Understanding Gluon Self-Interaction and Quark/Gluon Jet Differentiation

[Malamala]

Hello! Based on QCD we can have gluon self-interaction i.e. a vertex with 3 or 4 gluons. What were the experimental evidences by which the existence of these vertices was confirmed? Also, how does one differentiate between a quark and a gluon induced jet? Thank you!

 

[vanhees71]

Usually the three-jet events discovered at the end of the 1970ies by the DORIS experiment at DESY are taken as the discovery of gluons:

https://www.wikiwand.com/en/Gluon#/Experimental_observations

 

[Vanadium 50]

You mean TASSO, right?

 

[vanhees71]

Yes, indeed:

https://www.wikiwand.com/en/TASSO

 

[Malamala]

Usually the three-jet events discovered at the end of the 1970ies by the DORIS experiment at DESY are taken as the discovery of gluons:

https://www.wikiwand.com/en/Gluon#/Experimental_observations

Thank you for the reply. I will read the references in more details, but I am not sure I see how does this prove experimental evidence for the 3 and 4 gluon vertex. Based on the Feynman diagrams there, we either have some sort of bremsstrahlung, where one of the quarks produce a gluon (and hence we have 2 jets produced by quarks and 1 by a gluon) or the decay of that Y resonance producing 3 gluons, but the gluons don't come from a 3 gluon vertex, but from 3 quark-quark-gluon vertices. That is clear evidence for the existence of gluons, but I don't think it is evidence for the fact that the symmetry of QCD is SU(3) i.e. at the time it was expected that we need a boson to mediate the strong force, but it wasn't clear that the symmetry group of that interaction is SU(3) (which predicts 3 and 4 gluon vertices). As far as I can tell the references that you mentioned prove the existence of the gluon. I was actually more interested if there was an experiment with a clear signature of 3 and 4 gluon vertices, and how they figured that out?

Also, how about my second question, how do they differentiate between 3 jets produced by 3 gluons and 3 jets produced by 2 quarks and a gluon i.e. what is the experimental difference between a quark and a gluon jet? Thank you!

 

[Vanadium 50]

if there was an experiment with a clear signature of 3 and 4 gluon vertices, and how they figured that out?

Vertices are not real things. The four-gluon vertex is not gauge invariant by itself and it may not even be finite. The real things are amplitudes, or at least event rates.

 

[Malamala]

Vertices are not real things. The four-gluon vertex is not gauge invariant by itself and it may not even be finite. The real things are amplitudes, or at least event rates.

Thank you for your reply. I am not sure what you mean. Vertices are very real. When you have 2 quarks produced from a gluon, it is a vertex with a clear experimental signature (2 jets). I just came across the attached picture which partly answer one of my questions: they used that decay signature i.e. 4 jets to confirm the existence of 3 gluon vertices (this is from OPAL experiment at LEP). I am not sure what you mean by "not real", but that vertex seems very real and it has a clear experimental signature (i.e. 4 jets). This still doesn't answer my 4 vertex question, too. Based on this picture, I would imagine you could have 3 gluons coming from the original one (hence 4 gluon vertices) and 5 jets in the final state? Was this observed? If not, do we know experimentally, in any way, that the 4 vertices exists (or it is just inferred theoretically), in the same way we know about 3 vertices (i.e. a clear experimental signature)? And if so, could you point me towards that specific experiment?

 

[Vanadium 50]

I'm not going to argue with you.

 

[Malamala]

I'm not going to argue with you.

You don't need to argue... If you explain to me clearly why that 3 gluon vertex there is not real I would understand probably. You just stated that vertices are not real, that is not an explanation... Or you can just give me a link to some experiments proving what I am asking for...

 

[ChrisVer]

Also, how does one differentiate between a quark and a gluon induced jet?

In general, there are some differences between quark and gluon initiated jets. There are algorithms that use those differences to identify such jets. Just from a fast search I came across this paper, and for example you can see some diferentiating variables in Fig. 1:
https://arxiv.org/abs/1712.03634

The general way I've seen people approaching parameters in diagrams (e.g. the coupling strengths), is by trying to determine them statistically. I.e. you can assume that the strength is 0 (so the vertex doesn't exist) and compare your expectations with data that consists with jets in your final state. You would probably see that you also need the additional diagram to get a better description, and then by a fit you determine its value.
One extra thing is that you never access diagrams, they are a way to "encode" mathematical expressions. The only thing you have access to is the amplitudes that arise from adding the diagrams and taking their magnitudes squared.
You basically "never" see a photon decaying to electron positron, you see a photon, a Z and their interference decaying into this lepton pair.