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Pions or Gluons?

  1. Aug 30, 2004 #1
    I;ve heard 2 different things, and onw says that the strong force is mediated by the gluon, but also, I've also heard that it is mediated by pions. The strong force, AFAIK, is actually caused when the 3 quarks in a hadron, continually exchange gluons with each other. And if a red quark, emits a red-anti-green gluon, its neighbour, say a green quark, changes into a red quark, while the emitter changes into a green quark. So, this goes on and on and on, and somehow creates the strong force, this strong force, is yea, kinda strong, so it actually seeps out and there's a residual force holding the nucleus together.

    Is my understanding right on this one??

    And as for pions, which are mesons, how does it actually mediate the strong force? Is any one theory false? or is both of them somehow related?
  2. jcsd
  3. Aug 30, 2004 #2
    You are understanding quite correctly. At a fundamental level, the strong interaction is due to gluon exchange between quarks. In this way quarks make up hadronic particles such as the proton and the neutron. Now in a nucleus, the proton and neutron are stacked very close to each other, and this is due to meson exchange. Pions come inthree "isospin" sorts : neutral and electrically plus or minus 1. They are the lightest mesons, and are themselves made up of one antiquark and one quark. In our current undestanding of the nuclear force, the heavy proton and neutron (made up of three quarks) are hold together in the nucleus by meson exchange. This is absolutely analogous to what holds molecules together : the electric force holds the electrons to the nucleus in the atom, and a residual dipole force holds together atoms in molecules, this residual dipole force being due to exchange of pairs of electrons (Van de Vals force). In the nucleus, the force holding protons and neutrons together is a residual force due to meson (pion) exchange.
  4. Aug 30, 2004 #3
    Hmm, I see, Then in this case, where do the pions come from??
  5. Aug 30, 2004 #4


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    Isn't it Van der Waals's?
  6. Aug 31, 2004 #5
    It is indeed Van der Waals.

    Gluons are the force carriers of the strong force between quarks. For example three quarks form baryons (like neutrons and protons) and two quarks form mesons (like pions). The way these three or two constituent quarks are bound toghether is determened by the fact that the colour of each quark must sum up to white. I mean by this that for a baryon or meson there is NO net colour-charge.

    Pions,which are built out of a quark-anti-quark-combination, are also force carriers of the strong force as determened by Yukawa. They mediate the residual strong force which binds protons and neutrons toghether to form a nucleus. basically two hadrons (the name for baryons and mesons, or all particles that feel the strong force) come close enough together, so that the constituent quarks of one hadron can directly feel forces from constituent quarks of the other hadron

    So gluons hold quarks together to form hadrons (baryons and mesons)
    Pions hold hadrons together to form atomic nuclei. Pions , themselves are bound together by the strong force because they are mesons. Gluons are quanta of the strong-interaction, so they are elementary particles on themselves.

    Last edited: Aug 31, 2004
  7. Sep 9, 2004 #6
    yes, I guess I know that, but is there a relation to the creation of pions and the constant changing of colour inside the nucleon? Like how is the process of colour-changing within the hadron "create" the strong force and bind the nucleons together, and where do pions come in this whole "colour-changing" picture within the nucleon?

    Isn't this constant colour-changing caused by the gluon exchange within the nucleon responsible for holding nucleons together?
  8. Sep 9, 2004 #7

    I don't really understand what your exact question is ??? I mean, seems to me that I already answered to these questions... The role of both gluons and pions plus the big difference between then is already explained in the previous posts.

    What is it that you want to know?

    Sorry for my ignorance...
    I do really wanna help you out

  9. Sep 9, 2004 #8
    yea, I think you have explained pretty well the role of gluons and pions and both are part of the strong force which keeps the nucleus in place.

    Actually, I'm more interested in not what is the big difference between gluons and pions, but in what way are they similar ! In this case, similar as in, both of them mediate the strong force in the nucleus, (even though the gluon, being the gauge boson is the true mediator of it) and both of them binds nucleons together, so how is the process of continuous colour-changing within the nucleon related to pion exchange, since both processes are responsible for holding the nucleus together?

    I always hear people talk of these 2 processes as 2 different processes, but how are they related since they perform the same function?
  10. Sep 9, 2004 #9
    Pions do NOT hold the nucleons together, that's the job of the gluons. Don't mix them up. The strong force and the residual strong force are two different processes. Suppose two nucleons come very close to eachother, so that the quarks of nucleon one can feel the quarks of nucleon two. This "feeling" is ofcourse a manifestation of energy. Once the interaction between those "valence quarks" is "strong enough" a quark-anti-quarkpair will be created out of this energy, since energy and mass are equivalent. This quarkpair is called the pions (lightest meson), which certainly have mass, gluons not ofcourse.

    Basically gluons and pions are of totally different physical nature, though they mediate to some extent the same force. ofcourse the colour-neutrality has to be respected at all time during quark-interactions

    Last edited: Sep 9, 2004
  11. Sep 9, 2004 #10
    This is a very difficult question, and nobody has been able to provide a totally satisfactory answer yet. If you already know waht effective forces are, I suggest you take a look at this recent paper, (Quark structure and nuclear effective forces, P.A.M. Guichon, A.W. Thomas nucl-th/0402064) and don't bother read the remaining of my post. I quote a recent paper, but the literature is so vaste, that in any case I would have to be partial. I also strongly propose this very complete document by professor Jacek Dobaczewski.

    If you don't : there are several (many) models of the nuclear forces. Roughly speaking, none of them cares too much about strictly deriving the inter-nucleons force from the inter-quarks forces, because it is too difficult and with reasonable arguments, one can avoid this strict derivation and still have a very efficient model for all purpose. These models which are efficient but not strictly derived from the fundamental interaction are called effective models. There is for instance the so-called shell model, constructed by analogy with the electronic structure of atoms. This model can be enhanced by geometrical considerations, including vibrations and rotational excitations, as well as the very important pairing property from which one classify nuclei in 4 categories, according to the even or odd number of protons or neutrons. This is also associated with multipole expansions, as in EM. As you can see, this is very semi-classical physics. You can also take a look here.

    A general feature of those effective forces is that they are the result of a sublte balance between attraction and repulsion which are both large but (as you can guess) attraction is slightly stronger.

    If you want to make a model based on pion exchange, you must be slightly more elaborating : you need a lagrangian, based on isospin symmetry which relates protons and neutrons, and include in the lagrangian a coupling to the pion field (which is a vector).
  12. Sep 9, 2004 #11
    so is it when valence quarks interact, a pion would be formed? Is it something like "pulling" a quark out from a hadron, the "gluon rubberband" kinda thing? Like when i pull a quark out from a hadron, this gluon rubberband gains energy as it stretches, but when it snaps, the energy is gone to create a new meson as well as a quark to replace the quark which was being "pulled out".

    Is that how it works?

    and to the first part that pions do not hold nucleons together. actually, i meant that do pions hold nuclei together? mistake on my part here, is that what you were trying to say??

    and to Humanino:

    Sorry dude, I've read the site and I can't really understand much. My math is not really up to standard, and I don't think I would be able to understand mathematical explanations, and was hoping for a more physical one.

    But thanks anyway....

  13. Sep 10, 2004 #12
    I am giving you as much information as I can on the nuclear strucure and how it is linked to the fundamental QCD interaction. You are asking very relevant questions but I fear nobody knows the answer for sure.

    That would be so great if we could understand it at such an intuitive level. We know pions participate. The neutron for instance constantly fluctuates to a proton+negative pion in the nuclear medium. We also know that mesons in the nuclear medium have an effective mass which is not their proper/rest mass.

    If you think again in terms of effective interactions, there are two dominant meson fields, one of which is a scalar and the other one is a vector. They are not real mesons one could observe outside the nuclear medium. This illustrate the fact that you need to consider all meson species participating together to the true interaction. Of course since the pions are the lightest, their contribution must be important.

    The interaction field is the effective one. It is not a physical meson field. So the interpretation in terms of physical fields corresponding to particles (which one could observe in vacuum, the true particles) is far from straightforward.

    Now if you want to grasp an representation in terms of "How do those effective fields emerge from the fundamental gluon and quark fields ?" this is desperatly hopeless. If you succeed, you might get the Nobel prize and many people here including me would dream to become your student.
  14. Sep 10, 2004 #13
    Well, eeuuuh, no quark is pulled away, you gotta forget that idea. What happens is that when two quarks come "relatively" close to eachother their situation is described by some kind of potential , expressing the energy of their interaction, right ??? Just like two charges of opposite sign feel eachother through the Coulomb-potential.

    This energy is used in order to create the two quarks out of the QCD-vacuum. This might seem strange, yet it is a very well established way of thinking in QFT. Look at the electron in beta-decay for example.

    Be aware of the fact that i am not saying that one quark is created. That would take extremely high energies because of the asymptotic behaviour of the strong force.

    marlon :approve:
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