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The Incredible Shrinking Proton

  1. Jan 25, 2013 #1
    Some researchers seem to have measured the proton as being 4% smaller than previously thought:


    What is the reason for this?

    This is the second time the experiment has been conducted. If it is carried out yet again, and yields the same strange result, then what are we to make of it?

    Is there any speculation on what is going on here?
    Is it possible that muons have some previously unknown interaction with the proton? Otherwise, what else could it be?
  2. jcsd
  3. Jan 25, 2013 #2


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    Systematic and random errors are hard to estimate. People always underestimate their error bars.
  4. Jan 25, 2013 #3
    The 1.082 femtometer size of the proton is a red herring. It doesn't really mean much. I mean, it means something, of course, but the real issue is what the hell is quark confinement, what are all these gluons doing in my soup?, and can someone please tell me what 2/3 charge means? I mean for god's sake. If that's not enough, two words....sea quarks.
  5. Jan 28, 2013 #4
    It is still an issue however that different measurements seem to give different (charge) radii of the proton. The pessimist would say that there is something wrong with the measurement or error treatment, whereas the optimist would say that it might be due to new physics which affects muon-proton interactions differently. It might not have much to do with fractional charges or confinement but it still needs to be solved.
  6. Jan 28, 2013 #5


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    As the difference occurs with different measurement methods and the significance is high, I don't think it is a problem with the individual measurements itself. It might be new physics, but I think the easiest explanation is a problem in theoretic calculations relating the measured values to a proton radius.
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