Has photoproduction of gluons been observed?

In summary, the author is asking if a process with an initial state of two photons has been observed that leads to an end state with two (or more) gluons.
  • #1
ohwilleke
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I am wondering if a process with an initial state of two photons leading to an end state with two (or more) gluons has been observed.
I am wondering if a process with an initial state of two photons leading to an end state with two (or more) gluons has been observed.

I have seen some papers (like this one) that seem to suggest that this is the case, but none of them say so as clearly as I would like to be confident that this is true.
 
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  • #2
Since nobody has seen a free gluon, the answer to your question as posed is no.
If you had a different question in mind, you should ask it.
 
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  • #3
Vanadium 50 said:
Since nobody has seen a free gluon, the answer to your question as posed is no.
If you had a different question in mind, you should ask it.
Well, no one can "see" any particles, we detect resonance peaks which we interpret as a particle.
 
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  • #4
You're not helping. We can see protons, for example, just fine.
 
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  • #5
Vanadium 50 said:
You're not helping. We can see protons, for example, just fine.
Do you want to say you see them directly?
You see an image, but it doesn't mean you can see them without your equipment.
Anyway, I searched google for "image of a proton" and couldn't find one, do you have one?
 
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  • #6
Vanadium 50 said:
Since nobody has seen a free gluon, the answer to your question as posed is no.
If you had a different question in mind, you should ask it.
I think this proves too much. By that reasoning, no one has ever observed an up, down, strange, charm, or bottom quark, since there is likewise no evidence of free quarks that are not top quarks (in each case using the term quark to include antiquarks in this sentence).

But, of course, in a reasonable interpretation of what "observed" means in this context, we have seen, for example, experimental evidence of photoproduction of some quark-antiquark pairs (e.g. here), a process which is simply the inverse of quark-antiquark annihilation, which is one of the first things taught in quantum mechanics.

There are all sorts of particles in high energy physics that one only observes indirectly, for example, through their decay products, after statistically removing known background "noise" from other know processes, for example. But we still "observe" those particles.

"Observed" does not literally mean "see" with your technologically enhanced eyes, in this context. Instead, it means that there is strong experimental evidence that it has occurred (five sigma is a bit arbitrary, but the usual standard for such things, however, if there were 3 or 4 sigma experimental evidence for the existence of such a process that would be worth mentioning too). Observe is in contrast to merely theoretically predicting that something exists without experimentally evidence supporting its existence, like the Higgs boson at 125 GeV pre-2012.

The question is whether there is strong experimental evidence supporting the existence of the process or not.
 
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  • #7
I'm with V50 here. The experimental measurement will always be hadrons. Which hadrons are you looking for? We have seen photoproduction of hadrons, and diagrams with gluons contribute to them.
 
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  • #8
MathematicalPhysicist said:
Do you want to say you see them directly?
You're really not helping. The fact that protons are really, really tiny has nothing whatsoever to do with the question asked.
 
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  • #9
I show you an event ##\gamma \gamma \rightarrow \pi^+ \pi^-## and you say "Nope. Coulda been quarks." Then I show you an event ##\gamma \gamma \rightarrow \rho^+ \rho^-## and you say "Nope. Coulda been quarks." This game will get very old very quickly.
 
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  • #10
Vanadium 50 said:
I show you an event ##\gamma \gamma \rightarrow \pi^+ \pi^-## and you say "Nope. Coulda been quarks." Then I show you an event ##\gamma \gamma \rightarrow \rho^+ \rho^-##a nd you say "Nope. Coulda been quarks." This game will get very old very quickly.
What I am imagining (the numbers in the example are just made up) is something more like the SM predicts 6 events in a very large data set from quark production from ##\gamma \gamma \rightarrow \pi^+ \pi^-## and 3 events from quark production from ##\gamma \gamma \rightarrow \rho^+ \rho^-## and 2 events from other cases of photoproduction of quarks, for a total of 11 hadronic events predicted with an uncertainty of ± 1.

If the process ##\gamma \gamma \rightarrow gg \rightarrow \pi^+ \pi^-## and from other ##\gamma \gamma \rightarrow gg \rightarrow hadron^+ hadron^-## decay channels existed, however, the SM prediction would be an additional 7 hadronic events predicted with an uncertainty of ± 0.5, in particular proportions. Obviously a real experiment would involve a lot more technical detail, but that would be the basic kind of experiment I'm thinking about conceptually.

We look at our data set and identify 17 events with a distribution by hadron type which is close to the predicted mix of hadrons predicted for the combined processes in a Chi-square test.

This would be strong observational evidence of photoproduction of gluons.

What I am curious about is whether an experiment has been done and produced results like that which provide that kind of strong evidence.

Also, for that matter, I'm not entirely clear about whether the SM predicts a detectible number of events from such processes at all. In the alternative, I'd also be interested to know if someone calculated SM predictions and the predicted number of events in a scenario like the one above would be so small that the kind of experiment I use as an example wouldn't come close to being able to see particles created through the channel with current or near future technology, since if true, that would explain why no one has bothered to try to do that experiment.
 
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  • #11
OK, that's at least answerable. The answer is "no".
  1. In quantum mechanics, we add amplitudes and then square. Furthermore, the amplitudes may have different phases. Adding a new amplitude doesn't even necessarily make the probability go up.
  2. The process γγ→hadrons occurs at order α2 and the process γγ→gg → hadrons occurs at order α2αs2. That's 50-100x smaller. We cannot predict γγ→hadrons to that degree of accuracy.
  3. The dominant effect is not γγ→gg → hadrons but the interference between the non-gluonic and gluonic pieces. (a consequence of adding amplitudes) A discrepancy between the measurement and the pure QED prediction does not require adding the process of interest.
  4. Furthermore, there is a non-perturbative piece to the calculation called the form factor, which is essentially the wavefunction overlap of the constituent partons, and this is determined from experiment, A discrepancy would then be absorbed into the form factors.
 
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  • #12
Vanadium 50 said:
OK, that's at least answerable. The answer is "no".
  1. In quantum mechanics, we add amplitudes and then square. Furthermore, the amplitudes may have different phases. Adding a new amplitude doesn't even necessarily make the probability go up.
  2. The process γγ→hadrons occurs at order α2 and the process γγ→gg → hadrons occurs at order α2α[sSUP]2[/SUP]. That's 50-100x smaller. We cannot predict γγ→hadrons to that degree of accuracy.
  3. The dominant effect is not γγ→gg → hadrons but the interference between the non-gluonic and gluonic pieces. (a consequence of adding amplitudes) A discrepancy between the measurement and the pure QED prediction does not require adding the process of interest.
  4. Furthermore, there is a non-perturbative piece to the calculation called the form factor, which is essentially the wavefunction overlap of the constituent partons, and this is determined from experiment, A discrepancy would then be absorbed into the form factors.
Thanks. That's helpful and answers the question I had.
 
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  • #13
Vanadium 50 said:
OK, that's at least answerable. The answer is "no".
  1. In quantum mechanics, we add amplitudes and then square. Furthermore, the amplitudes may have different phases. Adding a new amplitude doesn't even necessarily make the probability go up.
  2. The process γγ→hadrons occurs at order α2 and the process γγ→gg → hadrons occurs at order α2αs2. That's 50-100x smaller. We cannot predict γγ→hadrons to that degree of accuracy.
  3. The dominant effect is not γγ→gg → hadrons but the interference between the non-gluonic and gluonic pieces. (a consequence of adding amplitudes) A discrepancy between the measurement and the pure QED prediction does not require adding the process of interest.
  4. Furthermore, there is a non-perturbative piece to the calculation called the form factor, which is essentially the wavefunction overlap of the constituent partons, and this is determined from experiment, A discrepancy would then be absorbed into the form factors.
Nevertheless, γγ→gg crosssection has been already studied theoretically (for example here: https://arxiv.org/abs/1310.6701), and at some point I guess the effect won't be negligible at least in ultraperipheral collisions at LHC.
 
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Related to Has photoproduction of gluons been observed?

1. What is photoproduction of gluons?

Photoproduction of gluons is a process in which gluons, which are the particles responsible for the strong nuclear force, are created through the interaction of photons (particles of light) with other particles.

2. Why is it important to observe photoproduction of gluons?

Observing photoproduction of gluons can provide valuable insights into the strong nuclear force and its role in the structure and interactions of subatomic particles. It can also help us better understand the behavior of matter at extremely high energies.

3. How is photoproduction of gluons observed?

Photoproduction of gluons is typically observed through high-energy particle collisions in particle accelerators, such as the Large Hadron Collider (LHC). Scientists analyze the resulting particles and their properties to determine if gluons were produced.

4. Has photoproduction of gluons been observed before?

Yes, photoproduction of gluons has been observed in several experiments, including at the LHC and the HERA accelerator in Germany. These observations have provided important evidence for the existence and behavior of gluons.

5. What are the potential implications of observing photoproduction of gluons?

Observing photoproduction of gluons could have significant implications for our understanding of the fundamental forces and particles that make up our universe. It could also lead to advancements in technology and energy production through the development of new particle accelerators and other technologies.

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