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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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  • Maybe they mixed imperial with metric units in some part of the process. At least they could had enjoyed to get back home and say "Honey, i shrunk the proton"
  • by idontgno ( 624372 ) on Friday January 25, 2013 @11:24AM (#42690933) Journal
    that it was the cold water.
    • that it was the cold water.

      Or old age.

  • easy (Score:1, Interesting)

    by AbrasiveCat ( 999190 )
    The universe it growing (including our meter sticks) and the proton is staying the same size.
    • Re: (Score:3, Interesting)

      But what are our meter sticks made of? Why would they grow with the universe if they are made of particles that stay the same size?

      • Re: (Score:3, Informative)

        by Anonymous Coward

        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.

        • But if protons are the same size and the apparent shrinkage is due to our meter sticks expanding, why would they be expanding? Is it the forces between particles that are changing? Energy levels? Why would particles be further apart now than before?

          Anyway, I don't think the universe, or protons, have changed size by 4% in a few decades so this discussion is a bit pointless. They just used two different methods of measuring the size, and one or both of those methods are wrong. Which could yield interesting n

          • There's that word again. "Shrinkage." Why are things shrinking in the future? Is there a problem with the Universe expanding?
            • option 1: the entire universe has expanded in the last couple decades, including the distance between every particle, but not the size of protons.
              option 2: protons have shrunk in the last couple decades
              option 3: one of our measuring methods is wrong for a reason we don't yet understand.
              Hint: those are sorted in reverse order by ridiculous over-complication.
              If you hadn't misplaced your razor, Mr. Occam, you'd have already eliminated the first one on your own.

              • My comment was in jest, referencing a well-known quote from the movie "Back to the Future". Thanks for making me explain it!!
                • My comment was in jest, referencing a well-known quote from the movie "Back to the Future". Thanks for making me explain it!!

                  Sorry, in my last time travel I've passed by the film studios where they made that movie. Well, I did some mistake. Fortunately it did not do very much damage, but one effect is that this sentence is now missing from the movie. Sorry about that.

                • Sorry for ruining the moment! I didn't get the reference because none of the most important words are the same, so I think we can classify it as "obscure."
                  Perhaps next time you can help a nerd out by citing Doc brown or something. That's probably the least quotable line in the whole trilogy to start with, because who even said, "heavy" beside McFly? I barely got the humor when I was watching the movie, let alone out of context.

                  And there are people who have propose shrinking matter as a valid hypothesis

        • Re:easy (Score:4, Interesting)

          by WillgasM ( 1646719 ) on Friday January 25, 2013 @12:10PM (#42691505) Homepage
          Wait. I thought the whole point of this experiment was that they extrapolate the size of the proton by actually measuring the size of the orbital. So it's not necessarily that the proton has gotten smaller, just that muons orbit closer than electrons. If anything, they've taken some of the empty space out of the atom.
          • 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 PKFC ( 580410 )

              So if this new measurement is related to charge in the proton, could it be an impact of how close or perhaps more likely how fast the muon orbited the proton? As muons (and taus) are heavier than electrons, I would expect them to move slower around the proton which could cause a shift in how the quarks are positioned or interact within the proton. Mind you the muon is huge compared to the up or down quarks. And this is where my brain stops processing once I read that quantum chromodynamics is responsible fo

          • by jonadab ( 583620 )
            Protons are subatomic phenomena. As such, they don't actually have macroscopic-style physical attributes like size, shape, color, texture, orientation, etc. -- at least, not in quite the way you think of them when you think of macroscopic objects having those attributes. When we speak of the "size" of a proton, we're actually just talking about the distance (possibly the mean distance, possibly the minimum observed distance) at which they participate in certain kinds of interactions. _Which_ interactions
    • Re: (Score:1, Funny)

      by ve3oat ( 884827 )
      Exactly. To their great credit, the protons have never raised their debt ceiling. So while the universe around them inflates out of control, the protons stay the same and still prosper. (There's a lesson here somewhere but the type on my screen is now so small that I can't read it anymore.)
    • Re:easy (Score:5, Insightful)

      by Anonymous Coward on Friday January 25, 2013 @11:47AM (#42691191)

      uh sure, if the prior method for measuring was also showing the reduced size, but it's not ... so how does "the universe expanding" explain two simultaneously different measurements? besides, if the universe were expanding and protons weren't, i don't think our meter sticks would be expanding.

      how the hell did this get +5 anyway ... brainless mods

      • Will mods please raise this comment to +5? The GP clearly didn't RTFA or didn't understand the situation, FFS.
        If there actually were a 4% shrinkage over less than a hundred years, then how big was the proton hundreds of millions of years ago? How about 13.7 billion years ago? Use your heads, please!

      • by bondsbw ( 888959 )

        But what exactly do you mean by "simultaneous"?

        Obviously, the measurements are being done from different inertial reference frames.

      • > besides, if the universe were expanding and protons weren't, i don't think our meter sticks would be expanding.

        Perhaps you are missing the fact that the meter sticks are made of atoms, not protons. An expanding electron shell, and resulting inter-atomic distance might go some way towards explaining the meter stick phenomenon.

        But just to argue the other side as well, astronomic evidence [] suggests that the universe is expanding. That we can tell this means that we have some metric that is NOT expanding.

      • Re: (Score:2, Informative)

        by Anonymous Coward

        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 comput

      • Re:easy (Score:5, Insightful)

        by rasmusbr ( 2186518 ) on Friday January 25, 2013 @02:31PM (#42693397)

        how the hell did this get +5 anyway ... brainless mods

        Anyone's who's read an amateur physics forum knows that the expanding scale universe "model" is reinvented several times a year by isolated eager guys armed with high school diplomas, apocryphal tales about Einstein and quotes by Galileo. It's one of those ideas that seem obviously true for several seconds until you actually think about it.

        Here's a tip: The age of simple discoveries in mature sciences is over. That's why they're called mature. Unless you've spent years studying physics intensely while getting frequent feedback from experienced physicists, your chances of making minor contributions to physics are infinitesimally close to zero. Any idea that you quickly stumble upon based on your high school or college Physics 101 understanding has literally been thought, tried and discarded a thousand times before by physicists.

        • Dude, you're probably right but it's almost inhumane to offhandedly discount any new idea presented by physics students. Sure it's a rough world out there but there are new fields of study which still are looking for answers. See new physics.

    • Re:easy (Score:5, Insightful)

      by mrsquid0 ( 1335303 ) on Friday January 25, 2013 @12:10PM (#42691493) Homepage

      It does not work that way. Things like metre sticks are held together by the electromagnetic force, which is decoupled from the expansion of the Universe. This means that objects in the Universe do not expand, they just move along with the expansion. If everything in the Universe expanded with the Hubble flow then we would never be able to detect the Hubble flow. Only spacetime expands, not what is sitting around in spacetime.

      The explanation for the unexpected small size of the proton is probably something to do with the way that muons interact with protons. We assume that electrons and muons interact with protons in exactly the same way, but this is a hypothesis. There is very little observational evidence supporting the idea that electrons and muons behave in exactly the same way when they are bound to an atomic nucleus. The problem with this idea is that it requires that particle physics be extended beyond the standard model. It is also possible that the problem is something much more mundane, like a faulty connection somewhere in the experimental setup. We need an independent verification of this result before we start rewriting the textbooks.

      • by Anonymous Coward

        "This means that objects in the Universe do not expand, they just move along with the expansion... Only spacetime expands, not what is sitting around in spacetime."

        I'm not sure I understand this. Maybe you can help me with my confusion? It sounds like you are saying this:

        If a proton and electron are seperated by 100 ly, then the distance between them will expand by X% because new space is "appearing" in between them.
        If they are seperated by 1 ly, it is the same.
        If they are seperated by 10^6 m, it is the sam

        • The expansion of the Universe does not create new spacetime, it just stretches the existing spacetime.

        • The difference is whether the objects are bound. The distance between far-away galaxies is arbitrary. The size of an atom is determined by quantum mechanics and quantum electrodynamics.

          Imagine two objects lying on the floor. If you drag them away from each other, their distance grows. However if they are connected with a string, the distance won't grow. The string holds them together.

        • It's not correct to think of objects as being passive points attached to an actively expanding grid which carries them along. Objects (masses) are primary participants in the shaping of spacetime and not simply being dragged along. In other words, we shouldn't think of the expansion of the universe as causing faraway galaxies to move away from us. Rather, the fact that faraway galaxies are moving away from us is the expansion of the universe (and not a symptom). Everything is moving apart from each other. G

      • the electromagnetic force is decoupled from the expansion of the Universe

        Citation please.

        • I did not say this very well. A better way of putting it is that molecular bonds (in fact, any of the four fundamental forces) are stronger than the expansion of the Universe over short distances. This is why you and I and my pint of beer do not expand along with spacetime. We are sitting in spacetime and are held together by the four fundamental forces. It is a bit like the way that a marble sitting on a rubber sheet does not expand when one stretches the rubber sheet.

          • This whole discussion is completely null anyway, since the FRW's "expanding universe" scale factor only applies to a universe filled with a perfectly homogeneous mixture of baryons, radiation, and dark matter.

            This is true overall of the universe, such that its scale factor is growing and taking galaxy clusters apart from one another. However the metric surrounding gravitationally bound objects is not dialating the same way, because the local matter density is high enough to stop it and because the FRW eq
    • Unless it is happening randomly (shrinks at a certain rate for a few years, then stops or expands for time etc) I think there would be detectable changes in the Sun's output over time if that was the case because the rate of nuclear fusion would change (reduce, I think).

    • I have a faint feeling, judging by comments, that we have a major case of 'whoosh' here.
    • Protons are growing as well... along with everything else.

  • by GovCheese ( 1062648 ) on Friday January 25, 2013 @11:37AM (#42691085)
    Subway Corporate announces that their foot-long measurements were unfortunately based on accepted assumptions of larger protons.
  • by whoda ( 569082 ) on Friday January 25, 2013 @11:40AM (#42691107) Homepage

    It's old physics that we haven't figured out yet, but thought we had.

  • If the universe is expanding everywhere, and if this included the space between protons and atomic particles, then this result would be due to the length we use to measure being larger than it was before. So it's possible that the proton isn't getting smaller, but that everything else in the universe is expanding with the expansion of the universe. Is there anything that precludes this as a possibility?
    • Re: (Score:2, Informative)

      by Anonymous Coward

      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 a

    • 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.

      • Re: (Score:3, Insightful)

        by jalvarez13 ( 1321457 )

        It reminds me of the Michelson-Morley experiment. Back then no one understood why an experiment that should have given different results for the speed of a ray of light failed to do so. As we know today, the constant speed of light is the basis for Einstein's relativity theory and has been proved right many times.

        Could this be one of those moments?

        • by ceoyoyo ( 59147 )

          Except that, contrary to Slashdot belief, Michelson-Morley type interferometer experiments (there were a lot, not just one) were done to help choose between a whole bunch of theories, some of which predicted a difference and some of which didn't.

    • Is there anything that precludes this as a possibility?

      Yes. That would mean a continuous growth, not a sudden change when a different method is used. Not to mention that a 4% change in a few years would mean that the proton was enormous around the time of the dinosaurs, even. If the proton was shrinking that quickly relative to collections of atoms, we would need an overhaul of a great deal of the current body of science.

    • Re: (Score:3, Interesting)

      by WillgasM ( 1646719 )
      You are exactly the opposite of right. They don't actually measure the proton, they measure an orbital and do some math to determine the size of a proton. They would expect a muon to orbit at the same distance since it has the same charge as an electron, but they're getting a smaller sized orbital and therefore determining that the proton has shrunk. In reality, protons are the same size, and we're stumped as to why muons are behaving differently than electrons. If anything, muonic hydrogen has less empty s
      • 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.

      • Are you sure?

        I thought the ground state orbital radius was proportional to the reciprocal of the mass of the orbiting particle?

        But it's so long since I did this that I might just be talking rubbish and I certainly cannot remember how to derive it from the Schroedinger equation.

        But it also presumably relies on the approximation that the orbiting particle is essentially massless when compared to the proton which also may not hold for the proton-muon ratio.

        Twenty years ago I could have answered this definitive

        • lol, no actually, I'm not sure. I'm just going off what I read in TFA and my limited understanding of particle physics. However, I'm willing to bet that protons aren't actually shrinking when orbitted by muons, but rather our current understanding of muon electrodynamics is less than complete.
        • by ceoyoyo ( 59147 )

          You're right. The idea is that the wavelength of the orbiting particle, no matter what it is, has to fit around the lowest orbit.

    • > So it's possible that the proton isn't getting smaller, but that everything else in the universe is expanding with the expansion of the universe.

      A functioning universe is actually a very, very precariously balanced animal. The Anthropic Principle was developed essentially to explain this. (Quick and horribly inaccurate summary: the only way to get around the apparent design is by assuming that there are other "worlds," other "realms" or other universes, each with a different collection of physical laws

  • Maybe the additional 4% was just lens flare.
  • The extra mass of the muon is holding it in a tighter orbit around the nucleus. We've been kidding ourselves with all this Higgs Boson crap.
  • It has not been unheard of in science that a fundamental measurement is initially in error. It has also been known for that error to not be correctly quickly, but rather slowly over time. It seems that many researchers will accept a value as accurate. However, when new measurements do not agree, sometimes the new more accurate value is not reported, but rather a value that is close to the original measurement, but shifted in the direction of the new better measurement. As no one really knows what the mo
  • by Anonymous Coward on Friday January 25, 2013 @12:24PM (#42691729)

    My first idea would be that the muon does indeed shrink the proton. After all, the proton is not some solid body, but consists of interacting charged quarks. The muon has a higher probability to be inside the proton (that's exactly why it is useful for measuring its size), and thus lowers the charge density there (it adds some negative charge density to the proton's positive charge density). The electrostatic repulsion inside the positively charged proton should certainly affect its size; decreasing that repulsion due to the partial screening by the muon should therefore allow the proton to shrink a bit. Not much, but maybe enough to explain the difference.

    • is it more likely that placing a muon in orbit causes the nucleus to shrink, or that we're flawed in our assumption that muons will orbit at the same distance as electrons?
      • 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. []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.

  • 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

      My understanding of the QED calculations, is they are more "straightforward," just a really bitch in terms of effort to actually take beyond the first order or two of expansion. In principle, with an army of grad students, could carry out the calculation to more orders and double check it is converging as fast as expected. I've seen this done in some other QED calculations, carried out to some insane number of orders along with some computer assistance to generate the exhaustive list of loop diagrams need

    • I believe most computations of the bound state are currently just assuming things about them (charge is a point source, nothing about quarks).

      This is my suspicion as well, specifically about the charge distribution. I think a 4% effect could easily be explained using a model with distributed charge.

      • by radtea ( 464814 )

        This is my suspicion as well, specifically about the charge distribution. I think a 4% effect could easily be explained using a model with distributed charge.

        It has been my experience that posts where the poster annouces their "suspicions" are almost alway gibberish, displaying a profound ignorance of the most basic elements of the subject they have suspicions about. It really is a useful litmus test, to the extent that I think /code should be modified to automatically down-mod any post that contains phrases like "I suspect that" and "my suspicion".

        It's not that there's anything wrong with expressing doubts. It's that this specific way of putting it seems to b

    • May be someone just didn't tighten a fiber optic cable somewhere.

      Why are some people so jumpy on the news of one experiement's measurement of a quanity that contradicts all earlier evidence? I'm not trying to be an asshole, but sorry, haven't we learned our lesson yet? [] I will concede that they have data dating to 2003, so we could have a real thing there and it is good to give them time to review their stuff. Still, apparently no other measurement has shown anything similar, or I'd assume that they'd have m

  • by gestalt_n_pepper ( 991155 ) on Friday January 25, 2013 @12:48PM (#42692087)

    Now, they tell me.

  • ... cut once.

  • "The unlikely but tantalizing alternative is that this is a hint of new physics."
    It's just like we learned in math and science class in school. If your experiment or equation doesn't result in what you were predicting, claim it was accurate and make some shit up. Like dark matter for example. Some guys sitting here on Earth with computers and telescopes didn't measure the mass of the ENTIRE UNIVERSE quite right so...must be magical invisible matter we just made up on the spot! Protons shrunk? Must be ne
    • because if *everyone* does the experiment and the results don't match up with the theory, then there's something missing in the theory.

      In this case, taking the same measurement two different ways results in two different numbers, and the theory says they should match.

    • "New physics" is shorthand for "changes to our theories of physics and/or understanding of the universe". Not new laws of physics magically appearing into existence.

      The summary shows that new observations of protons show them to bea different size to what we were expecting based on current theories and models. Now lots of scientists have checked and replicated it, and the difference is still there. That means our old theories must have been wrong, and the universe must work differently than we though. And

  • Obviously, the beings running the simulation just changed a constant. Or maybe the computer our universe is currently running on has a manufacturing flaw in some of its hardware.

  • by Billy the Mountain ( 225541 ) on Friday January 25, 2013 @01:42PM (#42692759) Journal
    I think a proton is shaped like a dodecahedron, so of course it's going to measure up to 4% less depending on it's orientation.
  • I'm going to demonstrate my own stupidity (and lack of willingness to RTFA) ...

    What changes with this measure? I'm sure it has loads of things which it might affect, but I have no idea.

    So, what does a slightly smaller photon translate into?

  • Protons are not the size our current model suggests.
    Gravity doesn't work on large scales the way our current model suggests.
    We can't observer dark matter and dark energy the way our current model suggests.

    There's literally hundreds of other examples, but am I the only one who thinks the problem is our current model?

    I'm willing to wager dark matter/energy don't even exist. They are just made up to make the computer model work. Ridiculous.

    • by Anonymous Coward

      Explain gravitational lensing when there's no apparent mass causing it.

  • No idea whether this is garbage or has already been taken into account.

    In large scale orbiting systems, equivalent measurements relating to size are based on centre of masses, where the masses also govern the force controlling the orbit.

    In hydrogen atoms the masses still determine the orbit, but the forces are mediated by the charges rather than by the masses directly as gravity, which may not be in the same place as the masses and may be to some extent free to move in relation to the centre of mass
  • by Anonymous Coward

    Did they check the cables...? Perhaps, it's a loose one.

  • The muon sits much closer to the nucleus than an electron, so the charge of the muon is perhaps changing the shape of the proton, "squeezing it". Since the proton is made up of charged quarks, the ground state orbitals for the quarks could be somehow modified by the nearby muon charge. I'm totally guessing of course.

If all else fails, lower your standards.