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Many years ago, a discrepancy was found between the experimentally measured value of the muon magnetic moment, and the theoretically calculated value. Shockingly, most physicists were blase about it. It was no big deal to them. They dismissed it as either an experimental error, or some mistake in the QCD calculations, even though error bars have been painstakingly computed for both of them, and the discrepency still survived up to a few sigmas.

What is the current status of the anomalous muon magnetic moment?

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The measured value cant be "anomalous". Its precise or not. The anomality is the difference between some theoretic value and the real value. –  Georg Apr 17 '11 at 11:58
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@Georg Please read the first two paragraphs in en.wikipedia.org/wiki/Anomalous_magnetic_moment . It is standard terminology, the "anomalous" is versus the magnetic dipole moment coming from the solution of Dirac's equation, the first order feynman diagram. Higher order corrections introduce an "anomaly" to the magnetic moment and in principle could be used to examine/limit/find new physics. –  anna v Apr 17 '11 at 12:45
    
@AnnaV, maybe this is Wiki or even physics terminology, but that shows the verbal deficiencies of the writers, not more. –  Georg Apr 17 '11 at 12:53
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Georg It is a historical terminology. First everybody thought that the solutions of Dirac's equations was the end all about magnetic moments. Then the experiments came out with a difference not acceptable within errors. Then higher order corrections came into play and people tried to calculate the correction to the Dirac moment. The anomaly is versus the Dirac g-2 expectation. –  anna v Apr 17 '11 at 14:22
    
I had wanted to say a few words about the early results from the Brookhaven experiment. They were going around giving talks with a preliminary value 4+ sigma away from the theory...I saw one of them. Then later they admitted they'd added some of the diagrams with the wrong phase. Does anyone have a reference to this history? I haven't been able to find it. –  dmckee Apr 17 '11 at 22:20
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2 Answers 2

One can find a number of references by googling. Here is one: http://arxiv.org/PS_cache/hep-ph/pdf/0102/0102122v2.pdf

The discrepancy between measurements and standard model calculations for the muon anomalous magnetic moment is of the order of 2.6 sigma. There are interpretations that this is a signal for new physics.

If you look at the discussion of standard deviations needed to declare a discovery in particle physics you will find that 5 sigma is really necessary. 2.5 sigma is barely at the level of "interesting" That is why there is not much discussion and you observe :

Shockingly, most physicists were blase about it.

It was no big deal to them except :interesting, more data needed.

If it were 4 sigma, they would be jumping up and down with excitement and speculating about the masses of new physics intermediate particles.

An example is the accurate data from LEP that allowed to calculate the top mass, before it was observed at Fermi lab, from differences in expectations.

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I dont understand why you didnt considered the 2007 paper (mentioned in my answer- 3.5$\sigma$) –  Helder Velez Apr 17 '11 at 13:19
    
I had not found it. It makes no difference to the argument, 3.5 sigma is still too small. –  anna v Apr 17 '11 at 14:24
    
3.5 is small? There are papers going around every month now for any new measurement from Tevatron that is about 3 sigma. Forward-backward asymmetry before, now the 150 GeV particle, and so on. Lots of physicists are actually jumping up and down already at 2 sigma these days ;) –  Marek Apr 17 '11 at 17:22
    
@Marek I know, and lots of physicists are wrong. I have said it before: back in the time of resonance counting we had a 4sigma mupi resonance in a neutrino experiment. Lots of jumping up and down and then hiding, though if you go to the archived data and do the same analysis the resonance will be there. It was not taking into account the systematics seriously which when done correctly lowered the significance. I suspect the same is true of the two last announcements where the errors shown are consistent with being just statistical ones. –  anna v Apr 17 '11 at 17:46
    
that's true of course. Still, I don't see anything bad about the excitement. People are producing lots of results to explain these new would-be-signals, most of which will be completely unneeded; but this is not so different from the normal research work. You never know whether you are not working on a dead end... –  Marek Apr 17 '11 at 19:42
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WP Anomalous_magnetic_moment_of_the_muon 3.4$\sigma$ standard deviation (as of 2007)

is a contribution of effects of quantum mechanics, expressed by Feynman diagrams with loops, to the magnetic moment of that particle. ...its measurement provides a precision test of the Standard Model.

The experiment Home Page - E821 Muon (g-2) (as of 2004)
Overview of the experiment
Improved predictions as of 2007 : 3.4$\sigma$ :
Abstract

We update the Standard Model predictions of the anomalous magnetic moment of the muon..., incorporating the new $e_{+}e_{-}-> \pi\pi$ data obtained by CMD-2 and KLOE, as well as the corrected SND data, and other improvements. The prediction .. which corresponds to a 3.4 $sigma$ deviation from the measured value...

But the final Note added in proof:(page 10) points to 3.5$\sigma$ discrepancy
Future Experiments

I will take this fact in consideration when I read that SM is OK.
(there is a particle model by Douglas Pinnow (Our Resonant Universe) waiting to be read and commented. Have a glimpse of it here)

EDIT add
IMO, add: In the experiment the muons are obtained by p+p collisions; add: the proton 'spin crisis' antiquity; then the model defect hypothesis is enhanced. (both problems are linked, I think)

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3.4 sigma is still not enough for discovery –  anna v Apr 17 '11 at 12:48
    
$3.5\sigma$ is important if it persists after we repeat the experiment with much more data. That way it reveals a feature to be modeled instead of a bad feature of the measuring process. –  Helder Velez Apr 17 '11 at 16:02
    
ALEPH in 2002 published a 3 sigma Higgs signal. When the other three experiments looked, they found nothing, thus the doubling of the statistics just lowered the significance of the "discovery", which is the usual fate of below 4 sigma "signals". –  anna v Apr 17 '11 at 17:53
    
Time will tell us. But it started bad (your's 2.6) and after it got worst (3.5). –  Helder Velez Apr 17 '11 at 18:16
    
Well, if there is physics beyond the standard model, there should be a signal there. So lets hope it will get better and better as the statistics are increasing. –  anna v Apr 17 '11 at 18:57
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