The anomalous magnetic moment of the muon in the Standard Model: an update
34 comments
·May 28, 2025jiggawatts
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.
magicalhippo
> 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.
qnleigh
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.
raincom
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.
kayo_20211030
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
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
> 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.
vlovich123
> 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
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.
tmiku
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.
EA-3167
"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.
JumpCrisscross
The promise of new science from a muon collider [1] is compelling.
PaulHoule
Kinda dangerous: https://accelconf.web.cern.ch/p99/papers/THP52.pdf
bcoates
The idea of a medically relevant neutrino dose is absolutely wild
JumpCrisscross
Emphasis on "kinda." These are mitigatable concerns, not roadblocks.
jxjnskkzxxhx
Thank you for going against the anti-intelectualism that is so prevalent on HN.
jmyeet
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.
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curtisszmania
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refulgentis
[flagged]
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