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Mystery of the Shrunken Proton 171

ananyo writes "The proton, a fundamental constituent of the atomic nucleus, seems to be smaller than was previously thought. And despite three years of careful analysis and reanalysis of numerous experiments, nobody can figure out why. An new experiment published in Science only deepens the mystery. The proton's problems started in 2010, when research using hydrogen made with muons seemed to show that the particle was 4% smaller than originally thought. The measurement, published in Nature, differed from those obtained by two other methods by 4%, or 0.03 femtometers. That's a tiny amount but is still significantly larger than the error bars on either of the other measurements. The latest experiment also used muonic hydrogen, but probed a different set of energy levels in the atom. It yielded the same result as the Nature paper — a proton radius of 0.84 fm — but is still in disagreement with the earlier two measurements. So what's the problem? There could be a problem with the models used to estimate the proton size from the measurements, but so far, none has been identified. The unlikely but tantalizing alternative is that this is a hint of new physics."
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Mystery of the Shrunken Proton

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  • Re:easy (Score:3, Informative)

    by Anonymous Coward on Friday January 25, 2013 @11:54AM (#42691271)

    The vast majority of the space inside atoms is empty, determined by the size of the orbits of the electrons around the nuclei, which are essentially unaffected by the proton size. It would be like saying the sun doubled in size, but stayed the same mass: all the planets would still orbit at the same range (orbital distance is determined by mass and attractive force). The quantum case is a tiny bit more complicated, but this classical example illustrates the point.

  • by Anonymous Coward on Friday January 25, 2013 @12:00PM (#42691315)

    Duplicate with another similar post, but I'll bite on this one anyway.

    The simplest counter is that the old methods still get the old values.

    The more complicated answer has to do with the abundant consequences of expanding inter-atomic distances in a universe where attractive forces decrease in strength by the cube of the distance. A universal 4% increase in interatomic size should result in a ~12% decrease in magnetic and gravitic attraction. This would be very noticeable.

    There are even more complicated answers, but I don't feel like even doing the basic estimate math for those.

  • by ChromaticDragon ( 1034458 ) on Friday January 25, 2013 @12:00PM (#42691323)

    This doesn't appear to be a case where the measurement is changing over time. That is, it seems many here are misinterpreting the summary to suggest that things are different NOW relative to THEN.

    Instead, things are different if we measure THIS WAY vs. THAT WAY. But we can still go back and measure both ways. If we use the old method(s), we get the old result.

    That's what's creating the angst. Theorists cannot see why the two methods would differ. And they've checked and rechecked their work. Experimentalists have also checked and rechecked their work.

    This is one of those "that's funny" things that becomes rather interesting.

  • by slew ( 2918 ) on Friday January 25, 2013 @12:38PM (#42691955)

    Short answer is that I suspect the physics is not new, but something related to something we think we qualitatively know, but we don't really know how to bound the computational errors correctly in a complicated system.

    AFAIK, the QED computation techniques that are used to compute bound state of a proton (often modified ordered pertubation methods) aren't particularly convergent so many shortcuts are taken (e.g., use orders of different quantities like non-relativistic velocity, etc). By using a muon and a proton (instead of an electron and a proton), we are essentially replacing something we know more about (the electron) with something we know less about (muon), to try and compute something about something we don't know much about (the proton). Since we don't know much about protons yet, I believe most computations of the bound state are currently just assuming things about them (charge is a point source, nothing about quarks). I haven't read the paper yet, so it's hard to know what they are doing in the QED corrections.

    Maybe there is a slight chance that this simplistic system (muon+proton) can macroscopically exhibit something that hints that QCD confinement inside a proton or muon isn't perfect (e.g, the heavy quarks sortof show themselves in a way that we can measure) which would be some interesting new gluon physics that is currently beyond our particle collider reach. But in some ways this might just show us that the QED based adjustments we are making aren't good enough for the real system and we need some even harder to dream up QCD adjustments and it's hard to say that this would definitly be new physics, but perhaps just new math on old QCD physics....

  • by Anonymous Coward on Friday January 25, 2013 @12:42PM (#42692005)

    They would expect a muon to orbit at the same distance since it has the same charge as an electron

    Actually, no they don't. The whole point of using muons is that their orbitals would be much closer to the proton due to the muon's mass. The size of the orbitals and structure of the orbitals depends on the mass ratio between the two parts, and since the muon is much more massive than the electron, it was expected to have smaller orbitals, much smaller than 4%. And hence, it was expected the smaller orbitals would be more sensitive to structure of the proton. The discrepancy comes from the effects of the proton on the orbital not being quite what they expected from electron based measurements, not from just a change in the size of the orbital.

  • Re:easy (Score:2, Informative)

    by Anonymous Coward on Friday January 25, 2013 @01:07PM (#42692293)

    It is more complicated than that. The measurements using the muon yielded two different sizes, a size related to the distribution of charge within the proton, and a size related to the magnetic structure of the proton. The latter is in agreement with electron and spectroscopic measurements. It is only the first one related to the charge distribution of the proton that disagrees. This heavily points toward a slight discrepancy in the structure of the proton. This points toward improving work with computational QCD, which while having made some great strides, still has a lot of room for improvement.

  • by reverseengineer ( 580922 ) on Friday January 25, 2013 @01:11PM (#42692359)

    It's actually well known that muons do not orbit at the same distance at electrons (orbit in the quantum atomic orbital sense, of course, but since we're talking about hydrogen-like atoms, they can be described with the Bohr model). The calculations of energy levels do include the rest mass of the electron or muon as appropriate. The very reason to use muons in an experiment like this is their greater mass amplifies certain quantum electrodynamic interactions, allowing scientists to take experimental measurements of these interactions and plug them into QED calculations to determine basic physical properties (like the sizes of particles).

    In this case, they used a phenomenon known as the Lamb shift. [gsu.edu]Essentially, two energy levels that should be identical have a slight difference due to a self-interaction effect. This difference can be measured by spectroscopy.

    As they are both the same sort of particle (leptons), electrons and muons should behave identically in this experiment except for the 207 times greater rest mass of the muon, which is accounted for in the calculations. What this result suggests is either the Lamb shift of the electron and of the muon work the same and the experimental setup measures them differently somehow, or that they work differently and there is some sort of new interaction not being accounted for.

  • Re:easy (Score:5, Informative)

    by Anonymous Coward on Friday January 25, 2013 @01:17PM (#42692437)

    It is not so simple as a change in size of the orbital structure. First off, the point of the experiment was that the muon orbital would be much smaller. Second, they measured two different atomic transitions in the system, involving four different orbitals. It wasn't the over all size/energy of the orbitals that was under consideration, it was the relative energies involved in these transitions.

    The results of comparing the transition energies were done two different ways, one sensitive to the magnetic structure of the proton, the other sensitive to the charge structure of the proton. The former was in agreement with previous measurements of the magnetic size of the proton. The latter is the one that is off by 4% from older measurements. There wasn't some singular, overall change in the size of everything involved. Instead, this points to there being something wrong with the understanding of the charge structure of the proton, and hence that structure's predicted impact on the muon orbitals.

    Just changing sizes or talking about expansion wouldn't account for the second half of their results where they found agreement with past, electron based measurements.

  • by cusco ( 717999 ) <brian@bixby.gmail@com> on Friday January 25, 2013 @01:50PM (#42692863)
    The weird thing seems to be that it's not a single measurement that differs. Measured a couple of different ways gives one size, if you measure a couple of other ways you come up with another size. Consistently. It's as though you measured a board with a meter stick and it was 90 cm long, but when you measure with a tape rule it's 86 cm long.
  • Re:Global warming (Score:4, Informative)

    by FrangoAssado ( 561740 ) on Friday January 25, 2013 @05:09PM (#42695393)

    I don't think anyone said anything about the proton's mass, just the radius.

    A difference of 4% in the previously measured mass would be a much bigger story.

    The radius, on the other hand, has much less significance -- it even depends heavily on an arbitrary definition [wikipedia.org], since a proton doesn't have a definite boundary.

When a fellow says, "It ain't the money but the principle of the thing," it's the money. -- Kim Hubbard