5 ms·

The anomalous magnetic moment of the muon in the Standard Model: an update

36 points by evanb 1y ago

refulgentis 1y ago
[flagged]
jiggawatts 1y ago
The reason this is big news is that modern physics theories such as quantum electrodynamics and the Standard Model can be used to calculate certain measurements such as the anomalous magnetic moment of the electron to absurdly high precision, with prediction and experiment differing in only about one part per ten billion. Run the same calculations for the Muon, and... err... not so good, previously differing by 3.5 standard deviations. Either the theory is wrong, or the experiments are wrong. The former is very interesting, because Muons are easy to experiment on, and if we can find "new physics" in something so ordinary, then it's an "accessible" regime for conditions that can be reproduced in a lab (albeit a big one). This paper is saying that the discrepancy has been solved by using a more fancy set of computations and newer experiments at Fermilab. In other words: No new exciting physics. Still though, this is interesting because a mystery was solved, even if the answer is in some sense boring.
- kayo_20211030 1y ago
  Interesting. Quick, what's the correct response to the statement: "the standard model is wrong"? Generally, it's "it's not". Maybe it will be some day wrong, but glad to know it still holds.
  - staunton 1y ago
    At this point, "the standard model" basically means "state of the art particle physics without highly speculative stuff". People tell me that neutrino masses are part of the standard model now... So if the standard model is wrong, long live the right standard model. At least, perhaps until it takes a completely new paradigm to go further.
    - JumpCrisscross 1y ago
      > People tell me that neutrino masses are part of the standard model now People say stupid things. It’s a bit silly to blame that on the model.
      - staunton 1y ago
        Ok, how many free parameters are in the standard model? Ask five random physics professors, insist on an answer while declining to answer questions for clarification. I guarantee you get at least two answers, maybe three (that's assuming you manage to get an answer from each...). See also people giving various possible answers on physics stack exchange...
        JumpCrisscross 1y ago
        You’re acting like the SM is an unfalsifiable string theory that just revises all of its parameter every time the last batch is disproven. It’s not. SM is not fully developed. And we know where it is wrong or painfully silent, e.g. neutrinos and gravity. But it’s a rigorous theory, possibly the most rigorous our species has ever developed, with central tenets that have held to ridiculous levels of precision. Of course there are conflicting hypotheses at its frontier. That’s sort of what defines the frontier. But at its core, the SM is robust. So robust that we mostly don’t talk about it, obsessing—as science should—with the parts where it doesn’t fit together as perfectly.
        staunton 1y ago
        I'm saying that the standard model is evolving and there's no authority or institution that gets to definine what "the standard model" means. All of this is completely reasonable. Maybe it can evolve forever to accommodate new results (e.g. by adding new fields), likely it can't (it's hard to imagine a reasonable modification for breaking CPT symmetry, not that this is the best example). If at any point noone can figure out how to evolve it before a radically different theory emerges to explain current discrepancies, we'll have what Kuhn might call a "paradigm shift", say "the standard model is falsified", and invent a different name for the new theory. I even included this possibility in my original comment, so I'm not sure what gave you offense... That's completely unrelated to any given instance of this evolution being falsifiable. Each of them is, they stick around for a while and there aren't that many. All very proper, Popper is happy. Meanwhile, string theory hasn't produced a single prediction that was later observed experimentally and one can even argue whether it has produced any predictions at all...
  - EA-3167 1y ago
    "All models are wrong, but some are useful." or Granted, but good luck trying to find a new model that matches the tested predictions QM/SM makes, AND reveals new physics you can hope to test.
  - tmiku 1y ago
    I agree that it's not wrong, but I would certainly call it incomplete too. It's your choice which of those two points you want to emphasize, but calling the Standard Model "not wrong" with no mention of its incompatibility with General Relativity would feel disingenuous to me.
  - vlovich123 1y ago
    > Maybe it will be some day wrong, but glad to know it still holds. Why? The last time we got relativity and quantum mechanics which completely upended our standard of living and technological progress in the 20th century. Wouldn’t you be excited for finding out exactly how the current model is wrong and a better more accurate explanation for the workings of the universe?
    - kayo_20211030 1y ago
      I was just poking fun at the avalanche of breathless papers that claim some anomalous experimental result "breaks the standard model". Generally, it doesn't. Maybe someday it'll happen. If it does I would still expect any new paradigm to contains within it the kernel of the standard model: as relativity contains within itself the newtonian model, at certain scales and levels of accuracy; and as quantum mechanics contains within itself all the physics and chemistry of an earlier era. I hope the standard model isn't the ne plus ultra, the end, but it just keeps getting reaffirmed despite all the breaks-the-standard-model papers, which seem almost desperate in their pleading.
      - vlovich123 1y ago
        Well we know it's wrong & incomplete in pretty observable ways since gravity isn't part of it (theoretically incomplete) and neutrinos in the model are massless even though we know they have mass (observationally incomplete). But sure, a replacement model has to account for a lot of observations we know do hold for the SM, but I think it'll be closer to how the heliocentric simplified and improved on the Ptolemaic model by providing a simpler perspective that also simplified the math & got rid of various math constructs as "illusions". Similarly, it took time for the heliocentric model to be developed enough to make predictions of higher accuracy because for a long time the Ptolemaic model had much finer prediction powers because the math & supporting data to tune constants was more mature. I think the SM of particle physics is more likely to follow that path to fix the known problems than how special relativity degrades to Newtonian equations at non-relativistic speeds.
- JumpCrisscross 1y ago
  The promise of new science from a muon collider [1] is compelling. [1] https://arxiv.org/abs/2303.08533 https://arxiv.org/abs/2303.08533
  - jxjnskkzxxhx 1y ago
    Thank you for going against the anti-intelectualism that is so prevalent on HN.
  - PaulHoule 1y ago
    Kinda dangerous: https://accelconf.web.cern.ch/p99/papers/THP52.pdf https://accelconf.web.cern.ch/p99/papers/THP52.pdf
    - JumpCrisscross 1y ago
      Emphasis on "kinda." These are mitigatable concerns, not roadblocks.
    - bcoates 1y ago
      The idea of a medically relevant neutrino dose is absolutely wild
      - PaulHoule 1y ago
        It's not the neutrinos themselves that are dangerous, it is the showers that are produced when the neutrinos interact with 'shielding' material between the source and the victim. It's related to the problem in space colonization that the optimal amount of shielding is maybe 2 meters of lunar soil. Less than that and moderate energy particles are harmful. High energy particles mostly blast right through you but if one is stopped by a nucleus in the shielding, that nucleus will be blasted apart and particles from that could blow up more nuclei so a large amount of dangerous and penetrating (muons, pions) radiation is produced. It seems that some very high energy particles (neutrinos?) can pass through the earth and cause a shower, so it's not reasonable you can stop all of them. The earth's atmosphere is spread out over a long distance that gives particles a chance to decay so a shield that is better than the 2 meter shield would be difficult to construct so space colonists are going to accept a minimum level of radiation than we have on Earth.
- jmyeet 1y ago
  It's kind of amazing that the Stanndard Model, which by any objective measure has been a stunningly successful theory, can be both incredibly accurate and ludicrously inaccurate. The magnetic moment, which you mention, is an example of the latter, accurate to 8-10 significant digits. An example of the latter is the so-called "vacuum catastrophe" where QFT predicts the energy of a vacuum and is off... by 120 orders of magnitude. Like no one really knows why we have three generations of particles and what that means or why they're so massive. I only found out about hyperons [1] last year, where (at least) one down quark is replaced with a strange quark. And this matter has weird properties. IIRC the nuclei get smaller. Many years ago I'd assumed it was only a matter of time until we make significant progress merging quantum mechanics and gravity but honestly, I'm starting to have doubts. The universe is under no obligation to make sense or give up its secrets. Just like in maths, some things may be unknowable. [1]: https://en.wikipedia.org/wiki/Hyperon https://en.wikipedia.org/wiki/Hyperon
- raincom 1y ago
  Often times, anomalies to existing theories bring "scientific revolutions". In this case, it is good to see anomaly being resolved with fixing the lab and the computational apparatus.
- magicalhippo 1y ago
  > This paper is saying that the discrepancy has been solved by using a more fancy set of computations That's underselling an important point I think. Here's my understanding of it: There have been two methods to provide the theoretical value, one based on calculating loop corrections, the so-called data-driven approach, and one based on lattice QCD. From what I can gather, the point in the paper is rather that they stopped using the data-driven prediction, and relied solely on the lattice QCD prediction. This is unlike the previous paper where they combined the two predictions into one. The former method requires a bunch of measured data as inputs, like how frequently many different interactions happens in nature (their cross-section). It requires more theoretical work, figuring out the loop correction formulas, and all those measurements, but once you have that you can relatively quickly compute the result. What happened was that one very important cross-section was measured significantly better, but it disagreed with all former measurements. This despite very thorough cross-checking of the experiment. So when those new results were plugged in, the data-driven method gave a much worse prediction. Meanwhile, the latter method is more brute-force, and relies on simulating QCD on a lattice[1]. A much more complex version of those water simulations used in movies and such. Due to how the physics is, it's very expensive to simulate, and thus the simulations haven't been quite good enough. Recent improvements to the method has changed that, making its predictions in line with the measured value. The authors states fixing the data-driven method is of high importance, so hopefully this will be fully resolved in the future. At least that's my armchair understanding. [1]: https://en.wikipedia.org/wiki/Lattice_QCD https://en.wikipedia.org/wiki/Lattice_QCD
  - qnleigh 1y ago
    This is a great summary. Is it plausible that the discrepancy between the two approaches points toward something else beyond the Standard Model? My impression from reading was 'probably not, because it would seem very ad-hoc and unusual,' but I don't have an insider's perspective.
deleted 1y ago
[deleted]
buffer1337 1y ago
I am looking for collaborators on exploring whether geometric principles might provide a foundation for understanding how aspects of the Standard Model could emerge from simpler underlying structures. This is obviously an incredibly challenging area that many brilliant physicists have worked on, so approaching it as a learning exercise - happy to share details if anyone's interested in diving into theoretical physics rabbit holes. bufferoverflow (at) gmail.com
curtisszmania 1y ago
[dead]

36 points by evanb 1y ago

[flagged]

The reason this is big news is that modern physics theories such as quantum electrodynamics and the Standard Model can be used to calculate certain measurements such as the anomalous magnetic moment of the electron to absurdly high precision, with prediction and experiment differing in only about one part per ten billion. Run the same calculations for the Muon, and... err... not so good, previously differing by 3.5 standard deviations. Either the theory is wrong, or the experiments are wrong. The former is very interesting, because Muons are easy to experiment on, and if we can find "new physics" in something so ordinary, then it's an "accessible" regime for conditions that can be reproduced in a lab (albeit a big one). This paper is saying that the discrepancy has been solved by using a more fancy set of computations and newer experiments at Fermilab. In other words: No new exciting physics. Still though, this is interesting because a mystery was solved, even if the answer is in some sense boring.

kayo_20211030 1y ago
Interesting. Quick, what's the correct response to the statement: "the standard model is wrong"? Generally, it's "it's not". Maybe it will be some day wrong, but glad to know it still holds.
- staunton 1y ago
  At this point, "the standard model" basically means "state of the art particle physics without highly speculative stuff". People tell me that neutrino masses are part of the standard model now... So if the standard model is wrong, long live the right standard model. At least, perhaps until it takes a completely new paradigm to go further.
  - JumpCrisscross 1y ago
    > People tell me that neutrino masses are part of the standard model now People say stupid things. It’s a bit silly to blame that on the model.
    - staunton 1y ago
      Ok, how many free parameters are in the standard model? Ask five random physics professors, insist on an answer while declining to answer questions for clarification. I guarantee you get at least two answers, maybe three (that's assuming you manage to get an answer from each...). See also people giving various possible answers on physics stack exchange...
      - JumpCrisscross 1y ago
        You’re acting like the SM is an unfalsifiable string theory that just revises all of its parameter every time the last batch is disproven. It’s not. SM is not fully developed. And we know where it is wrong or painfully silent, e.g. neutrinos and gravity. But it’s a rigorous theory, possibly the most rigorous our species has ever developed, with central tenets that have held to ridiculous levels of precision. Of course there are conflicting hypotheses at its frontier. That’s sort of what defines the frontier. But at its core, the SM is robust. So robust that we mostly don’t talk about it, obsessing—as science should—with the parts where it doesn’t fit together as perfectly.
        staunton 1y ago
        I'm saying that the standard model is evolving and there's no authority or institution that gets to definine what "the standard model" means. All of this is completely reasonable. Maybe it can evolve forever to accommodate new results (e.g. by adding new fields), likely it can't (it's hard to imagine a reasonable modification for breaking CPT symmetry, not that this is the best example). If at any point noone can figure out how to evolve it before a radically different theory emerges to explain current discrepancies, we'll have what Kuhn might call a "paradigm shift", say "the standard model is falsified", and invent a different name for the new theory. I even included this possibility in my original comment, so I'm not sure what gave you offense... That's completely unrelated to any given instance of this evolution being falsifiable. Each of them is, they stick around for a while and there aren't that many. All very proper, Popper is happy. Meanwhile, string theory hasn't produced a single prediction that was later observed experimentally and one can even argue whether it has produced any predictions at all...
- EA-3167 1y ago
  "All models are wrong, but some are useful." or Granted, but good luck trying to find a new model that matches the tested predictions QM/SM makes, AND reveals new physics you can hope to test.
- tmiku 1y ago
  I agree that it's not wrong, but I would certainly call it incomplete too. It's your choice which of those two points you want to emphasize, but calling the Standard Model "not wrong" with no mention of its incompatibility with General Relativity would feel disingenuous to me.
- vlovich123 1y ago
  > Maybe it will be some day wrong, but glad to know it still holds. Why? The last time we got relativity and quantum mechanics which completely upended our standard of living and technological progress in the 20th century. Wouldn’t you be excited for finding out exactly how the current model is wrong and a better more accurate explanation for the workings of the universe?
  - kayo_20211030 1y ago
    I was just poking fun at the avalanche of breathless papers that claim some anomalous experimental result "breaks the standard model". Generally, it doesn't. Maybe someday it'll happen. If it does I would still expect any new paradigm to contains within it the kernel of the standard model: as relativity contains within itself the newtonian model, at certain scales and levels of accuracy; and as quantum mechanics contains within itself all the physics and chemistry of an earlier era. I hope the standard model isn't the ne plus ultra, the end, but it just keeps getting reaffirmed despite all the breaks-the-standard-model papers, which seem almost desperate in their pleading.
    - vlovich123 1y ago
      Well we know it's wrong & incomplete in pretty observable ways since gravity isn't part of it (theoretically incomplete) and neutrinos in the model are massless even though we know they have mass (observationally incomplete). But sure, a replacement model has to account for a lot of observations we know do hold for the SM, but I think it'll be closer to how the heliocentric simplified and improved on the Ptolemaic model by providing a simpler perspective that also simplified the math & got rid of various math constructs as "illusions". Similarly, it took time for the heliocentric model to be developed enough to make predictions of higher accuracy because for a long time the Ptolemaic model had much finer prediction powers because the math & supporting data to tune constants was more mature. I think the SM of particle physics is more likely to follow that path to fix the known problems than how special relativity degrades to Newtonian equations at non-relativistic speeds.
JumpCrisscross 1y ago
The promise of new science from a muon collider [1] is compelling. [1] https://arxiv.org/abs/2303.08533 https://arxiv.org/abs/2303.08533
- jxjnskkzxxhx 1y ago
  Thank you for going against the anti-intelectualism that is so prevalent on HN.
- PaulHoule 1y ago
  Kinda dangerous: https://accelconf.web.cern.ch/p99/papers/THP52.pdf https://accelconf.web.cern.ch/p99/papers/THP52.pdf
  - JumpCrisscross 1y ago
    Emphasis on "kinda." These are mitigatable concerns, not roadblocks.
  - bcoates 1y ago
    The idea of a medically relevant neutrino dose is absolutely wild
    - PaulHoule 1y ago
      It's not the neutrinos themselves that are dangerous, it is the showers that are produced when the neutrinos interact with 'shielding' material between the source and the victim. It's related to the problem in space colonization that the optimal amount of shielding is maybe 2 meters of lunar soil. Less than that and moderate energy particles are harmful. High energy particles mostly blast right through you but if one is stopped by a nucleus in the shielding, that nucleus will be blasted apart and particles from that could blow up more nuclei so a large amount of dangerous and penetrating (muons, pions) radiation is produced. It seems that some very high energy particles (neutrinos?) can pass through the earth and cause a shower, so it's not reasonable you can stop all of them. The earth's atmosphere is spread out over a long distance that gives particles a chance to decay so a shield that is better than the 2 meter shield would be difficult to construct so space colonists are going to accept a minimum level of radiation than we have on Earth.
jmyeet 1y ago
It's kind of amazing that the Stanndard Model, which by any objective measure has been a stunningly successful theory, can be both incredibly accurate and ludicrously inaccurate. The magnetic moment, which you mention, is an example of the latter, accurate to 8-10 significant digits. An example of the latter is the so-called "vacuum catastrophe" where QFT predicts the energy of a vacuum and is off... by 120 orders of magnitude. Like no one really knows why we have three generations of particles and what that means or why they're so massive. I only found out about hyperons [1] last year, where (at least) one down quark is replaced with a strange quark. And this matter has weird properties. IIRC the nuclei get smaller. Many years ago I'd assumed it was only a matter of time until we make significant progress merging quantum mechanics and gravity but honestly, I'm starting to have doubts. The universe is under no obligation to make sense or give up its secrets. Just like in maths, some things may be unknowable. [1]: https://en.wikipedia.org/wiki/Hyperon https://en.wikipedia.org/wiki/Hyperon
raincom 1y ago
Often times, anomalies to existing theories bring "scientific revolutions". In this case, it is good to see anomaly being resolved with fixing the lab and the computational apparatus.
magicalhippo 1y ago
> This paper is saying that the discrepancy has been solved by using a more fancy set of computations That's underselling an important point I think. Here's my understanding of it: There have been two methods to provide the theoretical value, one based on calculating loop corrections, the so-called data-driven approach, and one based on lattice QCD. From what I can gather, the point in the paper is rather that they stopped using the data-driven prediction, and relied solely on the lattice QCD prediction. This is unlike the previous paper where they combined the two predictions into one. The former method requires a bunch of measured data as inputs, like how frequently many different interactions happens in nature (their cross-section). It requires more theoretical work, figuring out the loop correction formulas, and all those measurements, but once you have that you can relatively quickly compute the result. What happened was that one very important cross-section was measured significantly better, but it disagreed with all former measurements. This despite very thorough cross-checking of the experiment. So when those new results were plugged in, the data-driven method gave a much worse prediction. Meanwhile, the latter method is more brute-force, and relies on simulating QCD on a lattice[1]. A much more complex version of those water simulations used in movies and such. Due to how the physics is, it's very expensive to simulate, and thus the simulations haven't been quite good enough. Recent improvements to the method has changed that, making its predictions in line with the measured value. The authors states fixing the data-driven method is of high importance, so hopefully this will be fully resolved in the future. At least that's my armchair understanding. [1]: https://en.wikipedia.org/wiki/Lattice_QCD https://en.wikipedia.org/wiki/Lattice_QCD
- qnleigh 1y ago
  This is a great summary. Is it plausible that the discrepancy between the two approaches points toward something else beyond the Standard Model? My impression from reading was 'probably not, because it would seem very ad-hoc and unusual,' but I don't have an insider's perspective.

deleted 1y ago

[deleted]

buffer1337 1y ago

I am looking for collaborators on exploring whether geometric principles might provide a foundation for understanding how aspects of the Standard Model could emerge from simpler underlying structures. This is obviously an incredibly challenging area that many brilliant physicists have worked on, so approaching it as a learning exercise - happy to share details if anyone's interested in diving into theoretical physics rabbit holes. bufferoverflow (at) gmail.com

curtisszmania 1y ago

[dead]