Parton? quarks? what is the difference?

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Discussion Overview

The discussion centers on the differences and relationships between the parton model proposed by Richard Feynman and the quark model introduced by Murray Gell-Mann. Participants explore the theoretical implications of both models, their historical context, and their relevance to particle physics, particularly in relation to nucleons and deep inelastic scattering experiments.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Historical

Main Points Raised

  • Some participants note that the parton model describes pointlike constituents within nucleons, while the quark model incorporates SU(3) symmetry, suggesting a deeper theoretical framework.
  • Others argue that both models ultimately describe the same phenomena from different perspectives, with the parton model being more focused on dynamics and the quark model on symmetry.
  • A participant questions the interaction of electrons with gluons during scattering experiments, noting that gluons are massless but still carry energy.
  • There is a discussion about the historical context of Gell-Mann's views on quarks, with some suggesting he did not initially believe in their physical existence, viewing them instead as a classification tool.
  • One participant mentions that the parton model was developed in response to deep inelastic scattering experiments, while the quark model was more theoretical in nature.
  • Another participant highlights the distinction between the original quark model and the current understanding of quark theory, specifically Quantum Chromodynamics (QCD).
  • There is a reference to the idea that Feynman may have used the term "parton" to differentiate his model from Gell-Mann's quark model, with some humor about Gell-Mann's perspective on the parton model.

Areas of Agreement / Disagreement

Participants express differing views on the implications and interpretations of the parton and quark models, with no clear consensus reached regarding their distinctions and relationships. Some agree on the foundational aspects of both models, while others emphasize their differences and the historical context of their development.

Contextual Notes

Participants note the complexity of the discussions surrounding the models, including the historical perspectives of their developers and the evolving understanding of particle physics. There are references to unresolved questions about the nature of particles and interactions within the context of Quantum Field Theory and QCD.

Vazier
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hey i am having a problem problem with understanding the difference between the parton model for the structure of the atom proposed by Richard Feynman and the quark structure... it would be really helpful if someone could give me good links where both of the them are explained in a simple language...
 
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Vazier said:
i am having a problem problem with understanding the difference between the parton model for the structure of the atom proposed by Richard Feynman and the quark structure...
The parton model for the nucleons (protons & neutrons) was proposed by Feynman to describe the dynamics. Gell-Mann introduced the quark model from ideas of symmetry. Only later was it realized that Feynman's partons are also Gell-Mann's quarks. Both models describe the same thing from different perspectives.
 
humanino said:
The parton model for the nucleons (protons & neutrons) was proposed by Feynman to describe the dynamics. Gell-Mann introduced the quark model from ideas of symmetry. Only later was it realized that Feynman's partons are also Gell-Mann's quarks. Both models describe the same thing from different perspectives.

I might be wrong but I think there is a big difference in that the parton model only refers to the fact that there had to be pointlike constituents in the hadrons. Quark theory says that too but in addition it introduces the SU(3) symmetry. So the quark model goes much beyond the parton model.
 
okay... i get it that they are from a different perspective but the quark goes into more details of the working... i got another question... i was trying to read up more about this topic and found that there was a experiment done by James Bjorken at Stanford Linear Accelerator where he shot electrons at protons and found three points of deflection in a proton... but i read that a proton was also made up of gluons... don't the electrons get deflected due to collison with gluons... i know that gluons are massless but they are still particles that carry energy... and also could you expain to me what is meant by SU(3) symmetry...
 
nrqed said:
the parton model only refers to the fact that there had to be pointlike constituents in the hadrons. Quark theory says that too but in addition it introduces the SU(3) symmetry. So the quark model goes much beyond the parton model.
I would not agree with that. If I remember correctly, Gell-Mann really did not believe that "quarks" were in there. He used the symmetry group powerful methods and in particular predicted the omega minus. Feynman's partons on the other hand have little to say about the hadron specrum. They are designed to address the deep inelastic scattering experiments that were about the be performed at SLAC.

Unfortunately, we do not have Gell-Mann Nobel lecture available, so it is hard to know exactly what he thought at this point. It might be that he did not want to assume our space-time picture still hold deep inside hadrons, questioning what we call particles altogether. One must recall that, before QCD, many people were ready to abandon QFT as it was believed it could not describe strongly interacting systems.
 
Vazier said:
found three points of deflection in a proton
One must not over simplify either.
i read that a proton was also made up of gluons... don't the electrons get deflected due to collison with gluons... i know that gluons are massless but they are still particles that carry energy
At first order, electrons and photons do not interact with gluons. Gluons are mainly detected via second order effects.
could you expain to me what is meant by SU(3) symmetry...
Do you know what is the SU(3) group ? 3x3 special unitary matrices ? SU(3) can refer to several symmetries : the fundamental one of QCD or the approximate one of flavor. One must recall that the fundamental SU(3)-QCD symmetry was proposed by Greenberg, and it not the Eightfold-way SU(3)-flavor one.
 
Vazier said:
... and also could you expain to me what is meant by SU(3) symmetry...

Bruce Schumm's "Deep Down Things" is an excellent treatment of the concepts underlying the Standard Model including Lie groups, Lie algebras, U(1), SU(2), SU(3) and gauge theory and so on. The book really gets at the meaning of the various concepts for a person who isn't a physics or mathematics student. There is a very nice discussion of Feynman diagrams and renormalization.

Also Schumm observes that Glashow wasn't making any claims that quarks existed just that they served as effective fictions in the symmetry.

X
 
xristy said:
Also Schumm observes that Glashow wasn't making any claims that quarks existed just that they served as effective fictions in the symmetry.
Glashow ? Did you mean Gell-Mann ? Glashow worked on electroweak part of the standard model, not QCD.
 
humanino said:
I would not agree with that. If I remember correctly, Gell-Mann really did not believe that "quarks" were in there. He used the symmetry group powerful methods and in particular predicted the omega minus. Feynman's partons on the other hand have little to say about the hadron specrum. They are designed to address the deep inelastic scattering experiments that were about the be performed at SLAC.

Unfortunately, we do not have Gell-Mann Nobel lecture available, so it is hard to know exactly what he thought at this point. It might be that he did not want to assume our space-time picture still hold deep inside hadrons, questioning what we call particles altogether. One must recall that, before QCD, many people were ready to abandon QFT as it was believed it could not describe strongly interacting systems.

I agree. It was not clear to me if by "quark theory", the OP meant the original quark model or the present quark theory, namely QCD. I was talking about the present accepted "quark theory", QCD. If we are talking about the original quark model as it was considered early on, I agree that the the model was a way to incoprorate SU(3) symmetry to organize the hadron spectrum.
 
  • #10
humanino said:
Glashow ? Did you mean Gell-Mann ? Glashow worked on electroweak part of the standard model, not QCD.

Yes of course. Posting while asleep evidently doesn't work to well :redface:
 
  • #11
I recently had a brief exchange with a friend on this topic, which I'll paste below for those who might be interested. His original message, which is no longer available, claimed that Feynman used the term "parton" instead of "quark" just to piss off Gell-Mann.

I wrote:
Earlier today I was reminded of your recent post on Feynman, Gell-Mann, quarks and partons, while talking with one of my advisors, Rich Brower, who happened to be a post-doc at Caltech in the early '70s and had some funny stories about the two of them.

I remember you writing something to the effect that Feynman called his theoretical point-like hadronic constituent particles "partons" instead of quarks just to piss off Gell-Mann, who had introduced quarks some years earlier. But Gell-Mann probably appreciated the different name (and may even have pushed for it), since he was convinced quarks were merely a convenient counting/classification trick, and did not exist as physical particles. Rich related an amusing saying of Gell-Mann's: "The parton model is just light-cone algebra plus mistakes" (i.e., the partons).

He responded:
i think i absorbed the thing about partons from Susskind's reminiscences, which i read years ago: [LINK REMOVED]. rereading it i find that what i said isn't quite correct: the parton idea, which Feynman inferred from scattering data, was originally different from the quark idea, which came out of SU(3) symmetry. it turns out that partons are quarks, but that's in hindsight.

Finally, "You're right about quarks vs. partons, but I didn't want to get into that on my plan, which is already physics-wonkish enough."

I'd include the url for the Susskind talk, but I'm not allowed to do so until I rack up a few more posts (I visit these forums rather infrequently, and should really be working).
 
  • #12
daschaich said:
I'd include the url for the Susskind talk, but I'm not allowed to do so until I rack up a few more posts
Very interesting. You can post the url in a verbose mode allowing us to guess what it actually is, such as (replace blanks by dots) :
www google com
edit
the point of you not allowed to post actual url is to prevent automatic robots to read the links, not humans :smile:
 
  • #13
Fair enough. Try edge.org/3rd_culture/susskind03/susskind_index.html
 
  • #14
According to an interview, Gell-Mann termed partons, "put-ons", and, clearly, was unhappy that his quarks were being ignored by Feynman.(See, The Second Creation, Crease and Mann, older but a good account of the birth of the Standard Model, and beyond to 1995)
Regards,
Reilly Atkinson
 

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