the $s$ quark, and consider the $u$ and $d$ massless. This is
a good approximation since $m_{u,d} \ll \Lambda_{QCD}$. Then $\mathscr{...
- 498
9
votes
1
answer
386
views
Are mesons color polarized?
The binding of quarks in mesons baffles me. It's an Occam's Razor thing.
Since a meson is a colorless, the simplest way to bind its two quarks together is to use a $U(1)$ Cartan subalgebra of $SU(3)$. ...
- 22.1k