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Elementary particle

  1. Apr 3, 2008 #1
    Do you think there is such a thing as an elementary particle, regardless of whether such a thing has been detected/measured?

    What do you conceive an elementary particle to be?

    Dan Howitt Ny
  2. jcsd
  3. Apr 3, 2008 #2
    I take the definition of elementary to be "point-like". In other words, no matter how much momentum you transfer to the particle (probing it at shorter and shorter distances), you see no structure. This is a working definition, and as such it suffers from what every other working definition may suffer: it changes with time. 150 years ago, we thought the atoms were elementary. 100 years ago we thought the nuclei were elementary. For as little as 50 years ago, we thought the proton was elementary. But as our technological prowess improves to probe deeper, we discover that some things that appeared elementary before start exhibiting structure. There are alternate definitions of elementarity based on the field theory, the string theory, and what not. But I like my working definition better than any of them. If you could only observe events that transfer less than 1 Mev of momentum, then the proton is indeed elementary, and you don't need to know anything about quarks and gluons.

    At the moment we believe all leptons, quarks, and the gauge bosons to be elementary. The evidence for the photon and the electron is very strong. The evidence for the muon and the neutrinos is a bit weaker, but still good. The evidence for quarks and gluons is not bad either. The evidence for W and Z is weak but improving.
  4. Apr 3, 2008 #3
    In condensed matter we play with a different definition: a particle is something that doesn't interact very much. The problem with the definition given by fermi above is in what counts as substructure. At higher and higher levels of confinement things become murky. It's easy to say that atoms are made of electrons and nucleons. But it's already becoming sticky to say that the nucleus is made of protons and neutrons, and it's pretty much an outright lie to say that protons and neutrons are made of quarks --- you'll never manage to knock a quark out and see it. At the same time, it's possible to create gigantic jets of heavy particles by banging an electron and positron together, but no one seems to consider those to be part of the substructure of the electron or positron.

    The question about elementary particles is not something physics is ready to answer properly yet.
  5. Apr 3, 2008 #4
    Well I guess we at the lab are a bunch of liers, because we do that everyday with DOE money from people taxes. We actually take quarks out hadrons and we test QCD which tells us that indeed, the quark goes out, travels on the light cone, and then goes back in the hadron. Of course this is a semi-classical picture of really quantum correlators, but I would call this "chat" a gross lie.
  6. Apr 4, 2008 #5
    Perhaps I was over stating the case. However, you cannot claim to have taken a quark out. Quarks are well-defined (though only as well-defined as a photon) in the infinity energy limit --- when it is most likely that new physics (unification, gravity, etc.) are important. Otherwise, you have a strongly interacting system in which to even consider quarks as particles lead you into divergent perturbation sums. I assume that your QCD calculations are done with lattice QCD? It seems an often understated point that lattice QCD does not proceed via the usually understood picture of QFT calculations --- in a sense it's far truer to the fields picture than the particles picture.

    Just to re-iterate, I don't think particle physics is wrong, or that it's worthless -- far from it. However, I do think particle physicists need to consider carefully what it is that they are actually probing. The complete reliance on scattering experiments biases the theory. For instance, even things like photons and electrons are really defined in the free limit, and technically they represent states with infinite extent in space and time. As such no experiment is really going to detect those states. However, in typical scattering experiments you measure sufficiently far from the regions of interaction that the particle states are pretty close (actually, the two don't converge --- but the measured values do; it's a slightly subtle point that I haven't completely understood the implications of). Thus, since scattering is all that is really ever done, the theory and concept is then reinforced, and seen as absolutely correct.

    Perhaps a concrete example: the W and Z bosons. They are the low energy, broken symmetry bosons which have eaten Goldstone modes. Therefore, at higher energies they would mix with each other and photons to produce some more symmetric set of electroweak gauge bosons. Would you then say that those are not elementary? But those also have no "substructure", as I understand the term.
  7. Apr 4, 2008 #6
    I CANT understand
  8. Apr 4, 2008 #7
    I don't have much time, but let me at least answer a few
    I beg to differ, we make such a claim.
    That is completely wrong
    • Quark are asymptotically free, true, but asymptotic freedom appears much before any quantum gravity or grand unification scale.
    • I claim we do make non-perturbative calculations at low energy where quarks are well defined

    Fine, just don't do perturbations.
    No, I am refering to theorems proven within QCD and called "factorisation theorems" which ensure that scattering occurs at the quark level in a certain mathematical limit. There are tests to make sure this limit is attained.

    Oh, but please !, be certain that no single week do we forget to discuss that. Seriously :smile:
  9. Apr 4, 2008 #8
    As Dirac once said, "this is not a question". :tongue2:
  10. Apr 4, 2008 #9
    Technically, the freedom occurs at infinite energy, but I take the point that the interaction becomes sufficiently weak for a free theory to become feasible far, far before that. I think we may be dancing around terminology here. If I understand you correctly, you're saying that even in the low energy limit you can identify states which are adiabatically connected to the free quark states. If so, I think I agree, but disagree as to whether these are then elementary --- rather I don't think the term elementary has a forceful meaning, since all particles are something like localised field excitations, just a matter of which field. So for instance at the energies that we can currently access in an accelerator we see certain fields which may (and some we think do) mix and become the same at higher energies.

    I'm afraid my knowledge of QCD stops before then --- but it sounds like what I said above --- adiabatic continuity of the states. Same thing is used in condensed matter to claim that electrons are moving freely in metals, which is again a very useful picture, but one has to be careful about identifying these electrons (which share quantum numbers with "real" electrons).

    Sorry if I appear patronising --- it is not intended. I'm sure your understanding of particle physics is beyond what I possess. However, you may be familiar with the caveats of terminology that someone not steeped in the field will not possess. It may be best that we quit this bickering, since we both agree on the practical aspects like the outcomes of actual experiments, and arguments about words are not physics!
  11. Apr 8, 2008 #10
    Really helpful.
  12. Apr 8, 2008 #11
    I am interested in these free quarks. Could you provide a peer reviewed reference?

    And for Dan, sorry that this thread got hijacked. I'm sure that from a practical perspective, elementary particles do exist... it's not "turtles all the way down", if you know what I mean. But from what we know about field theories, and particles as we know them are only approximations to field theories, that the known theories are probably effective theories of some better, underlying theory. That theory probably has spacetime as an emergent feature, and not a fundamental one. So if spacetime isn't a fundamental entity, then particles probably aren't either. It's hard to define the notion of a point particle without a space to put it in.
  13. Apr 8, 2008 #12


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    What happened with the classy "a particle is a state of the Poincare Group in Wigner sense"? Or a real pole of the S matrix?
  14. Apr 8, 2008 #13
    Ah, but then quarks are definitely not particles :smile:
    This is not strictly equivalent to the other because of quarks again, is it ?
  15. Apr 8, 2008 #14
    Individual quarks have colour, so how do these free quarks remain gauge invariant?
  16. Apr 8, 2008 #15
    Maybe I misunderstand, but depending on exactly what you mean it is either a very good question or a very bad one :smile:

    If it is a very good question, you may already ask it for the electron and EM gauge invariance ! in this case, you may notice that to resolve this one, we usually impose "periodic boundary conditions" which amounts to putting (mirror) charges at infinity.

    If it is a bad question, it may stem from a confusion between color singlet and gauge invariant states. On one hand, color singlet are invariant at all and may fly at infinity. On the other hand, gauge transformations will indeed re-define you quark field in the proper manner such that the lagrangian is still invariant.
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