# Gluons, where do they come from and where do they go?

1. Jan 26, 2014

### llynne

I want to learn more about gluons. I understand that they are what bind the neutrons and protons into a nucleus. Do they spontaneously arise from a neutron+ positron relationship or are they related to some process?

2. Jan 26, 2014

### Staff: Mentor

The gluons that mediate the color force between quarks are virtual gluons, as are the mesons that are responsible for binding neutrons and protons to each other in nuclei. Do you know much about virtual particles?

3. Jan 26, 2014

### llynne

I should by now, but not quite fitting all together yet. I need meson too? Mesons are gluons? Both are virtual? Born of the need to mediate the Other forces?

4. Jan 26, 2014

### Staff: Mentor

This has nothing to do with positrons. Do you mean protons?

Yes, they "spontaneously arise". Quarks (the parts in neutrons and protons) can "emit" and "absorb" them. Note the quotation marks, as those gluons are virtual particles.

Mesons consist of a quark and an antiquark, bound by the strong interaction (so they have gluons inside). They are not gluons.
Depends on the phenomenon you want to describe.

5. Jan 26, 2014

### Bill_K

Please give us an example of a situation involving a gluon that is not virtual.

6. Jan 26, 2014

### Staff: Mentor

I thought of the mesons (virtual as effective action of the strong force between hadrons, real if they fly through a particle detector).

The gluon discovery was based on 3-jet events. Those gluons were still virtual, but not so far away from the properties of real particles as far as I know.

7. Jan 26, 2014

### Hawkwind

Particles which carry the quantum number "colour" interact with each other by exchanging gluons ("strong force"). Proton and neutron however are neutral with respect to the colour quantum number. As already mentioned in other replys, the quarks do have colour - they are glued to each other by gluons.

The interaction which binds neutrons and protons to nuclei, is called "nuclear force". This is a residual of the strong force but electromagnetic and weak force contribute to the nuclear force, too.
Phenomenolgically, the nuclear force is often described by the exchange of mesons between the nucleons. However, in principle, the standard model of particles is the theoretical base for describing nuclear forces. However, calculations are a challenge.

8. Jan 26, 2014

### ChrisVer

With no offence- not knowing whether mesons are gluons or not, I doubt you'd know what virtual particles are all about. So an introductory book on elementary particles (eg Griffith's) would be recommended if you are interested in this field.

Last edited: Jan 26, 2014
9. Jan 26, 2014

### llynne

Thanks, ( oops I got proton mixed up, sorry) Is there any possibility that gluon's are a force or a field rather than a particle. The term virtual makes me want to focus on its function. If it mediates the strong force could it be interpreted in a different way?

10. Jan 26, 2014

### ChrisVer

Well, we've never seen gluons as we have never seen quarks in the first place... Quantum Chromodynamics that deals with the topic (of quarks/partons interacting among themselves with gluons) is just successful in allowing us to make predictions and explain what we see in experiments....
What different interpretation do you have in mind?

11. Jan 26, 2014

### Staff: Mentor

In quantum field theories, particles are interpreted as excitations of a field.

12. Jan 26, 2014

### llynne

I have nothing specific in mind, these were the questions I ask myself. The experiment being the same, and the results the same has anyone argued for alternative interpretations?

13. Jan 27, 2014

### Staff: Mentor

An alternative to the standard model would have to do two things before it would be taken seriously. First, it would have to agree with the experiments that have already been done, at least as well as does the standard model. Second, it would have to predict something that the standard model doesn't, so that we can perform an experiment to see which one works better.

So far no one has found any such alternative.

14. Jan 27, 2014

### Hawkwind

Getting curious: what exactly do you think is wrong in my post?

15. Jan 27, 2014

### Hawkwind

For instance effective field theories like chiral pertubation theory could be a candidate:
http://en.wikipedia.org/wiki/Chiral_perturbation_theory

These are promising attempts to describe the low energy region of strong interactions, where QCD perturbation theory is not applicable. BTW, this theory justifies the historical attempt to describe nuclear forces by the exchange of mesons:
Although not being a fundamental renormalizable quantum field theory, these effective theories are used a lot by theoreticians.

Last edited: Jan 27, 2014
16. Jan 27, 2014

### Staff: Mentor

I don't think he was referring to you in that post.

17. Jan 27, 2014

### ChrisVer

i was referring to OP

18. Jan 27, 2014

### llynne

I'm sure I read it, not sure what remains in my brain. I read about chirality several times but if I ask myself what it is, I find nothing. But I know how my brain works. I take in information and suddenly it comes together. I found myself asking isn't what a gluon does more like a field? Not really trying to display my ignorance but for a quick way to find if that has already been worked through.

19. Jan 27, 2014

### ChrisVer

As I said, it was a recommendation rather than an offence (I didn't call you stupid or anything- i don't believe in such a thing in the first place). Just a way for both sides to understand what each other is speaking about.
In the mathematical procedure, gluons appear because strong interactions can be described as a local SU(3) gauge theory.
So let's start from the analogy of the electromagnetic field. The electromagnetic field comes when you impose local U(1) gauge invariance. In general a U(1) transformation will be:
$Φ\leftharpoondown e^{iYa(x)}Φ$
If you put the transformed expression into the lagrangian, you will see that it's not invariant as it is... The kinetic term:
$L=∂_{μ}Φ ∂^{μ}Φ^{*}$
will give you an extra term containing the partical derivative of $a(x)$ since it's a local transformation and thus depends on spacetime $x$ (i denote $x^{μ}$ as just $x$ here). Nevermind, to avoid the whole maths, you will have to introduce a new field to keep the invariance, which in fact will change your partial derivative to covariant derivative... Furthermore new interactive terms will be allowed for your local U(1) transformation invariance (the term will correspond to the "electromagnetic" field).

If you do the same for SU(3) (by keeping in mind it's not an abelian group) you will get also another field that will correspond to its interaction with itself. That'll be the field of gluons.

Again it's a model (QCD) that works nicely in explaining what we see, and that's why we have it. For example it was at the first stages of studying nuclear forces that physicists put in action the String Theory. Of course (since it's a fact now) QCD dropped it out.
The problem is that at low energies, the coupling constant of QCD is not working perturbatively because it is too big to give any reasonable result (higher orders will be more important and stuff). In that region, someone uses effective theories like mesons. The energies of nuclei (~MeV) are in that region, so it would make no sense to use gluons as medians of interaction and uses the pion mesons interchanging models. At the energies of a proton (~1GeV) or more, QCD is a good perturbative theory (it works).

Last edited: Jan 27, 2014
20. Jan 27, 2014

### Staff: Mentor

Have you looking into what quantum field theories are?
Here's a link if you haven't.
http://en.wikipedia.org/wiki/Quantum_field_theory