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  1. - Top - End - #1
    Titan in the Playground
     
    Yora's Avatar

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    Question What's the big deal with the Muon g-2 experiment?

    I see people everywhere getting super excited about the Muon g-2 experiment, making such claims that there's been a major discovery, that new fields of particle physics are opening up, and that we might have discovered a fifth fundamental force.

    As I understand it, 20 years ago an experiment was done to measure the g value of muons and got a result of ~2.002.
    Now the experiment was repeated and they also got a result of ~2.002.

    So what? What has changed in our understanding of anything since last week? Why is this news?
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    Barbarian in the Playground
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    Default Re: What's the big deal with the Muon g-2 experiment?

    The error bars are smaller now.

    The measured value does not quite match the value predicted by the Standard Model, but with the previous experiment that difference could have been an error.
    With the more accurate measurements now, that's unlikely enough to assume the difference is real.

    That means something is wrong with the existing theory - thus, "new physics."

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    Ogre in the Playground
     
    ElfRangerGuy

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    Default Re: What's the big deal with the Muon g-2 experiment?

    The guy from Piled Higher and Deeper (PHD) has done a comic on it: https://physics.aps.org/articles/v14/47
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    Titan in the Playground
     
    Yora's Avatar

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    Default Re: What's the big deal with the Muon g-2 experiment?

    So the previously measured value was not too far off from the theoretical value to be statistically relevant? But the new measurement gives more confidence that the two are actually different?

    The comic doesn't say that, but there seems to be some implication of that on one of the pictures.
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    Bugbear in the Playground
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    Default Re: What's the big deal with the Muon g-2 experiment?

    The brookhaven experiment had a 3.7 sigma confidence, the fermilab one raised that to 4.2. Still not the 5 sigma to call it a done deal, but closer and a sign that this isn't just random noise confounding things. So it's exciting, but not quite time to break out the cake and party hats just yet.

    As for what we know now that we didn't know last week? Nothing, really. Even if we assume this to be true (which, again, is likely but we're still not as confident as we'd like to be), we don't know what caused the discrepancy. However, if this does happen to be a place where experiment diverges from theory, that'll be the first time in a good, long while that our major theories have failed to accurately predict experiments. It'll be a place for new ideas to sprout up and see which ones best match the new evidence. So it's less that we know something new today, and more that we've found more space to discover new things.

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    Ogre in the Playground
     
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    Default Re: What's the big deal with the Muon g-2 experiment?

    I can't say I fully understand it but here are a few interesting resources.

    The TL;DR is that there may be new physics at play. New particles, forces. Or maybe experimental/math error, but probably not.

    https://imgur.com/gallery/3xUSoK3 (info-graph)
    https://www.youtube.com/watch?v=O4Ko7NW2yQo (pbs spacetime)
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    Default Re: What's the big deal with the Muon g-2 experiment?

    Wobbling muons hint strongly at the existence of bizarre new physics
    Quote Originally Posted by Bad Astronomy
    The Standard Model predicts the muon's anomalous magnetic moment value to be 0.00116591810 (0.00000000043; like I said, very precise).

    The new experiment gets a value of 0.00116592061 (0.00000000041).
    Just to make the difference a bit more visible:
    0.001 165 918 10 (0.000 000 000 43) Predicted Value
    0.001 165 920 61 (0.000 000 000 41) Measured Value

    The prediction is in the range of ...1767 to ...1853
    The measured value is in the range of ...2020 to ...2102

    That is a minimum discrepancy of 4 times the error bars on both the measurement and the prediction.

    Here's what I've taken away from what I've read/heard

    Have they found a 5th force? No. Not yet.

    Have they found new particles? No. Not yet.

    But they've found something interesting that doesn't agree with our current model, and could possibly be explained by a new force and/or particle.
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    Troll in the Playground
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    Default Re: What's the big deal with the Muon g-2 experiment?

    Quote Originally Posted by Anymage View Post
    (...) So it's less that we know something new today, and more that we've found more space to discover new things.
    Very much to the point. Essentially something opposite to the discovery of Higgs boson: that experimental result fits the current models very well, so it was a happy moment to confirm the theory but a bit of a let down at the same time as it did not lead to anything new.

    Progress can be made only when we discover where our theories fail: special relativity and quantum mechanics were built, when it became evident that Newtonian mechanics fails when connected with electrodynamics and at atomic scales. Planck's results on black body radiation have also shown that electrodynamics itself does not work properly, which has driven people to build quantum electrodynamics right after the basics of quantum mechanics have been figured out. Through falsification of old theories we make room for better ones.

    In short, the muon experiment has shown us, where to dig for new discoveries.
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    Default Re: What's the big deal with the Muon g-2 experiment?

    If we plug in the experimentally measured value and work backwards does it break the standard model's predictive ability for other things?
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    Titan in the Playground
     
    Yora's Avatar

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    Default Re: What's the big deal with the Muon g-2 experiment?

    The much less reported story was that at the same time as the g-2 experiment was being done, a different research time had fed the original equations into a shiny new super computer to make a new, more accurate calculation of the predicted value.
    And that new calculated value matches up quite well with the new measured value.

    So it looks again like the original equations were correct. It's just that the old measurement and the old predicted value where not very good, but coincidentally happened to agree with each other 20 years ago.
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    Troll in the Playground
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    Default Re: What's the big deal with the Muon g-2 experiment?

    Quote Originally Posted by Yora View Post
    The much less reported story was that at the same time as the g-2 experiment was being done, a different research time had fed the original equations into a shiny new super computer to make a new, more accurate calculation of the predicted value.
    And that new calculated value matches up quite well with the new measured value.

    So it looks again like the original equations were correct. It's just that the old measurement and the old predicted value where not very good, but coincidentally happened to agree with each other 20 years ago.
    That's actually quite disappointing (a bit similar to the experimental finding of Higgs boson). It has been a really long time since we had an actual breakthrough to some completely new physics. There has been a huge progress in many fields including reconciliation of general relativity with quantum physics, but the basic theories are still standing tall despite the effort of so many people.
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    Halfling in the Playground
     
    Kobold

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    Default Re: What's the big deal with the Muon g-2 experiment?

    Quote Originally Posted by Yora View Post
    The much less reported story was that at the same time as the g-2 experiment was being done, a different research time had fed the original equations into a shiny new super computer to make a new, more accurate calculation of the predicted value.
    And that new calculated value matches up quite well with the new measured value.

    So it looks again like the original equations were correct. It's just that the old measurement and the old predicted value where not very good, but coincidentally happened to agree with each other 20 years ago.
    Could you provide a link to article about that story? As I understand the whole g-2 experiment goal was to reduce probability for something like that to happen, so it would be quite fun if that was the case in the end : )
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    Titan in the Playground
     
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    Default Re: What's the big deal with the Muon g-2 experiment?

    The paper in question is called "Leading hadronic contribution to the muon magnetic moment from lattice QCD" by Borsanyi, Fodor, and others. Didn't find a good article that spells out the details in simple language, but from what I picked up last month it comes down to the new more accurate measurement agreeing with the new more accurate prediction.
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    Halfling in the Playground
     
    Kobold

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    Default Re: What's the big deal with the Muon g-2 experiment?

    Heh, it is complicated, but I think I pinpointed main issue. I think what they claim is that previous measurement hadn't included proper "leading-order, hadronic vacuum polarization (LO-HVP) contribution", but I don't have a clue what that is : P

    Spoiler
    Show
    On the theory side, the dominant source of error is the leading-order, hadronic
    vacuum polarization (LO-HVP) contribution.
    To fully leverage the upcoming measurements,
    it is critical to check the prediction for this contribution with independent methods and to
    reduce its uncertainties. The most precise, model-independent determinations currently rely on
    dispersive techniques, combined with measurements of the cross-section for electron-positron
    annihilation into hadrons [36]. Here we use "ab initio" simulations in quantum chromodynamics
    and quantum electrodynamics to compute the LO-HVP contribution with sufficient precision
    to discriminate between the measurement of (g−2) and the dispersive predictions. Our result,
    [(g − 2)/2]LO−HVP = 707.5[5.5] 10−10, favors the experimentally measured value of (g − 2)
    over the results based on the dispersion relation. Moreover, the methods used and developed
    here will allow further increases in precision, as more powerful computers become available.
    (...)
    At this level of precision, all of the interactions of the standard model contribute. The leading
    contributions are electromagnetic and described by quantum electrodynamics (QED), but the one that
    dominates the theory error is induced by the strong interaction and requires solving the highly non-linear
    equations of quantum chromodynamics (QCD) at low energies. This contribution is determined by the
    leading-order, hadronic vacuum polarization (LO-HVP), which describes how the propagation of a virtual
    photon is modified by the presence of quark and gluon fluctuations in the vacuum. Here we compute this
    LO-HVP contribution to (g − 2)/2, denoted by a LO−HVP , using ab initio simulations in QCD and QED.
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