# Tag Info

34

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 thinking of baryons as groups of three quarks and start thinking of them as excitations in quantum fields - and in particular, excitations in all the quantum fields ...

26

This is covered by a few existing answers (see for example About free quarks and confinement) though surprisingly it doesn't appear that anyone has asked this exact question before. Anyhow, the answer is that the colour force is mediated by particles called gluons just as the electromagnetic force is mediated by photons. The difference is that while photons ...

16

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 amounts to three charges: weak hypercharge $Y$ for $U(1)_Y$ and weak isospin (total isospin $T$ and third component $T_3$) for the $SU(2)_L$. Some examples of ...

15

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. You may have heard that a proton is made from three quarks. Indeed here are several pages that say so. This is a lie — a white lie, but a big one. In fact ...

14

Color charge in the sense of "being blue, red, green" is not a quantum mechanical observable because the $\mathrm{SU}(3)$ gauge transformations mix the colors. This means it is meaningless to say "We have a blue particle", because we can perform a gauge transformation and then we "have a red particle". Since physical descriptions related by gauge ...

10

Global invariance under $SU(N)$ is equivalent to the conservation of $N^2-1$ charges – these charges are nothing else than the generators of the Lie algebra ${\mathfrak su}(N)$ that mix some components of $SU(N)$ multiplets with other components of the same multiplets. These charges don't commute with each other in general. Instead, their commutators are ...

10

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 of force carriers. These are two different things. For example, if you counted the number of gluons (the force carriers for the strong force), you'd get ...

9

You are correct to point out that there's no symmetry that forbids a state with isospin 3/2 and spin 1/2; in the nomenclature, this is also called a $\Delta$ resonance. The Particle Data Group lists two such particles, with mass 1620 MeV and 1910 MeV. They exist, but they are heavier than the spin-3/2 $\Delta$ at 1232 MeV. The reason why is isospin, ...

8

I asked this question a few weeks ago and was dissatisfied with most of the answers I found on the internet, so I eventually managed to procure a copy of Griffiths' excellent text on elementary particles (really, all of his texts are excellent) which includes a section exactly answering my question with what I was looking for. I decided then to answer it ...

8

Force is a classical concept. Even classically not all forces decrease with distance, as the forces acting on a spring show, which increase with distance. I do not see the connection with the three dimensions of space and the functional form of the potential of a force. The strong force is modeled with the exchange of gluons which are depicted in Feynman ...

8

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 table Charge is connected with the electromagnetic force. Flavor is assigned as a quantum number to each quark, and it is connected with the weak ...

7

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 by the $W^\pm$.

6

Statements like "The mass of the electron includes the mass of the electric field it generates" have to be taken veeery carefully. In quantum field theory we can calculate the backreaction of the electric field our electron creates on itself, which is the mass shift I think you talk about. For these calculations to make sense, we need to make use of ...

6

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 other answer states this is the regime where only quantum chromodynamics models can attempt to describe the wavefunctions of the quarks within the nucleons, ...

5

The color language is not really well-suited to understand why there are eight gluons. Here's why, however: The gluon field transforms in the adjoint representation of the color gauge group $\mathrm{SU}(3)$. The adjoint representation is a representation on the vector space of the generators of $\mathrm{SU}(3)$, the Lie algebra $\mathfrak{su}(3)$. An ...

5

In most cases, Reflected Sunlight determines the color of the sea. Tropical islands have turquoise seas because the water absorbs blue and reflects the red in the sunlight. If you go diving, you will notice that the water become bluer the deeper you go. This is because only the blue light waves are able to penetrate deep waters. Sometimes when there is a ...

5

The color charges are paired (color with anticolor), but there's no gauge invariant meaning to the identification of the color (RGB). And due to QM, the quark states are a superposition over all the colors (and antiquarks over the anticolors). The wikipedia page is pretty clear: http://en.wikipedia.org/wiki/Color_charge. As it notes, you also can't ...

5

No, confinement means that such a state cannot exist, or more precisely, it cannot have a finite energy/mass. If such a colored state had a finite energy, it would mean that far enough from the colored particle, the quantum fields very closely approach the vacuum state. But if that's so, you could always combine two such objects of opposite colors. The total ...

5

What you 'know' is misleading. The object will appear black to our eyes if it absorbs all photons in the visible portion of the spectrum, and further is not emitting significant numbers of photons (due to the Planck/blackbody radation laws, it's always emitting a few visible photons). An object is opaque because of scattering, or because of absorption. ...

5

We usually do not talk about a "color electric field" to begin with. The strong force is treated fully relativistically from its beginning. If you want, you can define a "color electric field" and "color magnetic field" from the strong field strength tensor $G_{\mu\nu}^a$ in exact analogy to electromagnetism as $E^a_i := G_{0 i}^a$ and $B^a_i := \epsilon_{... 5 If you take an electron and a proton there is a strong electromagnetic force between them because the electron has a charge of$-e$and the proton has a charge of$+e$. However suppose you combine the electron and proton into a hydrogen atom. The hydrogen atom has a net charge of zero so there is no strong electromagnetic force between two hydrogen atoms. ... 4 Correction$\Delta(1620) 1/2^-$is actually pretty well settled. (Thanks to rob.) Original Answer Actually, the Pauli exclusion principle can explain why there are no (uuu,ddd,sss) spin-1/2 ground states. In baryons, quarks have four degree of freedom: orbital, spin, flavor, color. As you already know, the quarks' total wave functions should be anti-... 4 What Luboš has written is totally right but I also understand that it does not completely anwser to your question. By the statement "color is conserved in QCD" you probably mean that there are three U(1) symmetries corresponding to red, green and blue color. You know it because you have seen a lot of QCD pictures such as this one where the colored lines ... 4 None of them. All of them (and the rho and eta, too). It doesn't matter and doesn't have a unique answer as the meson exchange idea is an effective theory. If you insist on trying to write down rules, then charged pion exchanges can only occur between protons and neutrons resulting in the nucleons exchanging types. On the other hand neutral pions can be ... 4 The colour charge of quantum chromodynamics is, as far as we can tell, not experimentally measurable, because of quark confinement. More specifically, quantum chromodynamical systems are always colour neutral. Quarks do have a color charge, but they are always observed in groups of two (color + anticolor) or three (red + green + blue) for which the total ... 4 After reading through the corresponding chapters in several books I think I'm now able to give a "semi-satisfactory" answer to my own question (and to understand Lubos first comment ;) ). I write semi-satisfactory, because I hope someone with a deeper understanding of these topics will give a better answer. My explanation is still a little bit heuristic, ... 4 When the gauge theory is quantized in the proper way, you cannot even meaningfully talk about the action of an$\mathrm{SU}(3)$transformation on the space of states because the quantization of a gauge theory essentially requires you to quotient out the gauge transformations (all of them, including the global ones), so that they are "do nothing" ... 4 We are both right! Following AccidentalFourierTransform's suggestion in the comments, Weinberg's "Quantum Theory of Fields, Vol. II" indeed provides the relevant explanation for the discrepancy between the ordinary conservation of the Noether current in a gauge theory by applying Noether's theorem to the global version of the gauged symmetry and the ... 4 High energy particle collisions involve the production of a plethora of different particles. One example is$\Delta^{++}\$, which consists of three up-quarks in the same spin state. Since quarks are fermions, Pauli's exclusion principle seems to be violated, unless there is another quantum number (we call it colour) in which the three up-quarks are different ...

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