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Quarks and Leptons: Fundamental or not?

  1. Nov 30, 2004 #1
    I've been trying to research this area but there doesn't seem to be much out there. Does any one know of any evidence or theory showing that quarks and leptons are or are not fundamental?

    I have heard of String Theory but does string theory actually suggest that quarks and leptons are made up of strings or is there an intermediate particle between strings and quarks and leptons?

    I've also come across a hypothesis called the MATPELUKA hypothesis. But I can't find any info on it. [Part (e) of this Document] Has any one else heard of this?
     
  2. jcsd
  3. Nov 30, 2004 #2

    Haelfix

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    I wouldnt trust every theory you come across on the internet.

    AFAIK, other than String theory the only one I am aware off is the theory of preons, and that suffers from severe theoretical problems.
     
  4. Dec 2, 2004 #3
    Do you have any links about Preons?

    Also, are there any arguments out there to argue that quarks ARE fundamental?
     
  5. Dec 2, 2004 #4
    I don't
    Here's a link about BPS preons and the BPS preon conjecture
    http://xxx.arxiv.cornell.edu/abs/hep-th/0312266
    Well, now that i'm here, I beg if someone can tell me what person proposed the preon model. I've searched for it for a long time without luck
     
    Last edited: Dec 3, 2004
  6. Dec 3, 2004 #5
    Here's a model that postulates that quarks are made of a particles called primons
    http://arxiv.org/PS_cache/hep-ph/pdf/9901/9901234.pdf
    The more important theory of particles that we have is the Standard Model, that postulates that quarks are pointlike particles. If is ever found that quark compositeness is true, it will be a revolution in the foundations of the SM
     
  7. Dec 3, 2004 #6

    Nereid

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    On the experimental/observational side, a quick summary might look like this:
    - the Standard Model (SM) is consistent with all such results, to date
    - in the SM, quarks and leptons are fundamental
    - however, there are several big questions which the SM doesn't address (of course, the questions themselves may turn out to be misconceived)
    - there are a number of solid experimental/observational results which hint at 'new physics', perhaps just beyond the domains within which the SM has been tested to date
    - however, none of these hints are at all strong wrt quarks and/or leptons being not fundamental.
     
  8. Dec 3, 2004 #7
    As an addendum to what Nereid wrote...A friend of mine at college is involved into an experimental study based upon a field theory that is effective. This means that quarks are NOT considered as the degrees of freedom of this theory. In this case, the field theory called Quantum Hadro Dynamics does a better job then QCD at explaining some experimental results.In QHD, the degrees of freedom are hadrons...

    I admit this sound vague but what i am trying to say is the working with fundamental degrees of freedom like quarks in QCD does not always give rise to the best theoretical model.

    regards
    marlon
     
  9. Dec 3, 2004 #8
    QHD is in fact much older than QCD. This is the traditional, effective approach. Historically, people went desperate about QFT exactly because QHD requires to introduce new fields for every single new hadron (or at least hadron multiplet). So yes, QHD might be very efficient, but does not tell you much about Nature at a fundamental level.

    Nereid's post is very clear !
     
  10. Dec 5, 2004 #9

    arivero

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    You can look around scholar for references to Barut & Bracken, Phys Rev D v 23 n 10 p 2454 , where internal structure to the electron was suggested. On the other hand, you could look for technicolor, which in some approaches could let you to think on substructure.
     
  11. Dec 7, 2004 #10
    Alejandro,
    my (scarce) understanding of the technicolor model is that it postulates the compositeness of the Higgs boson (formed by particles called techniquarks) but I have never heard that it suggested compositeness of quarks. If you have a reference I would appreciate it.
     
  12. Dec 15, 2004 #11
    If Abdus Salam was not the originator of a preon model that quarks are composites of smaller particles, he was one of a small group. You might want to read this address in which he predicts the discovery of the preon by 1990: http://nobelprize.org/physics/laureates/1979/salam-lecture.pdf
     
  13. Dec 15, 2004 #12
    Many thanks! Your link has lead me to discover that the preon model was proposed in 1974 by Salam. What I didn't know is that preons could be divided into chromons, flavons and somons
     
  14. Dec 16, 2004 #13

    ohwilleke

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    There are a number of studies which have shown some evidence of fractional electron charge, which would make sense if quarks were fundamental (with their 1/3rd charges), but electrons were not. The trouble is that while some of the fractional electron charge studies seem to show the expected -1/3 charge, others seems to show -1/5th charges or other unexpected values. I don't have the references at hand, but have seen at least three or four such studies.

    See, for example, http://www.bell-labs.com/news/1998/october/13/2.html discussing an award granted for discovering the fractional quantum Hall effect. The theoretical reason for this (i.e. is it substructure of an electron or some form of interaction effect with electrons remaining fundamental) is in flux. Dozens of papers in the few years alone address the topic theoretically.
     
    Last edited: Dec 16, 2004
  15. Dec 16, 2004 #14

    Nereid

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    IIRC, all but one of the studies were shown to have flaws and/or gave inconsistent results when repeated by different groups. The only one that seemed to emerge more or less OK was one with Nb spheres, a version of the Millikan oil drop experiment. The problem there was that no other, similar experiments found anything.

    Sorry, I don't have references either.
     
  16. Dec 16, 2004 #15

    ohwilleke

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    Nice charts showing the FQHE are shown here:
    http://www.warwick.ac.uk/~phsbm/fqhe.htm

    A survey of the leading FQHE theories is here:
    http://arxiv.org/PS_cache/cond-mat/pdf/0108/0108271.pdf

    An interesting theoretical discussion of the possibility of rare electron decay into a neutrino and a photon is discussed in this article: http://xxx.arxiv.cornell.edu/abs/hep-th/0312325

    A clear position is set out in C.S. Kalman, "Why quarks cannot be fundamental particles" here: http://arxiv.org/abs/hep-ph/0411313

    Recent experiments supporting the standard model are found here:
    http://arxiv.org/PS_cache/hep-ex/pdf/0408/0408004.pdf
     
    Last edited: Dec 16, 2004
  17. Jan 1, 2005 #16
    The trouble is that while some of the fractional electron charge studies seem to show the expected -1/3 charge, others seems to show -1/5th charges or other unexpected values. I don't have the references at hand, but have seen at least three or four such studies.

    I spent time reading up on the experiment that won the Nobel prize for discovering fractional charged particles. To the best of my recollection no plus or minus sign appeared in the paper, but as reference was made to 'electron field' a minus charge was indicated, However, one writer clearly stated that the one thing Tsui could not explain was "why the fractional charge was positive".
    I lost my files in a computer crash and have not repeated the search. Does anyone know the true situation?

    Before Tsui et al there was a paper published that gave a table of observed and predicted fractional charges. The main column was 1/3 2/5 3/7 4/9 etc with two shorter cols of different fractions on either side. (these were 'fractional charges' not the 'fractionally charged particles' proposed by Tsui et al..
     
  18. Jan 3, 2005 #17
    I'm pretty sure that quarks and leptons aren't fundemental. They probably have some type of substructure. Hey, "atom" in greek means uncuttable, but it was cut. Same with nucleons. Whether or not there constituents are "particles," as we know them, is beyond me.
     
  19. Jan 3, 2005 #18

    Nereid

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    Do you know of any good experimental results which even hint that they may not be? Please give us some links!
     
  20. Jan 4, 2005 #19
    Please revisite what I posted and try notice my logic here. I'm just saying that everything we've labled in the past as "fundemental" has ended up having a type of substructure. Assuming something is fundemental because it's the smallest building block you know of at the time is hasty and unnecessary.

    What evidence hints towards smaller constituents of matter? What evidence is there that says there isn't? I don't know of any evidence that says quarks and leptons cannot be made of smaller components. Perhaps (better yet there propably is) there some papers or experiments that do suggest quarks and leptons are fundemental I'm simply not aware of. If so, let me know.
     
  21. Jan 4, 2005 #20
    Entropy,

    in QCD, quarks are regarded as the fundamental degrees of freedom. Now, whether or not there is some model in which they can be treated as non-fundamental and therefore they exhibit some underlying structure is certainly not well established and therefore very speculative. What i am saying is this...if you wanna describe quarks in any way, QCD will do the best job at this moment. So basically this discussion is speculative and useless. Suppose quarks were not fundamental then their mutual properties wouldn't change because the model you would construct MUST respect the rules and results produced by QCD. Just like QM respects Newtonian physics in the right limit (you know, large enough objects).


    As to your last question : Just goto http://arxiv.org/ and type in QCD or something with quarks...Eitherway, the one will lead to the other...you see my point :wink:


    regards
    marlon
     
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