# Questions about the gluons, their color and anti-color charges and charge of quarks

1. Mar 12, 2012

### WilliamJ

I have been learning some stuff off of youtube, but I did not get a full enough understanding of how quarks have colors, and anti-quarks have anti-colors and how the gluons have to be colors and anti-colors and how it all works. Some of it doesn't make any sense.
The first question is about a red quark absorbing an red-antiblue quark and turning blue. I don't see how a gluon of red-anti-blue quark makes the red quark suddenly turn blue, not anti-blue, but I have heard that the quarks can only have a regular color and an anti-quark are the only quarks that can have an anti-color. "As a quark absorbs a gluon, it keeps its status as a quark or anti-quark. So what does that mean?

Also, if a blue quark ineracts with another quark of a different color say red, the qluon is transfered and the quarks swap their colors. This means that when two quarks exchange a gluon, they switch their colors, but what about when they are exchanging gluons that are like the red and and antiblue quarks? I don't even know how to address the question. And the question lingers, what about quarks and anti-quarks and gluons that are colors and ant-colors and how they relate?
The whole thing of quarks and anti-quarks exchanging gluons which have a color and an anti-color and how only quarks can exchange colors with other quarks, and anti-quarks exchange anti-colors with anti-colors, by gluons which have a color and an anti-color makes no sense to me.
Sorry if this sounds so confusing, but the reason is is that I am confused about it.
If there is anyone out there who would answer my question in a way that clears up the confusion, I would appreciate it.
Sincerely,
WilliamJ

2. Mar 12, 2012

### Bill_K

Re: Questions about the gluons, their color and anti-color charges and charge of quar

Here's a diagram from Carl Brannen's site that explains it. Looks complicated at first, but shows it better than any other diagram I can find. Time goes from left to right. Color is conserved, which means that if you pick a red line say, you can follow it all the way through the diagram and come out the other side. Quarks are represented by single lines and have one color. If the arrow points to the right it's a quark, if it points to the left it's an anti-quark. Gluons are represented by wavy lines and have two colors. If you draw arrows on the colored lines in a gluon you'll see that one arrow points to the right (a "color") and the other arrow points to the left (an "anti-color")

At the left we initially have three quarks, red green and blue. The first thing that happens is that that the green quark emits a green anti-red gluon. The green anti-red gluon strikes a red quark and turns it into a green quark. And so on!

3. Mar 12, 2012

Re: Questions about the gluons, their color and anti-color charges and charge of quar

You're right, this particular scenario cannot occur. A red quark could emit a red-antiblue gluon to become a blue quark - sort of like "+red - (+red + -blue) = +blue" - but for a red quark to become blue by absorbing a gluon the latter must be blue-antired.
It means that, apart from its colour, all of the other charges and internal quantum numbers of the quark remain the same. Eg if its flavour is up it remains up. And, in particular, gluons cannot change quarks into antiquarks, just as photons cannot change (eg) electrons into positrons.
This is one and the same thing - quarks change colours by exchanging gluons. The word "exchanging" as used here is not intended to imply that each interaction involves sending gluons in both direction. Only one gluon need be transferred in each interaction.

Gluons can be also exchanged between a quark and an anti-quark. For example a green quark can emit a green-antiblue gluon (thus becoming blue) and the gluon can then be absorbed by an antigreen antiquark, turning the latter antiblue. This is a typical interaction within a meson - notice how the anti-quark both starts and ends having the anti- of the quark's colour.
The simple picture of red-antiblue type gluons unfortunately isn't the whole story.

In fact, in addition to the six gluons of the colour - anti-other-colour combinations, there are two further ones which can loosely be described as "neutral". Their colour combinations can be described mathematically as

$1\ /\ \sqrt{2}\ (r\bar{r}\ -\ g\bar{g})\ \ \ \$and$\ \ \ \ 1\ /\ \sqrt{6}\ (r\bar{r}\ +\ g\bar{g}\ -\ 2b\bar{b})$​
These can be exchanged between any quarks or anti-quarks that have at least one of their (anti-)colours.

Even the above, however, is still a very oversimplified picture. To really understand this you really do have to have at least a basic understanding of the SU(3) symmetry group. There is quite a good introduction to gluon types and exchanges in "Elementary Particle Physics" by Alessandro Bettini.

Last edited: Mar 12, 2012
4. Mar 20, 2012

### WilliamJ

Re: Questions about the gluons, their color and anti-color charges and charge of quar

These are the questions I have about your reply. I noticed that at the end of the reply some equations were written that I did not have the foggiest idea of what to make of them. You are much more advanced, so if you reply, make sure to keep it really simple. I have trouble understanding if only anti-quarks can have anti-colors and if only quarks can have colors. I’m also confused with knowing when you know which color/anti-color gluons are used in order to change colors.

Red quarks emit a red-antiblue gluon and becomes a blue quark.
“+red – (+red + -blue) = +blue”
Red quarks absorb a blue-antired quark and becomes a blue quark.
What is the equation here? And are their equations like this for all of the quarks changing colors? And am I right about what you are saying so far with emission and absorbtion of gluons?

Ok, I see that the gluons can’t change any of the properties of quarks besides its color charge. Also, and especially important is that a gluon cannot change a quarks be turned into an anti-quarks. I know that particles and antiparticles can pop up, created in pairs, like when a gama ray with at least 1.02 MeV can create electrons and positrons. This is called pair production. Also the gama rays, when in pair production that is energy being turned into mass (or is it matter, what is the diff?),. The opposite is when anti-particle pairs at rest the annihilate one another they produce (I haven’t checked over my notes in a while) particles of pure energy like photons and energy.
In any case, that was a detour.
Now I know that only one gluon is exchanged at a time. However I am not sure how to deal with this twist that is mesons that are composed of quarks and antiquarks. I’m still unsure most of the details about how colors and their exchanges from emissions and absorbtions of gluons work. I was already interested in figuring out more about how colors work with baryons and mesons, but I need more clarity about quarks, antiquarks, color and anti-color changes work and how colors change. So it’s going to take a while to get this figured out in order to understand how I can apply it to the hadrons’ quarks.
If you reply, please break it down really, really simply. This is a complicated subject and I am having a hard time figuring it out.

5. Mar 20, 2012

### WilliamJ

Re: Questions about the gluons, their color and anti-color charges and charge of quar

I know that this is a lot to ask, but I am having an unusually difficult time learning about the strong force, and I see that you are very knowledgeable about this topic, enough to help me make sense of it all. I really need some help with this so if you could help explain it to me I’d appreciate it.

I do not understand how this diagram works. I don’t even really have words to describe the questions I have about it. Is this diagram a standard diagram of how all quarks exchange their colors? I see that the straight lines represent quarks with their different colors, and I can see how the gluons have color/anti-color combinations that exchange the colors between quarks in a way, but I still don’t completely get it. I see that a line (quarks?) that point to the right is a quark, and that a line that points to the left is called an anti-quark. However, the only line that points left in this diagram is the green on the bottom right of the diagram which is part of the circle with the green and red quarks have their colors going in opposite directions. The thing is, is that I have heard from youtube.com that (as far as I can remember) only anti-quarks can have anti-colors and the part of the diagram I have just talked about right before this sentence states basically (when looking at the diagram) that there is one possible anti-particle in this diagram. Yet again, is this diagram a standard illustration? It doesn’t seem so because of the fact that there is only one anti-quark in this diagram as far as I can tell. Also from the diagram and from what you have said is that “Color is conserved, which means that if you pick a red line say, you can follow it all the way through the diagram and come out the other side.” I don’t see how this is, but then again, I am having a hard time making sense of this. I understand that the gluons come in color/anti-color combinations, and that has to do with the changes of colors between the different colored quarks.

“You wrote this:”At the left we initially have three quarks, red green and blue. The first thing that happens is that that the green quark emits a green anti-red gluon. The green anti-red gluon strikes a red quark and turns it into a green quark. And so on!”

Then you can have a red quark emit a green anti-red gluon, this green anti-red gluon is then absorbed by a green quark and turn it red, but how do you deal with the place where there is a green anti-quark in the diagram.

Also, how do you follow each one of the colors, one at a time through the diagram, from the left to the right, how is color charge conserved, and in what way does the conservation of color charge resemble the conservation of electric charge?

6. Mar 21, 2012

### Bill_K

Re: Questions about the gluons, their color and anti-color charges and charge of quar

WilliamJ, There's no special significance to the diagram that Brannen drew. It's just a random sample of what might happen, consistent with the rules. Three quarks are incoming on the left, three quarks are outgoing on the right. So it depicts a proton that comes in, bounces around awhile, and then continues on its way. It would be more interesting to show a proton emitting a pion or something, but I couldn't find any like that.
This is the easy part. Place your pencil at the beginning of the red line and follow it. It just keeps on going. It never dead ends, and it never forks into two red lines. This illustrates color charge conservation, showing that "red color" is never created or destroyed.

You're right, there's only one anti-quark in the picture, the green one at the loop marked A. Green moving to the left is called 'anti-green'. Inside each of the gluons there is one line moving to the right (the color) and another one moving to the left (the anti-color).

7. Mar 21, 2012

Re: Questions about the gluons, their color and anti-color charges and charge of quar

Unfortunately it just isn't possible to answer all your questions in layman's language, but here are a couple of things.
“+red + (-red + +blue) = +blue”

As you may have guessed, a red-antiblue gluon is just the antiparticle of the blue-antired one.
Yes, and gluons can do this too. For example our red-antigreen gluon can produce a red quark together with a green antiquark. But both must be of the same flavour, eg up and anti-up, down and anti-down, etc. The quark must take the gluon's colour, and the anti-quark its anti-colour.

A further twist is that, unlike photons which carry no electric charge, gluons carry colour charges so can also interact with other gluons. For example a red-antiblue gluon can emit a red-antigreen gluon and in doing so become green-antiblue. This makes a huge difference to the physical phenomena that result from the strong force, and is ultimately the reason why quarks and anti-quarks are always confined within hadrons, and can never be observed alone.

Last edited: Mar 21, 2012
8. Mar 24, 2012

### WilliamJ

Re: Questions about the gluons, their color and anti-color charges and charge of quar

Gluons come in antiparticles. For example, red-antiblue gluon in the antiparticle of the blue anti-red.
Red quarks emit a red-antiblue gluon and becomes a blue quark.
1) +red - (+red + -blue) = +blue
Red quarks absorb a blue-antired quark and becomes a blue quark
2) +red + ( -red + +blue) = +blue
What happens to the opposite of 1), where the red quark emits the gluon, when the scenario turns into absorption.
What happens to the opposite of 2), where the red quark absorbs the gluon, when the scenario is turned into emission.
Are in this example between 1 and 2, do gluons and anti-gluons such the same even if the emission or absorption change I mentioned

I see that a gluon of a color-anticolor can cause a quark and its antiquark, like up and anti-up and down and anti-down, etc. The antiquarks get the anticolors and the regular quark gets the regular color. Why is this set up w/only a quark and its antiquark? Is this a particular category of mesons? Are quarks and anti-quarks like u and anti-up an example of flavors, not like up and down?
I don’t really understand how gluons interact and change the gluon colors. Which would be very beneficial to me to see examples of emission/absorption, basically many examples of the color changes by the gluons. If I actually see examples of how the quarks change colors and gluon charges, it will finally sink in. I hope this isn’t too redundant. I should already know the answer by how many replies I’ve gotten. Most things I get really easily about particle physics that I have been learning so about for so long, the whole color charge, etc. is a particularly difficult to master subject.
Sincerely,
WilliamJ

9. Mar 27, 2012

Re: Questions about the gluons, their color and anti-color charges and charge of quar

If by "the opposite of 1)" you mean a red quark absorbing a red-antiblue gluon, then, as I mentioned originally, it can't. That gluon can be absorbed only by (a) a blue quark (which then becomes red), (b) an antired antiquark, which then becomes antiblue, or (c) another gluon.
Very similar to why a photon can pair-produce e+e-, but not e+e+, e-e- or μ+e-. This is down to the conservation of (a) charges, and (b) baryon and lepton type numbers (respectively). Quarks have electric charges, but gluons none, so the total electric charge of the daughter particles must also be zero. An antiquark of course has the opposite electric charge to that of its quark counterpart (same flavour). All quarks also carry a 'flavour' number of ±1* (eg "up-ness", charm, strangeness, etc), antiquarks the opposite, and gluons again zero. (Electrons have electron number +1, positrons -1, and photons 0.) These quantum numbers are also conserved in all strong and electromagnetic interactions.

* by convention, the sign of a quark's flavour number is the same as that of its electric charge, and the corresponding antiquark's is the opposite. So for example a s quark has strangeness -1, and its antiquark +1, whereas c has charm +1, etc.
Mesons are composite particles formed from one valence quark and a valence antiquark. These then surround themselves with a cloud of virtual gluons and other quark/antiquark pairs.

The six quark flavours are up, down, charm, strange, top and bottom. Each has its corresponding anti. The flavours are commonly abbreviated to their initial letters, such as u or d. u, c and t have electric charge +2/3 and may collectively be referred to as 'up-type'. d, s and b have -1/3 and are sometimes called 'down-type'.

In a meson, the valence quark and antiquark must have opposite colours, but need not be of matching flavours. For example a π+ meson is an up with an anti-down - note the charges are +2/3 -(-1/3) = +1. It also has upness of +1 and downness of +1. (Historically, upness and downness were combined into another quantum number called isospin, whose numerical value is half the total upness and downness.)
Yes, unfortunately you do need to study the maths of SU(3) to really understand this deeply.

10. May 1, 2012

### WilliamJ

Re: Questions about the gluons, their color and anti-color charges and charge of quar

When quarks change colors would the following be an example of how all quarks change color charge?

Red Turns Into Blue:
red - (red + -blue) = blue
blue + (red + -blue) = red

Blue Turning Into Red:
blue - (blue + -red) = red
red + (blue + -red) = blue

11. May 1, 2012

### WilliamJ

Re: Questions about the gluons, their color and anti-color charges and charge of quar

Am I completely wrong? I need to know exactly how to illustrate the color changes between quarks, with the addition and subtraction, like the absorption and emission, the example given. Do I have them in the wrong pairs, like may be all of the emissions and absorptions of red need to be in one group, and all of the emissions and absroptions of blue need to be in one group?. I need to know whether I did it right or if I did it wrong, and if I did it wrong, how is it supposed to be denoted to describe the color exchange between quarks?

12. May 1, 2012

### WilliamJ

Re: Questions about the gluons, their color and anti-color charges and charge of quar

I think I understand how a red quark can absorb a gluon turning it blue, and how a red quark can emit an anti-gluon and turn blue, but that in both situations there is only a red quark changing into blue, not swapping out colors between two quarks.
For example:
red - (red +-blue) = blue
red + (blue -red) = blue
These are the two original red colored quarks which turn blue. But how can you combine different emissions and absorptions like denoted here, to make it show how two quarks can exchange colors?
I'm still needing a bit of clarification.

13. May 1, 2012

### Staff: Mentor

Re: Questions about the gluons, their color and anti-color charges and charge of quar

In every reaction, the sum "red minus antired" (and similar for blue and green) is conserved, which means that it is the same before and after the reaction.

Therefore, a red quark can emit a (red, anti-blue) gluon and become blue:
Before the reaction, we have 1 red (quark) and 0 blue.
After the reaction, we have 1 red (in gluon) and 1-1=0 blue (blue in quark, anti-blue in gluon).

This is another way to express "red = (red-blue)+blue", where (red-blue) is the gluon. The left side has the initial state (the red quark) and the right side has the final state (gluon+quark).

Now, this (red, antiblue) gluon can be absorbed by a blue quark, which becomes red:
"blue + (red-blue) = red"

This image has this process: It is below the "C". The horizontal axis is time.

Of course, this is just a model, and should not be taken too seriously. It is not meaningful to say "at time x, this quark emitted a gluon which hit another quark later".

14. May 21, 2012

### WilliamJ

Re: Questions about the gluons, their color and anti-color charges and charge of quar

I now fully see, how a red colored quark can turn blue; thank you Adrian for showing me how to illustrate emission and absorption of the red colored quark, in a simple way, like addition and subtraction; that answered it. Like:

+red – (+red + anti-blue) = + blue (where a red quark turns into a blue quark by the red colored quark emitting the gluon of red anti-blue without the color any absorption by any other colored quark)

+red + (+blue + anti-red) = + blue (where a red quark turns into a blue quark by the red colored quark absorbing the gluon of blue anti-red color without any given emission of the gluon by any other specific colored quark before it was absorbed as in this situation).

However, these are just possible emissions and absorptions of gluons from all color charged quarks, not the actual swapping of color charge between two quarks.

Thank you for answering this question, I have looked at all of the possibilities of how emission and absorption of gluons work on all of the possible colored quarks, where they change colors. When I am talking about emission and absorption, I am not talking about exchanging gluons between quarks, but rather quarks of any color emitting or absorbing gluons changing the color of the quark . I am writing this down on note cards and I am memorizing them and looking for patterns, as well as seeing some interesting rules that describe how these emissions and absorption of the gluons work.
I need some help understanding how one quark can emit a gluon and the other absorbs it and the colors of the two quarks at the get go exchange their colors between them. I know I should look at specific examples, but for some reason, I can’t picture any in my mind. Wait, I think I may have just worked out a picture of it with addition/subtraction but I’m not sure…..
I see red turning into blue and blue turning into red. The math is
red – (red + -blue) = blue (where a red quark turns red into blue)
blue + (red + - blue) = red (where a blue quark turns into a red)
Does this change a red quark to a blue quark and a blue quark into a red quark and that means they switch color, and the same gluon is used to exchange the color? Is this correct or incorrect when saying that the color charge is exchanged between the two quarks by the emission and absorption of a gluon, like in the example of the red quark turning into a blue quark and blue quark turning into a red quark, like just illustrated, where it just takes one emitting and the other absorbing the gluon, but it is only one colored quark emitting and the other color quark absorbing. As already shown:

When a red quark turns into a blue quark with emission and the blue quark turns into a red quark with absorption it is:

red – (red + -blue) = blue
blue + (red + -blue) = red

That was one way for the red and blue quarks to exchange colors.

The other way and opposite way for red and blue quarks to exchange colors is:

blue – (blue + -red) = red
red + (blue + -red) = blue

It seems that it takes one specific quark to emit and the other to absorb, but it also seems to me that it would make sense that it could be opposite, where the red could turn into blue with emission of a gluon and blue turning red with absorption of the same gluon. I am confused about how to even word this. I’ve been spending so much time trying to make this make sense in an easy way so you can understand it, but I don’t know how.

If this is not the case where both colored quarks can switch which one absorbs the gluon and which one emits it, and vice versa, then it will be like what I have written next which is where only one quark can emit a specific gluon and the other quark absorbs it, and the rest of what I have written below is true. If not then I’ve wasted a lot of time writing it, but I will keep trying until I get it.

This may be redundant of some of what I have already written; I think that this attempt at trying to explain what I am talking about may do it:
Do the quarks exchange the change of colors between each other by emission and absorption of gluons? Can any colored quark and any other colored quark generate the right gluon to swap color charge? Does the quark emitting the gluon cause the right gluon (with the appropriate color/ant-color charge) in order to switch colors with the absorbing quark? And also can it be that the quark absorbing the gluon determining the color/anti-color gluon? Is it both? And how would the correct gluon be “chosen” by the quarks in order to exchange color? If this is so, does there exist potential gluons with the right potential color charge for each scenario (with absorbing and emitting) so that the any two quarks can exchange colors? Is this is so with potential color exchanges of two differently colored quarks, with potential difference creating for the electron and proton and mass attracting mass, these color charges and gluons are energy? It creates field (need a little more definition of what a field is E = F*d and F=ma). Also, can quarks of the same color charge emit and absorb gluons leaving them with just the original color, say red? I feel this is not so, but it is worth asking.
All mesons have quark and anti-quark combinations, quarks can have only have a color charge and only anti-colors can be on anti-quarks. Are there all possible combinations of quarks and anti-quarks making up all the types of mesons? I also see with the example of a green up quark and an anti-green anti-up quarks are also due to the gluons. How do gluons work in the situation of the mesons with color charge and anti-color charge, and specifically the situation of the green up quark and an anti-green anti-up quark? And also, what about when a proton or neutron are turned into anti-particles (their quarks are the opposite of what they were before)? They are anti-quarks, and that means that the proton or the neutron have an anti-color?

15. May 21, 2012

### WilliamJ

Re: Questions about the gluons, their color and anti-color charges and charge of quar

This is my understanding about quarks, anti-quarks and color charge, is it correct, and if it is correct, then does each scenario’s rules work for all of the possibilities:
This swaps the colors between two differently colored quarks:
1.) If you have a red quark, it turns blue by emitting a (red + anti-blue) gluon:

red - (red + anti-blue) = blue

2.) and a blue quark turns red by absorbing the (red + anti-blue) gluon emitted by the red quark:

blue + (red + anti-blue) = red
The exact opposite which still swaps colors between two differently colored quark which uses in both cases gluons which is antiparticles:
1.) If you have a blue quark, it turns red by emitting a (blue + anti-red) gluon:

blue - (blue + anti-red) = red

2.) and a red quark turns blue by absorbing the (blue + anti-red) gluon emitted by the blue quark:

red + (blue + anti-red) = blue

This deals with quarks and anti-quarks exchanging gluons:

1.) If you have a green quark, it turns blue by emitting a (green + anti-blue) gluon:
green - (green + anti-blue) = blue
2.) and an anti-green antiquark turns anti-blue by absorbing the (green + anti-blue) gluon emitted by the green quark:
anti-green + (green + anti-blue) = anti-blue
How is the exact opposite of this written and how can the antiquark emit a gluon and the quark absorb it, and does there exist a mimicking of the preceding 1 and 2?

Gluons can produce a quark and an anti-quark like this:
1.) (red anti-green) gluon can produce a red quark and an anti-green antiquark.
(red + anti-blue) gluon emits a (red anti-green) gluon and becomes a (green + anti-blue) gluon:
(red + anti-blue) - (red + -green) = (green + anti-blue)
I see how a gluon can emit another gluon, winding up making another type of gluon, like this example, but how can a gluon absorb another gluon? Could I see an example of absorption of one gluon with another gluon which is the opposite of it’s the opposite of the preceding example 1.)?

But: both must be of the same flavor:
Up and anti-up or down and anti-down.
A question I have is that, as with the example of the (red anti-green) gluon it will produce a quark and an anti-quark, so does this mean, when it says that both must be of the same flavor or not, and, does that mean that the quark and anti-quark has can be like this: red up quark and an anti-green anti-up quark? However, I think that I have seen where a gluon can produce a green quark and an anti-green antiquark (am I remembering correctly?) Am I right to assume that as long as you have a pair of anti-particles (flavors of quark, like up and anti-up), that it doesn’t matter what other properties (like color) of the quark or anti-quarks have, they will still behave as antiparticles? Is the only way to determine antiparticles is to look at their opposite electrical charge, like electrons and positrons, protons and anti-protons, etc.? And gluons can act like the gama rays which, if they have a certain amount of energy or above, like1.02 MeV , they can go through pair production? Can all force carriers produce antiparticle pairs, and if so, what examples are there? Also for every pair production is there the opposite, annihilation? That seems to make sense to me, but I am unsure, and might as well have validation on that point and if all particles which have opposite charges, regardless of other properties, are they still anti-particles and will annihilate each other when in close proximity of each other. Also I’ve learned that antiparticle annihilation can result in other particle antiparticle pairs, as well as energy in different forms, like light. I might as well ask so that I have a correct understanding of it. There’s more questions, but I need to focus on the topic I am working on right now.
You have stated that: “unlike photons which carry no electric charge, gluons carry colour charges so can also interact with other gluons. For example a red-antigreen gluon and in doing so become green-antiblue. This makes a huge difference to the physical phenomena that result from the strong force, and is ultimately the reason why quarks and anti-quarks are always confined within hadrons and can never be observed alone.” I don’t understand that, but does that have to do with the reason for the mass defect (which I would need a better understanding of anyway), and how that might relate to the gluons, because the further apart you pull the quarks in a nucleus the more difficult it is to pull them apart, like a spring. My guess is that the further apart you pull the quarks, the more energy you have to put into the neutrons and protons, and that makes it heavier because energy is mass, but I could be way off target on that.
A question I have is that I know gluons hold together the quarks in baryons and mesons. I am particularly interested in knowing how protons and neutrons which are baryons hold together in the nucleus. What I mean is this: do the gluons not only hold together the quarks which make up the proton and neutron, but do they also hold the protons and neutrons together in the nucleus as well? You say that quarks are held together by gluons. And if this is so, then is it not only the quarks being pulled apart, but also the neutrons and protons being pulled apart in the nucleus and that has to deal with the mass defect? The act of dumping F*d = E into the pulling apart, you are dumping more energy into the system, and that would make it more massive, but this only makes sense if not only do quarks hold together in the nucleus by gluons, which is true, and protons and neutrons are being pulled apart that way by an outside force, which if neutrons and protons are held together by the gluons that would explain it.

16. May 21, 2012

Re: Questions about the gluons, their color and anti-color charges and charge of quar

Don't have a lot of time for posting tonight, but here are one or two quick responses:
• Yes, one quark emitting a gluon that is then absorbed by another swaps the colours of those quarks.
• A quark can only emit a gluon that carries the colour that quark started with (NB I am ignoring here the more complicated cases of the 'neutral' gluons).
• A quark can only absorb a gluon that carries the anticolour of that quark (ditto).
• For anti-quarks, the corresponding rules apply but for colour read anticolour and vice versa. Eg an antigreen antiquark can emit an x-antigreen gluon and become anti-x, where x could be either red or blue.
• In a meson with a red quark and anti-red anti-quark, the quark can still emit a red-antigreen gluon, and the anti-quark absorb this, leaving the quark green and the anti-quark anti-green. Similarly with red-antiblue.
• All force carriers can produce pairs, for example the weak W- boson can produce an electron and an antineutrino. (W bosons also carry charge, hence why the particles are not exact antiparticles.)
• For the strong and electromagnetic forces, charges, as well as various other quantum numbers such as particle flavours, are always conserved. Eg a red quark has one unit of red. If it then emits a red-antigreen gluon, turning the quark green, and we add up the colours, we still have a total of one unit of red. The quark is now green but the anti-green on the gluon brings the total 'green-ness' back down to zero just as we started with.
• Actually, the behaviour of the gluons is what gives protons and neutrons most of the mass they start out with. Mass defects (nuclear binding energies) are the result of residual interactions between whole nucleons, a bit like the way Hydrogen bonding works between neutral water molecules. The attraction between nucleons is more complex than the gluon exchanges between individual quarks.