Gluons: Particles or Not? Evidence for their Particle Nature

[friend]

What evidence is there that gluons are even particles? They were originally proposed to supply an extra quantum number to comply with the Pauli exclusion principle. So gluons are supposed to carry a color-anticolor charge in order that the quarks in baryons would add up to the color white.

But if gluons are particles, then they spend some time traveling between quarks. And during that time between quarks, the color addition of quarks is not white. This is a bit like asking if electrons in a Helium atom must have opposite spins (Pauli exclusion principle), then what tells one electron what spin the other electron has?

 

[Vanadium 50]

Friend, one of the least endearing traits of your posts is your conclusion that since you don';t understand something, everybody else is doing it wrong.

Of course gluons are particles.

 

[friend]

I'm not at all suggesting that others are doing it wrong. I'm simply trying to understand how we can be so sure gluons are particles as opposed to some other interpretation. Why mere questions should put you on the defensive is beyond me. As I understand it, gluons only occur in baryons which are very small. And they are massless and so travel at the speed of light for very short distances. I don't understand how we could possibly distinguish gluons as a particle from some other instantaneous process. Would glueballs prove they are particles? Or maybe it is necessary for gluons to interact with other gluons in order to obtain correct QCD results. Wouldn't that prove that they are something other than some relationship between quarks?

 

[ChrisVer]

There are some pedagogical/historical papers around if you google for it (e.g. gluon discovery).
Here's an example of what comes up: https://arxiv.org/abs/1409.4232
So the story goes like this, once we collided electrons to protons at high energies (deep inelastic scattering), so that the electron could probe interactions with the proton constituents. What we saw was that the electrons interacted (via photons) with the "partons" (valance and sea quark particles which can interact with photons). Those measurements suggested that the partons carried about half of the momentum of the proton (suggesting the existence of neutral partons which could not interact via photons). At the same time and for many years, QCD as a theory (which predicted the gluons) was doing a good job in giving predictions or explaining what we saw in strong interactions (e.g. the prediction of charm quark followed by the discovery of charmonium mesons).
The discovery point of the gluon came with the electron-positron collisions, e^+e^- \to q\bar{q}g (gluon-"Bremsstrahlung" process) which suggests 3 jets (a few of which were observed subsequently in several independent experiments) in the final state in the right kinematical region.
During that time, without any profound evidence (just suggestions), the debate of whether gluons existed or not was a reasonable one, and many people wouldn't or would believe in its existence. Today though it is ridiculous to question their existence.

 

[mfb]

I'm not at all suggesting that others are doing it wrong.

Why did you choose that thread title then?

But if gluons are particles, then they spend some time traveling between quarks. And during that time between quarks, the color addition of quarks is not white.

You can't apply intuition from classical mechanics in quantum mechanics. As you see it leads to conclusions that don't make sense.
The overall baryon is color-neutral at all times, it doesn't make sense to ask which color a given quark has at a given point in time - you can't even distinguish them.

As I understand it, gluons only occur in baryons which are very small.

No, they are in all hadrons.

Would glueballs prove they are particles?

That's like asking if a second moon of Earth would prove Earth has gravity. I guess it would but it is also completely unnecessary to look for it for that reason.

Or maybe it is necessary for gluons to interact with other gluons in order to obtain correct QCD results.

While true (there is gluon-gluon coupling) this is completely unrelated to your other questions.

Brehmstrahlung

Bremsstrahlung

 

[ChrisVer]

Bremsstrahlung

thanks! corrected it... I hate that word...

 

[friend]

While true (there is gluon-gluon coupling) .

OK. Is the perturbation method of QFT unique to particles? I assume gluons interactions are calculated perturbatively.

 

[mfb]

Is the perturbation method of QFT unique to particles?

That question doesn't make sense.

I assume gluons interactions are calculated perturbatively.

At high energies: Yes. At low energies that doesn't work as the coupling is too strong.

 

[friend]

That question doesn't make sense.

You know, in QFT where Feynman diagrams are used to help compute terms in the perturbative expansion for scattering amplitudes. But Feynman diagrams are a description of how "particles" interact, right. So QFT → perturbative expansion → Feynman diagrams → particles and only particles, right?

At high energies: Yes. At low energies that doesn't work as the coupling is too strong.

I'm thinking if ever Feynman diagrams can be used, then it proves we are talking about only particles. So if QCD used Feynman diagrams, then it must be referring to particles? But then why would glueballs not be more evident?

 

[mfb]

You know, in QFT where Feynman diagrams are used to help compute terms in the perturbative expansion for scattering amplitudes.

Not everything can be described as scattering process, and not all scattering processes can be calculated with perturbation theory.

But Feynman diagrams are a description of how "particles" interact, right.

Internal lines are virtual particles, they are not real (otherwise they wouldn't be called virtual).

To learn physics you need to start with the basics. Your questions are way ahead of your knowledge, that makes threads like this so problematic and relatively useless. The questions are based on misconceptions, and even in cases where they have a proper answer you would need much more background knowledge to understand that answer properly.

 

[friend]

Don't be afraid to venture a guess as to what real things these equations are actually talking about? I'm trying to get at the underlying ontology of it all. Why this math and not some other? I have a pretty good start, but I'm stuck at gluons. I'm trying to understand if they exist on their own or if they are only relationships between quarks . With electrons it's easy because we can observe their effects. But it seems we must rely on interpretations of math for quarks and gluons. If the same math holds (perturbation expansion) for quarks and gluon as for electrons, then it seem the math is pointing to particles. The trouble seems to be that we can write a wave function for an electron (for a particle moving), but we can't seem to write a wave function for quarks and gluons. So I'm not so sure about what they are, so I'm asking for help.

 

[Vanadium 50]

but we can't seem to write a wave function for quarks and gluons

Incorrect. Maybe you can't.

 

[jim mcnamara]

Okay. Enough. If one of the physicists wishes to add reading material at an appropriate level to the thread please let me or another mentor know so that can happen. Because:

This thread content does not really seem PF quality. Thread locked.