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• CommentRowNumber1.
• CommentAuthorUrs
• CommentTimeFeb 6th 2018
• (edited Feb 6th 2018)

there is an old article (Berends-Gastman 75) that computes the 1-loop corrections due to perturbative quantum gravity to the anomalous magnetic moment of the electron and the muon. The result turns out to be independent of the choice of (“re”-)normalization (hence what they call “finite”).

I have added a remark on this in the $(g-2)$-entry here and also at quantum gravity here.

• CommentRowNumber2.
• CommentAuthorUrs
• CommentTimeJan 2nd 2019

added a paragraph with literature pointers on the latest discrepancies between theory and experiment (here)

• CommentRowNumber3.
• CommentAuthorUrs
• CommentTimeJan 16th 2019

added pointer to possible explanation of the anomaly in the anomalous magnetic moments via leptoquarks

• CommentRowNumber4.
• CommentAuthorUrs
• CommentTimeJan 23rd 2019

added the table by Lyons (here), kindly pointed out by David C., which argues that the detection-threshold for anomalies in the anomalous magnetic moment of the muon should be $4 \sigma$ instead of the conventional $5 \sigma$ – which would mean that the current experimental significance should already be counted as detection.

• CommentRowNumber5.
• CommentAuthorDavid_Corfield
• CommentTimeJan 24th 2019

Looking over some of the writings by experimentalists, you see how sensitive degrees of plausibility are to new information, theoretical or experimental. E.g., for this anomaly on this page, Dorigo seems to have gone from enthusiasm Muon G-2: The Anomaly That Could Change Physics, And A New Exciting Theoretical Development to doubt Gravitational Effects Explain Muon Magnetic Moment Anomaly Away!! in a few months.

Then people in comments of the second post cast doubt on this explaining away. Is that largely agreed now that GR doesn’t explain away the anomaly?

• CommentRowNumber6.
• CommentAuthorUrs
• CommentTimeJan 24th 2019

Yes, that GR argument was nonsense, and both the people running the experiment as well as independent theorists were very quick to point out that it couldn’t possibly be true, even without looking at the details. Then a little later somebody identified the erroneous step in the computation (that was somebody from Zagreb active on PhysicsForums, incidentally, he wrote a 2 page article about it.).

There was a comprehesive sum-up of the whole situation recently on the arXiv, but now I don’t have the link.

But generally I want to insist again that it is crucially important in science to keep experimental results strictly independent of theoretical expectations. Otherwise we fall back into darkness to before the age of enlightment.

Our “excitement” and all these soft human factors that you tend to stress will depend on all kinds of things, but before we even start getting into that, we need to have an idea with which independent objective certainty experimentalists are seeing a signal or not.

• CommentRowNumber7.
• CommentAuthorDavid_Corfield
• CommentTimeJan 24th 2019

What is it about this topic that makes for this strange gap between us? I just don’t recognise your way of describing what I say.

Our “excitement” and all these soft human factors that you tend to stress…

What is that about? When did I ever speak of excitement?

Did you know before this conversation that there is a difference between global and local p-values due to the Look Elsewhere Effect? Is that one of my “soft human factors”, or is it that you had to learn something about data analysis? I hope you’ve picked something up from some of these articles we’ve looked at. I certainly have.

But something I haven’t shifted a jot on is that doing the best with the information available is a far from simple process. There are many components to this information and typically this needs a team of experts to bring their distributed expert knowledge to the table. Dorigo errs above by moving too quickly and not waiting for the theoreticians to adjudicate. He should have had higher prior expectation that that theoretical explaining away would be found to be spurious. He wouldn’t have had to wait long in that case.

We should always look to use the best expertise we can access so as to have the most accurate expectations that: the instruments are working as intended, that systematic errors have been reduced, that the predictions are done correctly, that we understand correctly how the search for a signal is conducted … This has nothing to do with “excitement” and it’s not “soft”.

What does the “hardness” of a sigma level amount to if the “soft” understanding of the experiment changes? When you heard in 2011 of the signal for neutrinos travelling faster than light, what did their “hard” $6 \sigma$ (p. 22) count for? Wasn’t it overwhelmingly likely that something had been overlooked in the measurement process? Isn’t the “hardness” in this case coming from a reasonable unwillingness on theoretical grounds to give up on the speed of light as greatest velocity? But are you going to call that unwillingness “soft” and “human”? Or is not something that any rational (unexcited) theoretician will maintain given what was known in 2011?

• CommentRowNumber8.
• CommentAuthorUrs
• CommentTimeMar 3rd 2020
• (edited Mar 3rd 2020)

added pointer to today’s

• Sz. Borsanyi, Z. Fodor, J. N. Guenther, C. Hoelbling, S. D. Katz, L. Lellouch, T. Lippert, K. Miura, L. Parato K. K. Szabo, F. Stokes, B. C. Toth, Cs. Torok, L. Varnhorst:

Leading-order hadronic vacuum polarization contribution to the muon magnetic moment from lattice QCD (arXiv:2002.12347)

Surprisingly, our result eliminates the need to invoke new physics to explain the measurement of $(g_\mu - 2)$.

• CommentRowNumber9.
• CommentAuthorUrs
• CommentTimeMar 10th 2020

ah, but look at Fig 14 of today’s

• Christoph Lehner, Aaron S. Meyer, Consistency of hadronic vacuum polarization between lattice QCD and the R-ratio (arXiv:2003.04177)

I have included that into the entry (here)

• CommentRowNumber10.
• CommentAuthorUrs
• CommentTimeJun 11th 2020

added pointer to today’s

• T. Aoyama et al., The anomalous magnetic moment of the muon in the Standard Model (arXiv:2006.04822)
• CommentRowNumber11.
• CommentAuthorUrs
• CommentTimeApr 7th 2021

added pointer to Fermilab’s announcement (here) of – after years of re-measuring – confirming Brookhaven’s observation of the anomaly. Fermilab now states it at 4.2 sigma.

(From minutes ago. The server itself is down, so no details yet.)

• CommentRowNumber12.
• CommentAuthorDavid_Corfield
• CommentTimeApr 7th 2021

I saw a presentation on yt. Impressive!

• CommentRowNumber13.
• CommentAuthorUrs
• CommentTimeApr 7th 2021

So apparently the story now is that after 20 years of discrepancy, Nature (the journal) timed a “theory” paper to appear just today (lattice QCD, hence computer simulation, really), which claims that the experimental value that has now been confirmed is actually predicted by the standard model after all: doi:10.1038/s41586-021-03418-1.

I can’t judge this. But I do notice that the authors had a lot to worry about for this timing stunt to work out. Hopefully they had enough resources left to scrutinize their actual results.

In contrast and curiously, this morning on the arXiv in arXiv:2104.02632 is another lattice QCD team announcing that the (deviating) theoretical prediction stands.

In any case, I guess it means there is more debate to be had.

• CommentRowNumber14.
• CommentAuthorDavid_Corfield
• CommentTimeApr 7th 2021

Hmm, falsification is trickier than Popper made out.

• CommentRowNumber15.
• CommentAuthorUrs
• CommentTimeApr 8th 2021

I am wondering about the mindset of a group who a) claims to have the most important computation in the field for 20 years, b) chooses to not inform their colleagues who are doing the most important experiment in the field for 20 years and c) instead choose to drop the bomb right during their colleagues’s press conference. Is this behaviour driven by desire for insight? Or is this the spirit of social media infesting the holy halls of science?

• CommentRowNumber16.
• CommentAuthorDavid_Corfield
• CommentTimeApr 8th 2021

It certainly doesn’t look good.

Is this a possibility elsewhere? LHC establishes what seems to be a flavor anomaly and some theoreticians redo SM predictions to remove it? Or is the muon anomaly a particularly difficult place to extract predictions?

• CommentRowNumber17.
• CommentAuthorUrs
• CommentTimeApr 8th 2021
• (edited Apr 8th 2021)

Right, so in principle this what happens all the time, that the “theorists” (as they are called in this business, meaning those who compute numerical predictions of theories) keep improving their computations hence refining their predictions, while the experimentalists keep improving their measurements.

The issue is that in these indirect precision experiments, it is typically the hadronic loop effects which cause the big problem on the theory side because these – as we have been amplifying elsewhere – are not actually theoretically understood well at all (due to the last Millennium Problem) and hence require those “lattice QCD” computer simulations – which in themselves are much like experiments (computer experiments), and which in themselves need a lot of theoretical discussion for their interpretation. This (and hence ultimately the openness of the Millennium Problem) is the main source of systematic uncertainties in this business, and it is the reason why it is possible that theory groups can disagree with each other so much, and why it is possible to claim out of the blue that all previous predictions were wrong after all, because of a “new lattice effect!”, previously underestimated.

So it’s not shocking that a theory group comes out claiming that apparent deviation from experiments shrinks with a new lattice computation.

• CommentRowNumber18.
• CommentAuthorUrs
• CommentTimeApr 8th 2021

added pointer to the actual publication:

• B. Abi et al. (Muon g−2 Collaboration), Measurement of the Positive Muon Anomalous Magnetic Moment to 0.46 ppm, Phys. Rev. Lett. 126, 141801 2021 (doi:10.1103/PhysRevLett.126.141801)

$\,$

Exposition:

Priscilla Cushman, Muon’s Escalating Challenge to the Standard Model, Physics 14, 54, April 2021 (web)

• CommentRowNumber19.
• CommentAuthorDavid_Corfield
• CommentTimeApr 8th 2021

This Nature news article

In the Nature study, Zoltan Fodor at Pennsylvania State University in University Park and his collaborators recalculated the quark contributions from scratch with a simulation technique called lattice quantum chromodynamics (lattice QCD). The technique had not previously been included in g – 2 predictions because lattice QCD was not mature enough to give high-precision results. Fodor and his team managed to improve the precision, and found g – 2 to be larger than the consensus value, and much closer to the experimental measurement.

Seems an odd way of putting things. If some technique known to provide greater precision was previously unavailable, shouldn’t the error bars from the earlier methods have been wider?

• CommentRowNumber20.
• CommentAuthorUrs
• CommentTimeApr 8th 2021
• (edited Apr 8th 2021)

One pleasant effect of the increasing flavour/muon anomalies is that Resonaances is coming back to life (Adam Falkowski’s HEP musings).

• CommentRowNumber21.
• CommentAuthorDavid_Corfield
• CommentTimeApr 8th 2021

Yes, that’s a good article. Sounds like the onus is now on the “theorists” you mention in #17. Non-overlapping error bars is awkward.

Then we await the theorists (without scare quotes) to solve the Mass gap problem, etc.

• CommentRowNumber22.
• CommentAuthorUrs
• CommentTimeApr 9th 2021

Here is an interesting commentary on the role of the lattice computations in $(g-2)_\mu$. The bottom line of that comment is (with detailed citations offered for all claims) that disregarding the BMW article at this point both followed the prescribed rules and remains justified also in hindsight.

Notice in particular item 4 in the comment, matching the assessment in #17 above of how the lattice computations are a bit of an experimental mystery all in themselves.

• CommentRowNumber23.
• CommentAuthorDavid_Corfield
• CommentTimeApr 9th 2021

His point 1 is interesting too.

1 – Even if the theoretical determination of the hadronic vacuum polarization (HVP) moves the theory prediction to match the experimentally measured muon g-2, this just shuffles the tension to other parts of the electroweak constraints (with similarly sized tension) https://arxiv.org/abs/2003.04886

• CommentRowNumber24.
• CommentAuthorUrs
• CommentTimeApr 9th 2021
• CommentRowNumber25.
• CommentAuthorUrs
• CommentTimeAug 18th 2021

• Tatsumi Aoyama, Masashi Hayakawa, Toichiro Kinoshita, Makiko Nio, Tenth-Order QED Contribution to the Electron $g-2$ and an Improved Value of the Fine Structure Constant, Phys. Rev. Lett. 109, 111807 (arXiv:1205.5368, doi:10.1103/PhysRevLett.109.111807)
• CommentRowNumber26.
• CommentAuthorUrs
• CommentTimeAug 30th 2021

added pointer to today’s:

• Josef Leutgeb, Anton Rebhan, Hadronic light-by-light contribution to the muon $g-2$ from holographic QCD with massive pions (arXiv:2108.12345)
• CommentRowNumber27.
• CommentAuthorUrs
• CommentTimeOct 20th 2021

added pointer to today’s:

• Song Li, Yang Xiao, Jin Min Yang, *A pedagogical review on muon $g-2$, Modern Physics 4 (2021) 40-47 (arXiv:2110.04673)