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Critical distance for quark exchange

  1. Nov 19, 2004 #1
    I was wondering if their is a critical distance for quark exchange. say you have a proton and a neutron in the nucleus. When the quarks get very close to each other they exchange from the proton to the netron. they can no longer tell the differance between its up quark and the neutron's up quark so they exchange.

    Is this at all true.
    Last edited: Nov 20, 2004
  2. jcsd
  3. Nov 20, 2004 #2
    quark exchange ?


    If you're talking about n->p+e+neutrino process, I don't understand the exchange of quarks. In this case, there is "just" a change of nature of the quark (d->u+e+neutrino) but no exchange.

    I assume this is not the answer you were wondering, so try to develop your question and I will try to help you furthermore.

  4. Nov 20, 2004 #3
    you gotta be careful here. The strong force holds baryons and mesons together and is mediated through gluon-exchange between the constituent quarks. I have studied models (dual abelian Higgs-model) where the distance between such quarks (in a baryon or meson) is estimated to be less then 0.7fm when we assume that the three quarks of the baryon are placed on the corners of a triangle. In this case the whole system is to be looked at as one three-body-problem. Bigger inter-quark-distances will give rise to a Y-shaped structure.

    The atomic nucleus is bound together by the residual strong force and is mediated by the lightest mesons called the pions. Now, when two baryons (like a proton and a neutron) come close enough to each other, the valence quarks of the first baryon will interact with the valence quarks of the second baryon. What happens is this : two valence quarks from the two different baryons will feel a linear potential between them. When they are pulled apart, the potential rises which leads to an unstable connection between the two quarks. This leads to the fact that the connection is broken apart and the available energy is used to form a quark-anti-quark pair which is the pion. Energy is converted into matter via E =mc² and when enough energy is available this created pair can exists for quite a while because of Heisenberg uncertainty. All these things are described by QFT, which needs to be seen as the unification of special relativity and QM because of the previous two reasons (E=mc² and Heisenberg uncertainty from QM). At distances of about 1fm, the quark pair creation speed is maximal, yielding a maximal interaction between baryons of the atomic nucleus.

    It needs to be said that these are result that are predicted by the denoted model above and other models will give you other numbers, allthough they are in the same range. If you wish i can give you a link to the model i mentioned as some sort of reference, but you are gonna have to know your QFT thoroughly because it is heavy material.

  5. Nov 23, 2004 #4
    what is a quark? i'm doing as-level physics and hardly understand anything you people say

    day without sunlight............is like........................well...................scary!!!
  6. Nov 23, 2004 #5


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    In the early 1960's Murray Gell-Mann proposed that subnuclear particles (protons, neutrons and various mesons) could be explained by the existence of three "states", each with an anti-state, that satisfied certain laws of group theory (they were "representations of the Lie group SU(3)"). His theory was called the Eightfold Way. He thus collated a lot of behavior that physicists already knew about into one tidy package, and actually predicted a new meson with it, but he refused to consider the states, which he called "quarks" to be real. He thought they were just mathematical abstractions. In this theory, the proton had three quarks, two of the kind called "up" and one of the kind called "down". And the neutron also had three quarks, two "downs" and an "up".

    About five years later, physicists shooting electrons at nuclei found evidence for three pointlike particles inside the proton and the neutron. It was natural to identify these phenomenal particles with the theoretical quarks, and when this program was fully carried out, the result was quantum chromodynamics (QCD). Quarks are now thought to be particles that make up the nucleons and mesons and obey the physical laws embodied in QCD.
  7. Nov 23, 2004 #6
    I made a few pictures to show the quarks in a nuetron and proton. From the picture you can not tell the which is exactly the proton. their are at least
    two different arrangements that give a proton and a neutron. So it seemes to me that when bayons get close enough to each other they can "exchange a quark.

    Also wouldent their be so called magnetic moments from the arrangement much like polar molecules?
    thanks matthew davis

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  8. Nov 23, 2004 #7
    Sorry but these pictures do not give the correct image of the quark-structures in a baryon or a meson.

    Correct a Y-shape or a triangle shape

    How do you come to this conclusion. Besides no quark is exchanged between baryons. A pion is exchanged. Read my first post here...

    Quarks are fundamental particles,...indeed they do exhibit charge but not like electrons give rise to currents...they exhibit colour-charge...

  9. Nov 23, 2004 #8
    I seperated them to show triangle shape. one group is the proton (with two up quarks one down). the other is a neutron(two down quark one up).

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  10. Nov 26, 2004 #9
    Quarks are not really like electrons in a metal which can go anywhere they like. Gluons rein them in, and their movements are very limited.
  11. Nov 27, 2004 #10
    This is untrue. First of all no gluons rein in quarks because they are elementary in nature (well for the QCD-people at least not for the string-people). Quarks interact with eachother via gluons and pions and they need to be seen as being drowned into a gluon-sea. Quarks can also move "quite" a distance (well, at the right lentgh-scales offcourse) when enough energy is applied on the quark-system. I refer to the way that quarkpairs are created via screening and flux-tube-lengthening...Remember that a PAIR will be created and no single quarks because of asymptotic freedom...

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