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35 votes
Accepted

How (or when) do gluons change the color of a quark?

The idea that baryons contain three quarks is a significant oversimplification wrong. It works for some purposes, but in this case it causes way more confusion than it's worth. So you should stop ...
David Z's user avatar
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17 votes
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How many kinds of "weak charges" exist?

The problem with "weak charges" is that electroweak symmetry is spontaneously broken. Before the symmetry breaking, electroweak symmetry is described by an $SU(2)_L \times U(1)_Y$ gauge group.This ...
Bosoneando's user avatar
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15 votes

How (or when) do gluons change the color of a quark?

The model you are thinking about is really rudimentary and cannot explain the dynamics of Quantum ChromoDynamics, QCD . In this link there is a better exposition of what a proton is, within QCD. ...
anna v's user avatar
  • 234k
12 votes

How many kinds of "weak charges" exist?

You're mixing a few things up here. When you say "three" for the strong force, you're counting the number of colors of quarks, but when you guess "three" for the weak force, you're counting the number ...
knzhou's user avatar
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11 votes
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What's the difference between Quark Colors and Quark Flavours?

One is talking quantum mechanics and attributed quantum numbers to elementary particles. A simple quantum number is charge and it it assigned to quarks ( and antiquarks) as +/-1/3 or +/-2/3 as in the ...
anna v's user avatar
  • 234k
11 votes
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Fundamental reason for the color and flavor group to be the same?

It's just a coincidence. Note that the original flavour $SU(3)$ permuting up, down and strange is broken by the differing masses, and the less badly broken $SU(2)$ is the isospin group. Both are thus ...
Toffomat's user avatar
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10 votes
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Is it possible for an isolated quark to exist if it's bound to one or more non-virtual gluons to render the system color neutral?

It boils down to whether one can make a color neutral particle with a quark and some colored gluons. The rationale for the concept of color can be highlighted with the case of the omega-minus, a ...
anna v's user avatar
  • 234k
10 votes
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Why $\pi^+$ meson possible combinations of quarks is only three?

In quantum mechanics the operation of swapping the charge is called charge conjugation and this operation swaps a particle for its antiparticle. So for example if we take an electron and swap the ...
John Rennie's user avatar
9 votes

Is it possible for an isolated quark to exist if it's bound to one or more non-virtual gluons to render the system color neutral?

You can prove that this is impossible using Young tableaux. The trivial (color-neutral) representation 1 must have 3 boxes in each vertical column. (You can prove this to yourself using the hook-...
Xerxes's user avatar
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8 votes
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Is color charge quantized?

Naively, color can vary continuously between the colors according to a gauge transformation $\psi\mapsto \mathrm{e}^{\mathrm{i}\epsilon^a T^a}$ for some $\mathfrak{su}(2)$-valued object $\epsilon$, ...
ACuriousMind's user avatar
  • 126k
8 votes

Is the concept of bicolored gluons mathematically precise/meaningful? Please explain

Quarks have color charge, but antiquarks have anticolor charges. Baryons form color singlets by including three quarks with all three colors. But mesons form color singlets by having a quark and an ...
rob's user avatar
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7 votes
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In nonabelian gauge theory, does the ordinary or covariant derivative go into the statement of current conservation?

We are both right! Following AccidentalFourierTransform's suggestion in the comments, Weinberg's "Quantum Theory of Fields, Vol. II" indeed provides the relevant explanation for the discrepancy ...
ACuriousMind's user avatar
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7 votes
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Do quark charges repel one another?

Experimentally the charge distribution of protons and neutrons has been measured as a function of the radius. So the different charge content of the two nucleons does affect the distributions. As the ...
anna v's user avatar
  • 234k
7 votes

How many kinds of "weak charges" exist?

I would say two, which is pleasantly consistent with the $SU(2)$ structure of the weak force. One is the coupling strength with the $Z$ boson, and one is the weak isospin which is raised and lowered ...
rob's user avatar
  • 90.3k
7 votes

How emerges a color "magnetic" (chromomagnetic) field?

We usually do not talk about a "color magnetic field" to begin with. The strong force is treated fully relativistically from its beginning. If you want, you can define a "color electric ...
ACuriousMind's user avatar
  • 126k
7 votes

Do hadrons have color moments?

No, stable hadrons exist in color singlet states which are color neutral and have color distributions which are perfectly spherically symmetric. Thus, they have precisely zero color moment of any ...
KF Gauss's user avatar
  • 7,862
7 votes

Justifying the identification of eight gluons with the ${\bf 8}$ from ${\bf 3}\otimes{\bf 3}^*$

The ${\bf 8}$ not directely connected to the ${\bf 3}\otimes {\bf 3}^*= {\bf 1}\oplus {\bf 8}$ decomposition. Instead it comes from the fact that the gluons are labelled by the generators of the ...
mike stone's user avatar
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6 votes
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Why does confinement apply only to quarks, and not to nucleons?

If you take an electron and a proton there is a strong electromagnetic force between them because the electron has a charge of $-e$ and the proton has a charge of $+e$. However suppose you combine the ...
John Rennie's user avatar
6 votes

Confusions with gluons. How many of them are there?

1: The answer to any question along the lines of "why is this term not present in the Standard Model Lagrangian" is always the same: because it works experimentally. You could make a theory ...
SuperCiocia's user avatar
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6 votes
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Why is the $SU(4)$ group not suitable for describing color symmetry?

Simplest way? The $\Delta^{++}\sim uuu$ has to be a color singlet. It has spin 3/2, so it is flavor and spin symmetric. But fermion quarks need to be in a fully antisymmetrized state. Can you make an ...
Cosmas Zachos's user avatar
6 votes

Is it possible for an isolated quark to exist if it's bound to one or more non-virtual gluons to render the system color neutral?

No. In general, in any (exotic) hadron made of valence quarks, antiquarks and gluons, the number of quarks minus the number of antiquarks in absolute value must be divisible by 3. Then, add any number ...
Urb's user avatar
  • 2,638
6 votes
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Color vectors for antiquarks

From the above table, I can see that we need to have $F_3 \bar{r} = - \frac{1}{2} \bar{r}, \ \ F_3 \bar{g} = \frac{1}{2} \bar{g} \ \ F_3 \bar{b} = 0.$ ... What am I doing wrong? No. That's what you ...
Cosmas Zachos's user avatar
5 votes
Accepted

Why are 3 colors used in QCD?

The language of "colors" maps more-or-less straightforwardly to the actual group theory that controls the strong force: The gauge symmetry that is associated to the strong force is the unitary group ...
ACuriousMind's user avatar
  • 126k
5 votes

Why is color conserved in QCD?

What LuboŇ° has written is totally right but I also understand that it does not completely anwser to your question. By the statement "color is conserved in QCD" you probably mean that there are three U(...
Hoody's user avatar
  • 614
5 votes

Why do quarks and gluons have colour?

High energy particle collisions involve the production of a plethora of different particles. One example is $\Delta^{++}$, which consists of three up-quarks in the same spin state. Since quarks are ...
scaphys's user avatar
  • 824
5 votes
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How are quarks assigned color charge?

Colour is independent of flavour. An up quark can be red, green or blue. Since gluons also carry colour, the colour of a quark isn't fixed. When a blue quark interacts with a green quark (of whatever ...
James K's user avatar
  • 695
5 votes

What does "singlet state" mean in the context of colour charge, and do red, green and blue colours cancel?

Singlet, scalar, colorless, etc... all refer to the same thing: invariance under color transformations. That is, parameterizing your silly popular science r,b,g by a 3-valued index i, your mesons are ...
Cosmas Zachos's user avatar
5 votes

Do hadrons have color moments?

"Color moment" could mean two different things. One of them is certainly not possible, but I'm not sure about the other one: One is something like "mostly green on one side and mostly ...
Chiral Anomaly's user avatar
5 votes
Accepted

How is the colour quantum number not conserved in this process?

You have every right to be puzzled. To the left of the fermion loop you have a color octet, and to the right a color singlet. What is actually meant by strictly nonsense diagrams such as this is that ...
Cosmas Zachos's user avatar
5 votes

Can a complex wave function just be seen as two real functions describing a particle and antiparticle state?

No. For example if you take a free electron moving with momentum $\vec{k}$, then its wavefunction is complex $\psi \sim e^{i \vec{k}\cdot \vec{x}}$. If for some reason you had a prejudice that caused ...
Andrew's user avatar
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