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Latest Muon Measurements Hint at Cracks in the Standard Model

from the mysterious-muon-magnetic-moment dept.

Ars Technica

The Muon g-2 experiment (pronounced "gee minus two") is designed to look for tantalizing hints of physics beyond the Standard Model of particle physics. It does this by measuring the magnetic field (aka the magnetic moment) generated by a subatomic particle known as the muon. Back in 2001, an earlier run of the experiment at Brookhaven National Laboratory found a slight discrepancy, hinting at possible new physics, but that controversial result fell short of the critical threshold required to claim discovery.

Now, Fermilab physicists have completed their initial analysis of data from the updated Muon g-2 experiment, showing "excellent agreement" with the discrepancy Brookhaven recorded. The results were announced today in a new paper published in the journal Physical Review Letters.

Journal References:
1.) B. Abi, et al. Measurement of the Positive Muon Anomalous Magnetic Moment to 0.46 ppm [open], Physical Review Letters (DOI: 10.1103/PhysRevLett.126.141801)
2.) T. Albahri et al. (The Muon g−2 Collaboration) Magnetic-field measurement and analysis for the Muon g−2 Experiment at Fermilab [open], Physical Review A (DOI: 10.1103/PhysRevA.103.042208)
3.) T. Albahri et al. (Muon g−2 Collaboration)Measurement of the anomalous precession frequency of the muon in the Fermilab Muon g−2 Experiment [open], Physical Review D (DOI: 10.1103/PhysRevD.103.072002)

Original Submission

Related Stories

Muon G-2 Experiment Hints at Mysterious New Physics 20 comments

What do touch screens, radiation therapy and shrink wrap have in common? They were all made possible by particle physics research. Discoveries of how the universe works at the smallest scale often lead to huge advances in technology we use every day.

Scientists from the U.S. Department of Energy’s (DOE) Argonne National Laboratory and Fermi National Accelerator Laboratory, along with collaborators from 46 other institutions and seven countries, are conducting an experiment to put our current understanding of the universe to the test. The first result points to the existence of undiscovered particles or forces. This new physics could help explain long-standing scientific mysteries, and the new insight adds to a storehouse of information that scientists can tap into when modeling our universe and developing new technologies.

The experiment, Muon g-2 (pronounced Muon g minus 2), follows one that began in the ​‘90s at DOE’s Brookhaven National Laboratory, in which scientists measured a magnetic property of a fundamental particle called the muon.

The Brookhaven experiment yielded a result that differed from the value predicted by the Standard Model, scientists’ best description of the makeup and behavior of the universe yet. The new experiment is a recreation of Brookhaven’s, built to challenge or affirm the discrepancy with higher precision.

The Standard Model very precisely predicts the muon’s g-factor — a value that tells scientists how this particle behaves in a magnetic field. This g-factor is known to be close to the value two, and the experiments measure their deviation from two, hence the name Muon g-2.

[Update: This story appears to be a dupe of Latest Muon Measurements Hint at Cracks in the Standard Model; as there are already comments here, it will remain posted for discussion. --martyb]

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• standard model seems okstandard model seems ok(Score: 2, Interesting) by Anonymous Coward on Thursday April 08, @05:35AM (5 children)

by Anonymous Coward on Thursday April 08, @05:35AM (#1134670)

Peter Woit (writer of Not Even Wrong) points out that the theoretical calculation that is in contradiction with the experiment may be itself wrong: https://www.math.columbia.edu/~woit/wordpress/?p=12292 [columbia.edu]
A new calculation of the same number is actually in agreement with the experiment.

• Re:standard model seems okRe:standard model seems ok(Score: 3, Informative) by kvutza on Thursday April 08, @11:30AM (1 child)

by kvutza (11959) on Thursday April 08, @11:30AM (#1134731)

Welp, it is about not having a clearly correct theoretical prediction at this level preciseness: two ways to calculate one contribution differ and no one knows who is right. Notice that it is not about newer vs. older calculations, it is about two different methods.

BTW Another blog post that discusses it.
https://resonaances.blogspot.com/2021/04/why-is-it-when-something-happens-it-is.html [blogspot.com]

• Re:standard model seems ok(Score: 0) by Anonymous Coward on Thursday April 08, @09:15PM

by Anonymous Coward on Thursday April 08, @09:15PM (#1135010)

Have tgey tried the new Common Core way to solve the equation, instead of the "old method?"

• Re:standard model seems okRe:standard model seems ok(Score: 0) by Anonymous Coward on Thursday April 08, @01:19PM (1 child)

by Anonymous Coward on Thursday April 08, @01:19PM (#1134756)

I hear that they had to throw out the standard model because it didn't predict global warming with sufficient speed, risking their funding.

• Re:standard model seems ok(Score: 0) by Anonymous Coward on Thursday April 08, @02:03PM

by Anonymous Coward on Thursday April 08, @02:03PM (#1134773)

Ha, ha! The one thing that could finally bring down the Standard Model.

• Re:standard model seems ok(Score: 2) by FatPhil on Friday April 09, @11:00AM

> A new calculation of the same number is actually in agreement with the experiment.

$$a_\mu(BMW)=116591954(55)×10^{−11}$$
$$a_\mu(Exp)=116592061(41)×10^{−11}$$

OK, it's not in actual disagreement, but the error bars don't touch, so "actually in agreement" is a bit of a stretch. In fact, the bell-curves line up at such a tantalising position that a claim of them being in agreement is on an equally weak footing as a claim of them being in disagreement. So best to not make either assertion just yet.

Fortunately for the scientists, it definitely points to "we need to do more hard science to get closer to the truth".
--
I know I'm God, because every time I pray to him, I find I'm talking to myself.
• That's ok...(Score: 0) by Anonymous Coward on Thursday April 08, @05:41AM

by Anonymous Coward on Thursday April 08, @05:41AM (#1134674)

Always wanted an automatic

• standard modelstandard model(Score: 1, Offtopic) by melyan on Thursday April 08, @06:47AM (6 children)

by melyan (14385) on Thursday April 08, @06:47AM (#1134696) Journal

It's all social status. Consider using square root of the square difference as an example. Absolute deviation works better, especially when closed form solutions aren't needed (computer numerical methods).

• Re:standard modelRe:standard model(Score: 2) by PiMuNu on Thursday April 08, @08:21AM (2 children)

by PiMuNu (3823) on Thursday April 08, @08:21AM (#1134710)

That post makes no sense.

• Re:standard modelRe:standard model(Score: 0) by Anonymous Coward on Thursday April 08, @09:53AM (1 child)

by Anonymous Coward on Thursday April 08, @09:53AM (#1134719)

didn't you hear? GP doesn't have an ego. I think that happens below an IQ of 60 or so.
since it is you who are prejudiced to communicate with higher IQs, the misunderstanding is your fault.

• Re:standard model(Score: 1) by melyan on Monday April 12, @09:26PM

by melyan (14385) on Monday April 12, @09:26PM (#1136682) Journal

Nope. I'm in mensa.

• Re:standard modelRe:standard model(Score: 2) by crafoo on Thursday April 08, @11:46AM (2 children)

by crafoo (6639) on Thursday April 08, @11:46AM (#1134733)

but if I don't recursively use squares of square roots on float32, how will I get my numerical model to explode? Also, subtracting two approximately similar values is fun, especially inside square roots.

• Re:standard model(Score: 0) by Anonymous Coward on Thursday April 08, @12:29PM

by Anonymous Coward on Thursday April 08, @12:29PM (#1134741)

Nerd.

• Re:standard model(Score: 0) by Anonymous Coward on Thursday April 08, @02:33PM

by Anonymous Coward on Thursday April 08, @02:33PM (#1134784)

You can use float16 on GPU nowadays. Think of the speedup!

• cracks in the standard model(Score: 2) by FatPhil on Friday April 09, @11:05AM

The cracks appeared in the late 60s, before we'd even finished formulating the standard model. At least 2 Nobel prizes have been awarded for this. And this isn't a result which makes a measurement deviate from predictions by fractions of a part per billion, this is a result that makes measurements deviate from predictions by a factor of 3: https://en.wikipedia.org/wiki/Homestake_experiment
--
I know I'm God, because every time I pray to him, I find I'm talking to myself.
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