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A Are hadrons quasiparticles?

  1. Apr 20, 2016 #1

    Demystifier

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    First, let me ask moderators not to move this thread to the High Energy, Nuclear, Particle Physics forum, because I am interested in hadrons from a wider perspective, especially from the perspective of condensed-matter QFT.

    A hadron (e.g. proton, neutron, or pi-meson) is a complicated mixture involving 2 or 3 quarks and a sea of gluons. From perspective of non-perturbative QCD, one would say that it is a bound state of quarks and gluons. But how about condensed-matter perspective? Can we say that hadron is a collective particle-like excitation of quark and gluon fields? Or more specifically, can we say that hadron is a quasiparticle in the condensed-matter terminology? If it is not a quasiparticle, then what property of quasiparticles is missing in the case of hadrons?

    A wider goal of such questions is to better understand the similarities and differences between QFT concepts in high-energy and condensed-matter physics.
     
    Last edited: Apr 20, 2016
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  3. Apr 20, 2016 #2

    stevendaryl

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    A quick look using Google seems to show that people don't use "quasiparticle" to include "composite particle", but I'm not sure if there is a principled reason why not.
     
  4. Apr 20, 2016 #3

    A. Neumaier

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    no, it is a bound state, not a mixture.
    It has far too few constituents. To define a quasiparticle in the condensed matter sense you need a macroscopic amount of substance.
     
  5. Apr 20, 2016 #4

    Demystifier

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    Roughly how many gluons a hadron has?

    Why a macroscopic amount of substance is important?
     
  6. Apr 20, 2016 #5

    A. Neumaier

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    The first question has no answer since gluons are massless; hence there is an unlimited number of very soft gluons (which don't count in a statistical mechanics treatment). One has to sum these up into a coherent state. The contribution of hard gluons (which would count) is small; one can probably even neglect it to a good approximation.

    A macroscopic amount of substance is important in order that statistical mechanics (in the usual sense) is applicable. Formally, one has to perform a thermodynamic limit, in which the Bogoliubov transformation that defines the quasiparticles produces a representation inequivalent to the vacuum representation (in which the ordinary particles are defined).
     
    Last edited: Apr 20, 2016
  7. Apr 20, 2016 #6

    Demystifier

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    Why is it important that statistical mechanics is applicable? Are you saying that quasiparticles only make sense in the context of statistical mechanics?
     
  8. Apr 20, 2016 #7

    A. Neumaier

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    What would be your definition of a quasiparticle without statistical mechanics?
     
  9. Apr 20, 2016 #8

    Demystifier

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    A primary example of a quasiparticle I have in mind is a phonon, as described e.g. in Sec. 1.1 of Ben Simons's Concepts in Theoretical Physics which can be freely download here
    http://free-ebooks.gr/en/book/concepts-in-theoretical-physics [Broken]
     
    Last edited by a moderator: May 7, 2017
  10. Apr 20, 2016 #9

    A. Neumaier

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    The phonon makes sense only in a solid lattice, hence presumes a many-body system in the background.

    Of course you may consider relativistic QFT with cutoff as a strongly interacting many-body system of bare particles (a kind of ether) and the vacuum as its ground state. Then all elementary particles (not only the composite ones) become quasiparticles in this picture. The problem is that this picture makes no longer sense if the cutoff is removed.
     
    Last edited: Apr 20, 2016
  11. Apr 20, 2016 #10

    Demystifier

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    Why solid? I think there are phonons in fluids too. I agree with the importance of the many-body background, which leads us to the next point ...

    Yes, that philosophy is much more in the spirit I had in mind. But intuitively, hadron is somehow more "quasi" than quark or gluon, in the sense that it is more "emergent" and less "elementary". I am not sure if such intuition can be made to make more sense.

    Fine, but QFT without cutoff does not make sense for many other reasons, so let us work with the cutoff all the time.
     
  12. Apr 20, 2016 #11

    A. Neumaier

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    With a cutoff, the many-body background would be a non-covariant ether consisting of highly interacting bare particles with an interaction that depends heavily on the cutoff.

    On this background one can liken elementary bosons to phonons in a solid or fluid, elementary fermions to effective electrons in a conductor, mesons to Cooper pairs in a superconductor, and baryons in a more figurative way to a kind of ''Cooper triples'', since they are dressed versions of single particles, particle pairs, and triples. But I have no idea what one would gain from using this analogy (which apparently was widespread in the 1950s but since then lost ground). Maybe because you want to build a Bohmian mechanics on top of it? But then you have to explain why all the couplings are extremely sensitive to the cutoff!

    In practice, it is rather the opposite way, that one wants to liken quasiparticles to particles! Indeed, one works in condensed-matter physics with quasiparticles precisely since these behave somewhat like elementary quantum particles.
     
  13. Apr 20, 2016 #12

    Demystifier

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    I don't see why would that be a problem.

    That too, but I have other reasons as well:
    http://arxiv.org/abs/1505.04088
     
  14. Apr 20, 2016 #13

    atyy

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  15. Apr 20, 2016 #14

    A. Neumaier

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    It is of the same kind as the fine-tuning problem in grand unification. It is not a problem if you treat the interactions as God-given.
     
  16. Apr 20, 2016 #15

    A. Neumaier

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    Weinberg's notion of quasiparticles is different from that of condensed matter physics.
    One can see this from the abstract of his 1963 paper http://journals.aps.org/pr/abstract/10.1103/PhysRev.131.440 [Broken].
     
    Last edited by a moderator: May 7, 2017
  17. Apr 20, 2016 #16

    atyy

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    I was thinking in the sense that a "quasiparticle" is a particle in the low energy effective theory. Eg. if the electron is emergent, then it is a quasiparticle. So in that sense, Weinberg's quasiparticles are quasiparticles. (I see you said something like this in poist #11.)
     
    Last edited: Apr 20, 2016
  18. Apr 21, 2016 #17

    A. Neumaier

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    Weinberg's paper is not about low energy approximations. He calculates the bound states exactly from a bigger Hilbert space in which free quasiparticles are added artificially to be able to do an improved perturbation theory. The bound statres computed are complicated composites of the original particles and the added quasiparticles. This is very unlike what is done in condensed matter theory.

    On the other hand, post #11 assumes from the start that we have a cutoff (in order to be able to define the background), which means working in a truncated Hilbert space and then defines quasiparticles to be the bound states of the truncated description. This is analogous to thedressing of an elecron in condensed matter theory.

    Thus the two concepts of quasiparticles go into opposite directions.
     
  19. Apr 21, 2016 #18

    Demystifier

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    How else can interactions be given?
     
  20. Apr 21, 2016 #19

    A. Neumaier

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    Many people interested in grand unification or string theory believe that a really fundamental theory should have no tunable parameter (except those for which the predictions are fairly insensitive) but be completely determined by symmetry principles. To need the constants to 20 digits relative accuracy to get a prediction accuracy of 2 digits is considered by them as too unnatural for a fundamental theory. This is what is behind the so-called fine-tuning problem.

    If one works with a fixed value of cutoff and bare couplings, one has the same problem.
     
  21. Apr 21, 2016 #20

    Demystifier

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    OK, but I don't think that QFT is fundamental theory. And I am not alone, many high-energy physicists do not think that QFT is fundamental theory. Many (including Weinberg) hold the view that all quantum field theories are only effective theories. Such a view is also implicit in the Wilsonian approach to renormalization (group), which is rather condensed-matter-like in spirit, and yet quite popular among high-energy physicists.
     
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