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Thread: Experimental results indicate revision in the size of the proton

  1. #1 Experimental results indicate revision in the size of the proton 
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    Recently announced results challenge previous estimates of the size of the proton.

    https://muhy.web.psi.ch/wiki/index.php/Main/PR1
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    Quote Originally Posted by DrRocket View Post
    Recently announced results challenge previous estimates of the size of the proton.

    https://muhy.web.psi.ch/wiki/index.php/Main/PR1
    Interesting. I wonder if these results are compatible with QCD as we understand it, specifically numerical calculations via lattice QCD.
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    Quote Originally Posted by Markus Hanke View Post
    Interesting. I wonder if these results are compatible with QCD as we understand it, specifically numerical calculations via lattice QCD.
    I presume your question, in detail, is something like "Is the size of the proton determined by this experiment compatible with the usual value of the coupling constant for the strong interaction ?".

    Interesting question. I don't know.
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    That is interesting. But that just means that the charge of a proton is more localised than initially thought?
    In the age of information, ignorance is a choice.
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    Quote Originally Posted by Kerling View Post
    That is interesting. But that just means that the charge of a proton is more localised than initially thought?
    Sure, but the issue is what this means in terms of the understanding of the strong interaction among the quarks that are the source of the charge on the proton.
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    Quote Originally Posted by DrRocket View Post
    Sure, but the issue is what this means in terms of the understanding of the strong interaction among the quarks that are the source of the charge on the proton.
    Hm... That is very interesting. Maybe quarks shield eachother?
    In the age of information, ignorance is a choice.
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    Quote Originally Posted by Kerling View Post
    Hm... That is very interesting. Maybe quarks shield eachother?
    How so ?
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    Quote Originally Posted by Markus Hanke View Post
    How so ?
    Well, the only way how we can see quarks seperatly is if we pump in so much energy that we can see dots. For all other purposes these quarks have wavefunction with one another. Say we look just at the quarks 2*+2/3 and 1*-1/3. Then the distribution of this charge over a wavefunction can shield the charges from the outside world. Shielding effect - Wikipedia, the free encyclopedia
    So instead of taking the shielding like with protons and electrons. Maybe the principle applies to quarks aswell. I know too little about Gluon to know if this is possible. But why not? Don't see a good reason why not yet.
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    Quote Originally Posted by Kerling View Post
    Well, the only way how we can see quarks seperatly is if we pump in so much energy that we can see dots. For all other purposes these quarks have wavefunction with one another. Say we look just at the quarks 2*+2/3 and 1*-1/3. Then the distribution of this charge over a wavefunction can shield the charges from the outside world. Shielding effect - Wikipedia, the free encyclopedia
    So instead of taking the shielding like with protons and electrons. Maybe the principle applies to quarks aswell. I know too little about Gluon to know if this is possible. But why not? Don't see a good reason why not yet.
    In the classical situation one sees electrons in the outer shell "shielded" by electrons in the inner shells. Thus one has the approximate relationship, as noted in the Wiki article that you cited.



    Where is the number of protons in the nucleus and is the number electrons in lower orbits.

    It is pretty easy to visualize and justify this approximation.

    But in the case of they hydrogen nucleus one already has this in effect, the 4/3 charge being "shielded" by the -1/3 to arrive at the +1 charge that is observed.

    So, do you have in mind something beyond this, and if so, what ?
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    My best guess is that what is going on is a little of what you might call "anti-shielding." The quarks inside a proton are pretty free to move around so long as they don't go further apart. With muonic hydrogen, compared to ordinary proton-electron hydrogen, there is more negative charge inside the proton. [Since the muon is heavier than the electron, the radius of its atomic radius is smaller than the radius of the electron atom.] With more negative charge inside the proton, the +2/3 e quarks will be pulled a little toward the center, and the -1/3 e quark will be pushed a little out. 4/3 e positive charge beats 1/3 e negative charge, so the charge distribution will shrink a bit.

    The remaining question is whether this effect is large enough. The right way to explore this is presumably QCD on a lattice. It might be possible to get an approximate result using an old model, the MIT Bag model. This model has been abandoned, because it produced no explanation of confinement of quarks, which was the prime issue at the time. A long time ago I showed that the bag model gives a quantitative fit to electromagnetic form factors, so it is possible that it could give a decent indication of the effect of the muon in muonic hydrogen on the distribution of quarks in the proton involved.
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    Quote Originally Posted by mvb View Post
    My best guess is that what is going on is a little of what you might call "anti-shielding." The quarks inside a proton are pretty free to move around so long as they don't go further apart. With muonic hydrogen, compared to ordinary proton-electron hydrogen, there is more negative charge inside the proton. [Since the muon is heavier than the electron, the radius of its atomic radius is smaller than the radius of the electron atom.] With more negative charge inside the proton, the +2/3 e quarks will be pulled a little toward the center, and the -1/3 e quark will be pushed a little out. 4/3 e positive charge beats 1/3 e negative charge, so the charge distribution will shrink a bit.
    I think that I see how this might give one a somewhat polarized nucleus, though this might be a rather naive picture given my lack of any deep understanding of QCD. Would that result in a "smaller" charge distribution (i.e smaller radius for the proton) ?
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    Quote Originally Posted by DrRocket View Post
    I think that I see how this might give one a somewhat polarized nucleus, though this might be a rather naive picture given my lack of any deep understanding of QCD. Would that result in a "smaller" charge distribution (i.e smaller radius for the proton) ?
    Well, the experiment is measuring <R2> of the charge distribution; if you move a net amount of charge inward you will reduce that parameter. I think the major question is whether QCD allows enough movement to get the experimental result. Unfortunately I don't know the answer to that question since I haven't done any calculations, either in a model or in something closer to true QCD.
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