Understanding Gluon Self-Interaction and Quark/Gluon Jet Differentiation

In summary, gluon self-interaction refers to the interaction between gluons, which are elementary particles that carry the strong force, within the theory of quantum chromodynamics (QCD). This interaction plays a crucial role in understanding the behavior of quarks and gluons within hadrons, the particles that make up protons and neutrons. On the other hand, quark/gluon jet differentiation is the process of distinguishing between jets produced by quarks and those produced by gluons in high-energy particle collisions. This differentiation is important for accurately interpreting experimental data and studying the properties of the strong nuclear force. Overall, understanding gluon self-interaction and quark/gluon jet differentiation is crucial for gaining insights into the fundamental nature of matter
  • #1
Malamala
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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!
 
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  • #3
You mean TASSO, right?
 
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  • #5
vanhees71 said:
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!
 
  • #6
Malamala said:
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.
 
  • #7
Vanadium 50 said:
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?
 

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  • #8
I'm not going to argue with you.
 
  • #9
Vanadium 50 said:
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...
 
  • #10
Malamala said:
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.
 
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Related to Understanding Gluon Self-Interaction and Quark/Gluon Jet Differentiation

1. What are gluons and how do they interact with quarks?

Gluons are elementary particles that mediate the strong nuclear force between quarks. They are responsible for holding quarks together to form protons and neutrons, and for binding quarks and anti-quarks to form mesons. Gluons interact with quarks through the exchange of virtual gluons, which are constantly being emitted and absorbed.

2. How do gluon self-interactions contribute to the strong nuclear force?

Gluons have the unique property of being able to interact with themselves, unlike other fundamental particles. This self-interaction is a result of their charge and color properties. These interactions contribute to the strong nuclear force by creating a strong binding force between quarks and preventing them from breaking apart.

3. What is the significance of understanding gluon self-interaction?

Understanding gluon self-interaction is crucial for understanding the underlying mechanisms of the strong nuclear force and the behavior of quarks and gluons in high-energy collisions. It also plays a key role in the study of particle physics and the search for new particles and interactions.

4. How are quark/gluon jet differentiation and gluon self-interaction related?

Quark/gluon jet differentiation is the process of distinguishing between jets of particles produced by quarks and those produced by gluons in high-energy collisions. Gluon self-interaction is one of the factors that affects the properties of these jets, making it an important factor in the differentiation process.

5. What are some current research efforts focused on understanding gluon self-interaction and quark/gluon jet differentiation?

There are many ongoing research efforts aimed at understanding gluon self-interaction and quark/gluon jet differentiation. Some of these include experiments at particle accelerators such as the Large Hadron Collider, theoretical studies using mathematical models and simulations, and data analysis of high-energy collision events. These efforts are crucial for advancing our understanding of the fundamental forces and particles that make up the universe.

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