Parton? quarks? what is the difference?

[Vazier]

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...

 

[humanino]

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.

 

[nrqed]

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.

 

[Vazier]

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...

 

[humanino]

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.

 

[humanino]

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.

 

[xristy]

... 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

 

[humanino]

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.

 

[nrqed]

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.

 

[xristy]

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

 

[daschaich]

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).

 

[humanino]

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

 

[daschaich]

Fair enough. Try edge.org/3rd_culture/susskind03/susskind_index.html

 

[reilly]

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