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How can we be certain that particles such as quarks and leptons are in

  1. Jun 19, 2013 #1
    Centuries ago, Greek philosophers (such as Democritus) postulated that atoms were the fundamental building blocks of matter. Then, in the 1900s Rutherford along with others discovered that atoms consisted of electrons surrounding a nucleus made up of protons and neutrons. With the technological improvement of particle accelerators, a plethora of new "fundamental" particles were discovered in the 1960s and the years that followed.

    I can't help but wonder; how do we know that particles such as quarks and leptons are not in turn, made up of even smaller particles? Perhaps our accelerators have not reached energies capable of showing us such evidence.

    One should note, that my question originally arose because I was reading an article about the ILC, and it made a comparison between leptons and hadrons (in reference to the LHC). It claimed that producing collisions with leptons at weaker energies than the LHC could produce, would be ideal for research because leptons are fundamental particles whereas hadrons are not and could be subject to colliding at various angles (which in turn, could alter the results).
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  3. Jun 19, 2013 #2


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    We cannot be sure (and the LHC experiments look for possible signs of compositeness), but there is good evidence that the known particles are elementary:

    - the Lande g-factor of electrons (2.002319304) agrees with the prediction for elementary particles with an incredible precision (all 10 digits I quoted are correct). For composite particles (like protons and neutrons), the value can be "anything" (proton: 5.59, neutron: -3.86), a random agreement would be extremely unlikely.
    - the standard model is extremely successful, and it is based on the assumption that quarks and leptons are elementary
    - no substructure was found yet. If a particle with a mass of .5 MeV to a few GeV consists of multiple particles of (at least) a few TeV, the coupling between those particles needs significant fine-tuning to give such a low mass as result.
    - probably some more things I forgot
  4. Jun 19, 2013 #3


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    We don't, for sure. But every theory along those lines that's been developed so far (preons, technicolor, etc) has failed to agree with the observations (and it's not just a question of unreachable energies).

    One big advantage that lepton colliders have over hadron colliders is precision. When you collide protons, it's really a pair of quarks inside them that are colliding, and those quarks have a large spread in energy which you have no way of controlling and have to average over.
  5. Jun 19, 2013 #4


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    If you believe in string theory, everything IS made up of something more fundamental --- strings !

    I'm not holding my breath, though. it seems that after a zillion hours of really smart people working on it over something like 40 years, string theory has produced nothing but elegant theory and no substance.
  6. Jun 19, 2013 #5
    Thank you both for your responses.
    I'm not sure I understand what you mean, would you care to elaborate? It would be much appreciated.

    That's very interesting, I didn't know about the theorized preons. And thanks for the tidbit about the lepton colliders.
  7. Jun 19, 2013 #6
    If there appears to be observational/experimental evidence that quarks and leptons are not composite, and are indeed fundamental - shouldn't that dispel arguments for string theory? Or does the idea that such particles may be composed of more fundamental strings differ from the idea that quarks are not composite particles?
  8. Jun 19, 2013 #7


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    Well, some interesting mathematical tools to calculate some tricky problems.

    Well, consider a hydrogen atom: It has a mass of ~938 MeV, but you need just ~13 eV to see its substructure.
    Positronium is a bit better, just 1 MeV of mass and 6 eV needed to see its substructure.

    To improve that ratio, you would need a very strong coupling - a binding energy very close to the rest mass of the particles.

    You are looking for a particle with a mass ~MeV to GeV, with TeV of energy needed to see its substructure.
  9. Jun 19, 2013 #8
    String theory doesn't so much predict that quarks and leptons are *composed* of strings, it is more that they simply *are* strings. So instead of fundamental point particles you have fundamental strings.
  10. Jun 20, 2013 #9
    Okay, I think I understand. Thank you for sharing your knowledge.

    Oh, I see. Thanks for the clarification.
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